vim9.txt For Vim version 9.1. Last change: 2024 May 31 VIM REFERENCE MANUAL by Bram Moolenaar Vim9 script commands and expressions. Vim9 vim9 Most expression help is in eval.txt. This file is about the new syntax and features in Vim9 script. 1. What is Vim9 script? Vim9-script 2. Differences vim9-differences 3. New style functions fast-functions 4. Types vim9-types 5. Namespace, Import and Export vim9script 6. Classes and interfaces vim9-classes 9. Rationale vim9-rationale ============================================================================== 1. What is Vim9 script? Vim9-script Vim script has been growing over time, while preserving backwards compatibility. That means bad choices from the past often can't be changed and compatibility with Vi restricts possible solutions. Execution is quite slow, each line is parsed every time it is executed. The main goal of Vim9 script is to drastically improve performance. This is accomplished by compiling commands into instructions that can be efficiently executed. An increase in execution speed of 10 to 100 times can be expected. A secondary goal is to avoid Vim-specific constructs and get closer to commonly used programming languages, such as JavaScript, TypeScript and Java. The performance improvements can only be achieved by not being 100% backwards compatible. For example, making function arguments available in the "a:" dictionary adds quite a lot of overhead. In a Vim9 function this dictionary is not available. Other differences are more subtle, such as how errors are handled. The Vim9 script syntax and semantics are used in: - a function defined with the :def command - a script file where the first command is vim9script - an autocommand defined in the context of the above - a command prefixed with the vim9cmd command modifier When using :function in a Vim9 script file the legacy syntax is used, with the highest scriptversion. However, this can be confusing and is therefore discouraged. Vim9 script and legacy Vim script can be mixed. There is no requirement to rewrite old scripts, they keep working as before. You may want to use a few :def functions for code that needs to be fast. :vim9[cmd] {cmd} :vim9 :vim9cmd E1164 Evaluate and execute {cmd} using Vim9 script syntax and semantics. Useful when typing a command and in a legacy script or function. :leg[acy] {cmd} :leg :legacy E1189 E1234 Evaluate and execute {cmd} using legacy script syntax and semantics. Only useful in a Vim9 script or a :def function. Note that {cmd} cannot use local variables, since it is parsed with legacy expression syntax. See some examples of Vim9 script at 52.6. ============================================================================== 2. Differences from legacy Vim script vim9-differences Overview E1146 Brief summary of the differences you will most often encounter when using Vim9 script and :def functions; details are below: - Comments start with #, not ": echo "hello" # comment - Using a backslash for line continuation is hardly ever needed: echo "hello " .. yourName .. ", how are you?" - White space is required in many places to improve readability. - Assign values without :let E1126 , declare variables with :var: var count = 0 count += 3 - Constants can be declared with :final and :const: final matches = [] # add to the list later const names = ['Betty', 'Peter'] # cannot be changed - :final cannot be used as an abbreviation of :finally. - Variables and functions are script-local by default. - Functions are declared with argument types and return type: def CallMe(count: number, message: string): bool - Call functions without :call: writefile(['done'], 'file.txt') - You cannot use old Ex commands: :Print :append :change :d directly followed by 'd' or 'p'. :insert :k :mode :open :s with only flags :t :xit - Some commands, especially those used for flow control, cannot be shortened. E.g., :throw cannot be written as :th. vim9-no-shorten - You cannot use curly-braces names. - A range before a command must be prefixed with a colon: :%s/this/that - Executing a register with "@r" does not work, you can prepend a colon or use :exe: :exe @a - Unless mentioned specifically, the highest scriptversion is used. - When defining an expression mapping, the expression will be evaluated in the context of the script where it was defined. - When indexing a string the index is counted in characters, not bytes: vim9-string-index - Some possibly unexpected differences: vim9-gotchas. Comments starting with # In legacy Vim script comments start with double quote. In Vim9 script comments start with #. # declarations var count = 0 # number of occurrences The reason is that a double quote can also be the start of a string. In many places, especially halfway through an expression with a line break, it's hard to tell what the meaning is, since both a string and a comment can be followed by arbitrary text. To avoid confusion only # comments are recognized. This is the same as in shell scripts and Python programs. In Vi # is a command to list text with numbers. In Vim9 script you can use :number for that. :101 number To improve readability there must be a space between a command and the # that starts a comment: var name = value # comment var name = value# error! E1170 Do not start a comment with #{, it looks like the legacy dictionary literal and produces an error where this might be confusing. #{{ or #{{{ are OK, these can be used to start a fold. When starting to read a script file Vim doesn't know it is Vim9 script until the vim9script command is found. Until that point you would need to use legacy comments: " legacy comment vim9script # Vim9 comment That looks ugly, better put vim9script in the very first line: vim9script # Vim9 comment In legacy Vim script # is also used for the alternate file name. In Vim9 script you need to use %% instead. Instead of ## use %%% (stands for all arguments). Vim9 functions E1099 A function defined with :def is compiled. Execution is many times faster, often 10 to 100 times. Many errors are already found when compiling, before the function is executed. The syntax is strict, to enforce code that is easy to read and understand. Compilation is done when any of these is encountered: - the first time the function is called - when the :defcompile command is encountered in the script after the function was defined - :disassemble is used for the function. - a function that is compiled calls the function or uses it as a function reference (so that the argument and return types can be checked) E1091 E1191 If compilation fails it is not tried again on the next call, instead this error is given: "E1091: Function is not compiled: {name}". Compilation will fail when encountering a user command that has not been created yet. In this case you can call execute() to invoke it at runtime. def MyFunc() execute('DefinedLater') enddef :def has no options like :function does: "range", "abort", "dict" or "closure". A :def function always aborts on an error (unless :silent! was used for the command or the error was caught a :try block), does not get a range passed, cannot be a "dict" function, and can always be a closure. vim9-no-dict-function You can use a Vim9 Class (Vim9-class) instead of a "dict function". You can also pass the dictionary explicitly: def DictFunc(self: dict<any>, arg: string) echo self[arg] enddef var ad = {item: 'value', func: DictFunc} ad.func(ad, 'item') You can call a legacy dict function though: func Legacy() dict echo self.value endfunc def CallLegacy() var d = {func: Legacy, value: 'text'} d.func() enddef E1096 E1174 E1175 The argument types and return type need to be specified. The "any" type can be used, type checking will then be done at runtime, like with legacy functions. E1106 Arguments are accessed by name, without "a:", just like any other language. There is no "a:" dictionary or "a:000" list. vim9-variable-arguments E1055 E1160 E1180 Variable arguments are defined as the last argument, with a name and have a list type, similar to TypeScript. For example, a list of numbers: def MyFunc(...itemlist: list<number>) for item in itemlist ... When a function argument is optional (it has a default value) passing v:none as the argument results in using the default value. This is useful when you want to specify a value for an argument that comes after an argument that should use its default value. Example: def MyFunc(one = 'one', last = 'last') ... enddef MyFunc(v:none, 'LAST') # first argument uses default value 'one' vim9-ignored-argument E1181 The argument "_" (an underscore) can be used to ignore the argument. This is most useful in callbacks where you don't need it, but do need to give an argument to match the call. E.g. when using map() two arguments are passed, the key and the value, to ignore the key: map(numberList, (_, v) => v * 2) There is no error for using the "_" argument multiple times. No type needs to be given. Functions and variables are script-local by default vim9-scopes When using :function or :def to specify a new function at the script level in a Vim9 script, the function is local to the script. Like prefixing "s:" in legacy script. To define a global function or variable the "g:" prefix must be used. For functions in a script that is to be imported and in an autoload script "export" needs to be used for those to be used elsewhere. def ThisFunction() # script-local def g:ThatFunction() # global export def Function() # for import and import autoload E1058 E1075 When using :function or :def to specify a nested function inside a :def function and no namespace was given, this nested function is local to the code block it is defined in. It cannot be used in function() with a string argument, pass the function reference itself: def Outer() def Inner() echo 'inner' enddef var Fok = function(Inner) # OK var Fbad = function('Inner') # does not work Detail: this is because "Inner" will actually become a function reference to a function with a generated name. It is not possible to define a script-local function in a function. You can define a local function and assign it to a script-local Funcref (it must have been declared at the script level). It is possible to define a global function by using the "g:" prefix. When referring to a function and no "s:" or "g:" prefix is used, Vim will search for the function: - in the function scope, in block scopes - in the script scope Imported functions are found with the prefix from the :import command. Since a script-local function reference can be used without "s:" the name must start with an upper case letter even when using the "s:" prefix. In legacy script "s:funcref" could be used, because it could not be referred to with "funcref". In Vim9 script it can, therefore "s:Funcref" must be used to avoid that the name interferes with builtin functions. vim9-s-namespace E1268 The use of the "s:" prefix is not supported at the Vim9 script level. All functions and variables without a prefix are script-local. In :def functions the use of "s:" depends on the script: Script-local variables and functions in a legacy script do use "s:", while in a Vim9 script they do not use "s:". This matches what you see in the rest of the file. In legacy functions the use of "s:" for script items is required, as before. No matter if the script is Vim9 or legacy. In all cases the function must be defined before used. That is when it is called, when :defcompile causes it to be compiled, or when code that calls it is being compiled (to figure out the return type). The result is that functions and variables without a namespace can usually be found in the script, either defined there or imported. Global functions and variables could be defined anywhere (good luck finding out where! You can often see where it was last set using :verbose). E1102 Global functions can still be defined and deleted at nearly any time. In Vim9 script script-local functions are defined once when the script is sourced and cannot be deleted or replaced by itself (it can be by reloading the script). When compiling a function and a function call is encountered for a function that is not (yet) defined, the FuncUndefined autocommand is not triggered. You can use an autoload function if needed, or call a legacy function and have FuncUndefined triggered there. Reloading a Vim9 script clears functions and variables by default vim9-reload E1149 E1150 When loading a legacy Vim script a second time nothing is removed, the commands will replace existing variables and functions, create new ones, and leave removed things hanging around. When loading a Vim9 script a second time all existing script-local functions and variables are deleted, thus you start with a clean slate. This is useful if you are developing a plugin and want to try a new version. If you renamed something you don't have to worry about the old name still hanging around. If you do want to keep items, use: vim9script noclear You want to use this in scripts that use a finish command to bail out at some point when loaded again. E.g. when a buffer local option is set to a function, the function does not need to be defined more than once: vim9script noclear setlocal completefunc=SomeFunc if exists('*SomeFunc') finish endif def SomeFunc() .... Variable declarations with :var, :final and :const vim9-declaration :var E1079 E1017 E1020 E1054 E1087 E1124 Local variables need to be declared with :var. Local constants need to be declared with :final or :const. We refer to both as "variables" in this section. Variables can be local to a script, function or code block: vim9script var script_var = 123 def SomeFunc() var func_var = script_var if cond var block_var = func_var ... The variables are only visible in the block where they are defined and nested blocks. Once the block ends the variable is no longer accessible: if cond var inner = 5 else var inner = 0 endif echo inner # Error! The declaration must be done earlier: var inner: number if cond inner = 5 else inner = 0 endif echo inner Although this is shorter and faster for simple values: var inner = 0 if cond inner = 5 endif echo inner E1025 E1128 To intentionally hide a variable from code that follows, a block can be used: { var temp = 'temp' ... } echo temp # Error! This is especially useful in a user command: command -range Rename { var save = @a @a = 'some expression' echo 'do something with ' .. @a @a = save } And with autocommands: au BufWritePre *.go { var save = winsaveview() silent! exe ':%! some formatting command' winrestview(save) } Although using a :def function probably works better. E1022 E1103 E1130 E1131 E1133 E1134 Declaring a variable with a type but without an initializer will initialize to false (for bool), empty (for string, list, dict, etc.) or zero (for number, any, etc.). This matters especially when using the "any" type, the value will default to the number zero. For example, when declaring a list, items can be added: var myList: list<number> myList->add(7) Initializing a variable to a null value, e.g. null_list, differs from not initializing the variable. This throws an error: var myList = null_list myList->add(7) # E1130: Cannot add to null list E1016 E1052 E1066 In Vim9 script :let cannot be used. An existing variable is assigned to without any command. The same for global, window, tab, buffer and Vim variables, because they are not really declared. Those can also be deleted with :unlet. E1065 You cannot use :va to declare a variable, it must be written with the full name :var. Just to make sure it is easy to read. E1178 :lockvar does not work on local variables. Use :const and :final instead. The exists() and exists_compiled() functions do not work on local variables or arguments. E1006 E1041 E1167 E1168 E1213 Variables, functions and function arguments cannot shadow previously defined or imported variables and functions in the same script file. Variables may shadow Ex commands, rename the variable if needed. Global variables must be prefixed with "g:", also at the script level. vim9script var script_local = 'text' g:global = 'value' var Funcref = g:ThatFunction Global functions must be prefixed with "g:": vim9script def g:GlobalFunc(): string return 'text' enddef echo g:GlobalFunc() The "g:" prefix is not needed for auto-load functions. vim9-function-defined-later Although global functions can be called without the "g:" prefix, they must exist when compiled. By adding the "g:" prefix the function can be defined later. Example: def CallPluginFunc() if exists('g:loaded_plugin') g:PluginFunc() endif enddef If you do it like this, you get an error at compile time that "PluginFunc" does not exist, even when "g:loaded_plugin" does not exist: def CallPluginFunc() if exists('g:loaded_plugin') PluginFunc() # Error - function not found endif enddef You can use exists_compiled() to avoid the error, but then the function would not be called, even when "g:loaded_plugin" is defined later: def CallPluginFunc() if exists_compiled('g:loaded_plugin') PluginFunc() # Function may never be called endif enddef Since `&opt = value` is now assigning a value to option "opt", ":&" cannot be used to repeat a :substitute command. vim9-unpack-ignore For an unpack assignment the underscore can be used to ignore a list item, similar to how a function argument can be ignored: [a, _, c] = theList To ignore any remaining items: [a, b; _] = longList E1163 E1080 Declaring more than one variable at a time, using the unpack notation, is possible. Each variable can have a type or infer it from the value: var [v1: number, v2] = GetValues() Use this only when there is a list with values, declaring one variable per line is much easier to read and change later. Constants vim9-const vim9-final How constants work varies between languages. Some consider a variable that can't be assigned another value a constant. JavaScript is an example. Others also make the value immutable, thus when a constant uses a list, the list cannot be changed. In Vim9 we can use both. E1021 E1307 :const is used for making both the variable and the value a constant. Use this for composite structures that you want to make sure will not be modified. Example: const myList = [1, 2] myList = [3, 4] # Error! myList[0] = 9 # Error! myList->add(3) # Error! :final E1125 :final is used for making only the variable a constant, the value can be changed. This is well known from Java. Example: final myList = [1, 2] myList = [3, 4] # Error! myList[0] = 9 # OK myList->add(3) # OK It is common to write constants as ALL_CAPS, but you don't have to. The constant only applies to the value itself, not what it refers to. final females = ["Mary"] const NAMES = [["John", "Peter"], females] NAMES[0] = ["Jack"] # Error! NAMES[0][0] = "Jack" # Error! NAMES[1] = ["Emma"] # Error! NAMES[1][0] = "Emma" # OK, now females[0] == "Emma" Omitting :call and :eval E1190 Functions can be called without :call: writefile(lines, 'file') Using :call is still possible, but this is discouraged. A method call without eval is possible, so long as the start is an identifier or can't be an Ex command. For a function either "(" or "->" must be following, without a line break. Examples: myList->add(123) g:myList->add(123) [1, 2, 3]->Process() {a: 1, b: 2}->Process() "foobar"->Process() ("foobar")->Process() 'foobar'->Process() ('foobar')->Process() In the rare case there is ambiguity between a function name and an Ex command, prepend ":" to make clear you want to use the Ex command. For example, there is both the :substitute command and the substitute() function. When the line starts with substitute( this will use the function. Prepend a colon to use the command instead: :substitute(pattern (replacement ( If the expression starts with "!" this is interpreted as a shell command, not negation of a condition. Thus this is a shell command: !shellCommand->something Put the expression in parentheses to use the "!" for negation: (!expression)->Method() Note that while variables need to be defined before they can be used, functions can be called before being defined. This is required to allow for cyclic dependencies between functions. It is slightly less efficient, since the function has to be looked up by name. And a typo in the function name will only be found when the function is called. Omitting function() A user defined function can be used as a function reference in an expression without function(). The argument types and return type will then be checked. The function must already have been defined. var Funcref = MyFunction When using function() the resulting type is "func", a function with any number of arguments and any return type (including void). The function can be defined later if the argument is in quotes. Lambda using => instead of -> vim9-lambda In legacy script there can be confusion between using "->" for a method call and for a lambda. Also, when a "{" is found the parser needs to figure out if it is the start of a lambda or a dictionary, which is now more complicated because of the use of argument types. To avoid these problems Vim9 script uses a different syntax for a lambda, which is similar to JavaScript: var Lambda = (arg) => expression var Lambda = (arg): type => expression E1157 No line break is allowed in the arguments of a lambda up to and including the "=>" (so that Vim can tell the difference between an expression in parentheses and lambda arguments). This is OK: filter(list, (k, v) => v > 0) This does not work: filter(list, (k, v) => v > 0) This also does not work: filter(list, (k, v) => v > 0) But you can use a backslash to concatenate the lines before parsing: filter(list, (k, \ v) \ => v > 0) vim9-lambda-arguments E1172 In legacy script a lambda could be called with any number of extra arguments, there was no way to warn for not using them. In Vim9 script the number of arguments must match. If you do want to accept any arguments, or any further arguments, use "..._", which makes the function accept vim9-variable-arguments. Example: var Callback = (..._) => 'anything' echo Callback(1, 2, 3) # displays "anything" inline-function E1171 Additionally, a lambda can contain statements in {}: var Lambda = (arg) => { g:was_called = 'yes' return expression } This can be useful for a timer, for example: var count = 0 var timer = timer_start(500, (_) => { count += 1 echom 'Handler called ' .. count }, {repeat: 3}) The ending "}" must be at the start of a line. It can be followed by other characters, e.g.: var d = mapnew(dict, (k, v): string => { return 'value' }) No command can follow the "{", only a comment can be used there. command-block E1026 The block can also be used for defining a user command. Inside the block Vim9 syntax will be used. This is an example of using here-docs: com SomeCommand { g:someVar =<< trim eval END ccc ddd END } If the statements include a dictionary, its closing bracket must not be written at the start of a line. Otherwise, it would be parsed as the end of the block. This does not work: command NewCommand { g:mydict = { 'key': 'value', } # ERROR: will be recognized as the end of the block } Put the '}' after the last item to avoid this: command NewCommand { g:mydict = { 'key': 'value' } } Rationale: The "}" cannot be after a command because it would require parsing the commands to find it. For consistency with that no command can follow the "{". Unfortunately this means using "() => { command }" does not work, line breaks are always required. vim9-curly To avoid the "{" of a dictionary literal to be recognized as a statement block wrap it in parentheses: var Lambda = (arg) => ({key: 42}) Also when confused with the start of a command block: ({ key: value })->method() Automatic line continuation vim9-line-continuation E1097 In many cases it is obvious that an expression continues on the next line. In those cases there is no need to prefix the line with a backslash (see line-continuation). For example, when a list spans multiple lines: var mylist = [ 'one', 'two', ] And when a dict spans multiple lines: var mydict = { one: 1, two: 2, } With a function call: var result = Func( arg1, arg2 ) For binary operators in expressions not in [], {} or () a line break is possible just before or after the operator. For example: var text = lead .. middle .. end var total = start + end - correction var result = positive ? PosFunc(arg) : NegFunc(arg) For a method call using "->" and a member using a dot, a line break is allowed before it: var result = GetBuilder() ->BuilderSetWidth(333) ->BuilderSetHeight(777) ->BuilderBuild() var result = MyDict .member For commands that have an argument that is a list of commands, the | character at the start of the line indicates line continuation: autocmd BufNewFile *.match if condition | echo 'match' | endif Note that this means that in heredoc the first line cannot start with a bar: var lines =<< trim END | this doesn't work END Either use an empty line at the start or do not use heredoc. Or temporarily add the "C" flag to 'cpoptions': set cpo+=C var lines =<< trim END | this works END set cpo-=C If the heredoc is inside a function 'cpoptions' must be set before :def and restored after the :enddef. In places where line continuation with a backslash is still needed, such as splitting up a long Ex command, comments can start with '#\ ': syn region Text \ start='foo' #\ comment \ end='bar' Like with legacy script '"\ ' is used. This is also needed when line continuation is used without a backslash and a line starts with a bar: au CursorHold * echom 'BEFORE bar' #\ some comment | echom 'AFTER bar' E1050 To make it possible for the operator at the start of the line to be recognized, it is required to put a colon before a range. This example will add "start" and "print": var result = start + print Like this: var result = start + print This will assign "start" and print a line: var result = start :+ print After the range an Ex command must follow. Without the colon you can call a function without :call, but after a range you do need it: MyFunc() :% call MyFunc() Note that the colon is not required for the +cmd argument: edit +6 fname It is also possible to split a function header over multiple lines, in between arguments: def MyFunc( text: string, separator = '-' ): string Since a continuation line cannot be easily recognized the parsing of commands has been made stricter. E.g., because of the error in the first line, the second line is seen as a separate command: popup_create(some invalid expression, { exit_cb: Func}) Now "exit_cb: Func})" is actually a valid command: save any changes to the file "_cb: Func})" and exit. To avoid this kind of mistake in Vim9 script there must be white space between most command names and the argument. E1144 However, the argument of a command that is a command won't be recognized. For example, after "windo echo expr" a line break inside "expr" will not be seen. Notes: - "enddef" cannot be used at the start of a continuation line, it ends the current function. - No line break is allowed in the LHS of an assignment. Specifically when unpacking a list :let-unpack. This is OK: [var1, var2] = Func() This does not work: [var1, var2] = Func() - No line break is allowed in between arguments of an :echo, :execute and similar commands. This is OK: echo [1, 2] [3, 4] This does not work: echo [1, 2] [3, 4] - In some cases it is difficult for Vim to parse a command, especially when commands are used as an argument to another command, such as :windo. In those cases the line continuation with a backslash has to be used. White space E1004 E1068 E1069 E1074 E1127 E1202 Vim9 script enforces proper use of white space. This is no longer allowed: var name=234 # Error! var name= 234 # Error! var name =234 # Error! There must be white space before and after the "=": var name = 234 # OK White space must also be put before the # that starts a comment after a command: var name = 234# Error! var name = 234 # OK White space is required around most operators. White space is required in a sublist (list slice) around the ":", except at the start and end: otherlist = mylist[v : count] # v:count has a different meaning otherlist = mylist[:] # make a copy of the List otherlist = mylist[v :] otherlist = mylist[: v] White space is not allowed: - Between a function name and the "(": Func (arg) # Error! Func \ (arg) # Error! Func (arg) # Error! Func(arg) # OK Func( arg) # OK Func( arg # OK ) E1205 White space is not allowed in a :set command between the option name and a following "&", "!", "<", "=", "+=", "-=" or "^=". No curly braces expansion curly-braces-names cannot be used. Command modifiers are not ignored E1176 Using a command modifier for a command that does not use it gives an error. E1082 Also, using a command modifier without a following command is now an error. Dictionary literals vim9-literal-dict E1014 Traditionally Vim has supported dictionary literals with a {} syntax: let dict = {'key': value} Later it became clear that using a simple text key is very common, thus literal dictionaries were introduced in a backwards compatible way: let dict = #{key: value} However, this #{} syntax is unlike any existing language. As it turns out that using a literal key is much more common than using an expression, and considering that JavaScript uses this syntax, using the {} form for dictionary literals is considered a much more useful syntax. In Vim9 script the {} form uses literal keys: var dict = {key: value} This works for alphanumeric characters, underscore and dash. If you want to use another character, use a single or double quoted string: var dict = {'key with space': value} var dict = {"key\twith\ttabs": value} var dict = {'': value} # empty key E1139 In case the key needs to be an expression, square brackets can be used, just like in JavaScript: var dict = {["key" .. nr]: value} The key type can be string, number, bool or float. Other types result in an error. Without using [] the value is used as a string, keeping leading zeros. An expression given with [] is evaluated and then converted to a string. Leading zeros will then be dropped: var dict = {000123: 'without', [000456]: 'with'} echo dict {'456': 'with', '000123': 'without'} A float only works inside [] because the dot is not accepted otherwise: var dict = {[00.013]: 'float'} echo dict {'0.013': 'float'} No :xit, :t, :k, :append, :change or :insert E1100 These commands are too easily confused with local variable names. Instead of :x or :xit you can use :exit. Instead of :t you can use :copy. Instead of :k you can use :mark. Comparators The 'ignorecase' option is not used for comparators that use strings. Thus "=~" works like "=~#". "is" and "isnot" (expr-is and expr-isnot) when used on strings now return false. In legacy script they just compare the strings, in Vim9 script they check identity, and strings are copied when used, thus two strings are never the same (this might change someday if strings are not copied but reference counted). Abort after error In legacy script, when an error is encountered, Vim continues to execute following lines. This can lead to a long sequence of errors and need to type CTRL-C to stop it. In Vim9 script execution of commands stops at the first error. Example: vim9script var x = does-not-exist echo 'not executed' For loop E1254 The loop variable must not be declared yet: var i = 1 for i in [1, 2, 3] # Error! It is possible to use a global variable though: g:i = 1 for g:i in [1, 2, 3] echo g:i endfor Legacy Vim script has some tricks to make a for loop over a list handle deleting items at the current or previous item. In Vim9 script it just uses the index, if items are deleted then items in the list will be skipped. Example legacy script: let l = [1, 2, 3, 4] for i in l echo i call remove(l, index(l, i)) endfor Would echo: 1 2 3 4 In compiled Vim9 script you get: 1 3 Generally, you should not change the list that is iterated over. Make a copy first if needed. When looping over a list of lists, the nested lists can be changed. The loop variable is "final", it cannot be changed but what its value can be changed. E1306 The depth of loops, :for and :while loops added together, cannot exceed 10. Conditions and expressions vim9-boolean Conditions and expressions are mostly working like they do in other languages. Some values are different from legacy Vim script: value legacy Vim script Vim9 script 0 falsy falsy 1 truthy truthy 99 truthy Error! "0" falsy Error! "99" truthy Error! "text" falsy Error! For the "??" operator and when using "!" then there is no error, every value is either falsy or truthy. This is mostly like JavaScript, except that an empty list and dict is falsy: type truthy when bool true, v:true or 1 number non-zero float non-zero string non-empty blob non-empty list non-empty (different from JavaScript) dictionary non-empty (different from JavaScript) func when there is a function name special true or v:true job when not NULL channel when not NULL class when not NULL object when not NULL (TODO: when isTrue() returns true) The boolean operators "||" and "&&" expect the values to be boolean, zero or one: 1 || false == true 0 || 1 == true 0 || false == false 1 && true == true 0 && 1 == false 8 || 0 Error! 'yes' && 0 Error! [] || 99 Error! When using "!" for inverting, there is no error for using any type and the result is a boolean. "!!" can be used to turn any value into boolean: !'yes' == false !![] == false !![1, 2, 3] == true When using ".." for string concatenation arguments of simple types are always converted to string: 'hello ' .. 123 == 'hello 123' 'hello ' .. v:true == 'hello true' Simple types are Number, Float, Special and Bool. For other types string() should be used. false true null null_blob null_channel null_class null_dict null_function null_job null_list null_object null_partial null_string E1034 In Vim9 script one can use the following predefined values: true false null null_blob null_channel null_class null_dict null_function null_job null_list null_object null_partial null_string true is the same as v:true, false the same as v:false, null the same as v:null. While null has the type "special", the other "null_" values have the type indicated by their name. Quite often a null value is handled the same as an empty value, but not always. The values can be useful to clear a script-local variable, since they cannot be deleted with :unlet. E.g.: var theJob = job_start(...) # let the job do its work theJob = null_job The values can also be useful as the default value for an argument: def MyFunc(b: blob = null_blob) # Note: compare against null, not null_blob, # to distinguish the default value from an empty blob. if b == null # b argument was not given See null-compare for more information about testing against null. It is possible to compare null with any value, this will not give a type error. However, comparing null with a number, float or bool will always result in false. This is different from legacy script, where comparing null with zero or false would return true. vim9-false-true When converting a boolean to a string false and true are used, not v:false and v:true like in legacy script. v:none has no none replacement, it has no equivalent in other languages. vim9-string-index Indexing a string with [idx] or taking a slice with [idx : idx] uses character indexes instead of byte indexes. Composing characters are included. Example: echo 'bár'[1] In legacy script this results in the character 0xc3 (an illegal byte), in Vim9 script this results in the string 'á'. A negative index is counting from the end, "[-1]" is the last character. To exclude the last character use slice(). To count composing characters separately use strcharpart(). If the index is out of range then an empty string results. In legacy script "++var" and "--var" would be silently accepted and have no effect. This is an error in Vim9 script. Numbers starting with zero are not considered to be octal, only numbers starting with "0o" are octal: "0o744". scriptversion-4 What to watch out for vim9-gotchas Vim9 was designed to be closer to often used programming languages, but at the same time tries to support the legacy Vim commands. Some compromises had to be made. Here is a summary of what might be unexpected. Ex command ranges need to be prefixed with a colon. -> legacy Vim: shifts the previous line to the right ->func() Vim9: method call in a continuation line :-> Vim9: shifts the previous line to the right %s/a/b legacy Vim: substitute on all lines x = alongname % another Vim9: modulo operator in a continuation line :%s/a/b Vim9: substitute on all lines 't legacy Vim: jump to mark t 'text'->func() Vim9: method call :'t Vim9: jump to mark t Some Ex commands can be confused with assignments in Vim9 script: g:name = value # assignment :g:pattern:cmd # :global command To avoid confusion between a :global or :substitute command and an expression or assignment, a few separators cannot be used when these commands are abbreviated to a single character: ':', '-' and '.'. g:pattern:cmd # invalid command - ERROR s:pattern:repl # invalid command - ERROR g-pattern-cmd # invalid command - ERROR s-pattern-repl # invalid command - ERROR g.pattern.cmd # invalid command - ERROR s.pattern.repl # invalid command - ERROR Also, there cannot be a space between the command and the separator: g /pattern/cmd # invalid command - ERROR s /pattern/repl # invalid command - ERROR Functions defined with :def compile the whole function. Legacy functions can bail out, and the following lines are not parsed: func Maybe() if !has('feature') return endif use-feature endfunc Vim9 functions are compiled as a whole: def Maybe() if !has('feature') return endif use-feature # May give a compilation error enddef For a workaround, split it in two functions: func Maybe() if has('feature') call MaybeInner() endif endfunc if has('feature') def MaybeInner() use-feature enddef endif Or put the unsupported code inside an if with a constant expression that evaluates to false: def Maybe() if has('feature') use-feature endif enddef The exists_compiled() function can also be used for this. vim9-user-command Another side effect of compiling a function is that the presence of a user command is checked at compile time. If the user command is defined later an error will result. This works: command -nargs=1 MyCommand echom <q-args> def Works() MyCommand 123 enddef This will give an error for "MyCommand" not being defined: def Works() command -nargs=1 MyCommand echom <q-args> MyCommand 123 enddef A workaround is to invoke the command indirectly with :execute: def Works() command -nargs=1 MyCommand echom <q-args> execute 'MyCommand 123' enddef Note that for unrecognized commands there is no check for "|" and a following command. This will give an error for missing endif: def Maybe() if has('feature') | use-feature | endif enddef Other differences Patterns are used like 'magic' is set, unless explicitly overruled. The 'edcompatible' option value is not used. The 'gdefault' option value is not used. You may also find this wiki useful. It was written by an early adopter of Vim9 script: https://github.com/lacygoill/wiki/blob/master/vim/vim9.md :++ :-- The ++ and -- commands have been added. They are very similar to adding or subtracting one: ++var var += 1 --var var -= 1 Using ++var or --var in an expression is not supported yet. ============================================================================== 3. New style functions fast-functions :def E1028 :def[!] {name}([arguments])[: {return-type}] Define a new function by the name {name}. The body of the function follows in the next lines, until the matching :enddef. E1073 E1011 The {name} must be less than 100 bytes long. E1003 E1027 E1056 E1059 The type of value used with :return must match {return-type}. When {return-type} is omitted or is "void" the function is not expected to return anything. E1077 E1123 {arguments} is a sequence of zero or more argument declarations. There are three forms: {name}: {type} {name} = {value} {name}: {type} = {value} The first form is a mandatory argument, the caller must always provide them. The second and third form are optional arguments. When the caller omits an argument the {value} is used. The function will be compiled into instructions when called, or when :disassemble or :defcompile is used. Syntax and type errors will be produced at that time. It is possible to nest :def inside another :def or :function up to about 50 levels deep. E1117 [!] is used as with :function. Note that script-local functions cannot be deleted or redefined later in Vim9 script. They can only be removed by reloading the same script. :enddef E1057 E1152 E1173 :enddef End of a function defined with :def. It should be on a line by its own. You may also find this wiki useful. It was written by an early adopter of Vim9 script: https://github.com/lacygoill/wiki/blob/master/vim/vim9.md If the script the function is defined in is Vim9 script, then script-local variables can be accessed without the "s:" prefix. They must be defined before the function is compiled. If the script the function is defined in is legacy script, then script-local variables must be accessed with the "s:" prefix if they do not exist at the time of compiling. E1269 Script-local variables in a Vim9 script must be declared at the script level. They cannot be created in a function, also not in a legacy function. :defc :defcompile :defc[ompile] Compile functions and classes (class-compile) defined in the current script that were not compiled yet. This will report any errors found during compilation. :defc[ompile] MyClass Compile all methods in a class. class-compile :defc[ompile] {func} :defc[ompile] debug {func} :defc[ompile] profile {func} Compile function {func}, if needed. Use "debug" and "profile" to specify the compilation mode. This will report any errors found during compilation. {func} call also be "ClassName.functionName" to compile a function or method in a class. {func} call also be "ClassName" to compile all functions and methods in a class. :disa :disassemble :disa[ssemble] {func} Show the instructions generated for {func}. This is for debugging and testing. E1061 Note that for command line completion of {func} you can prepend "s:" to find script-local functions. :disa[ssemble] profile {func} Like :disassemble but with the instructions used for profiling. :disa[ssemble] debug {func} Like :disassemble but with the instructions used for debugging. Limitations Local variables will not be visible to string evaluation. For example: def MapList(): list<string> var list = ['aa', 'bb', 'cc', 'dd'] return range(1, 2)->map('list[v:val]') enddef The map argument is a string expression, which is evaluated without the function scope. Instead, use a lambda: def MapList(): list<string> var list = ['aa', 'bb', 'cc', 'dd'] return range(1, 2)->map((_, v) => list[v]) enddef For commands that are not compiled, such as :edit, backtick expansion can be used and it can use the local scope. Example: def Replace() var fname = 'blah.txt' edit `=fname` enddef Closures defined in a loop will share the same context. For example: var flist: list<func> for i in range(5) var inloop = i flist[i] = () => inloop endfor echo range(5)->map((i, _) => flist[i]()) # Result: [4, 4, 4, 4, 4] E1271 A closure must be compiled in the context that it is defined in, so that variables in that context can be found. This mostly happens correctly, except when a function is marked for debugging with :breakadd after it was compiled. Make sure to define the breakpoint before compiling the outer function. The "inloop" variable will exist only once, all closures put in the list refer to the same instance, which in the end will have the value 4. This is efficient, also when looping many times. If you do want a separate context for each closure, call a function to define it: def GetClosure(i: number): func var infunc = i return () => infunc enddef var flist: list<func> for i in range(5) flist[i] = GetClosure(i) endfor echo range(5)->map((i, _) => flist[i]()) # Result: [0, 1, 2, 3, 4] In some situations, especially when calling a Vim9 closure from legacy context, the evaluation will fail. E1248 Note that at the script level the loop variable will be invalid after the loop, also when used in a closure that is called later, e.g. with a timer. This will generate error E1302: for n in range(4) timer_start(500 * n, (_) => { echowin n }) endfor You need to use a block and define a variable there, and use that one in the closure: for n in range(4) { var nr = n timer_start(500 * n, (_) => { echowin nr }) } endfor Using :echowindow is useful in a timer, the messages go into a popup and will not interfere with what the user is doing when it triggers. Converting a function from legacy to Vim9 convert_legacy_function_to_vim9 These are the most changes that need to be made to convert a legacy function to a Vim9 function: - Change func or function to def. - Change endfunc or endfunction to enddef. - Add types to the function arguments. - If the function returns something, add the return type. - Change comments to start with # instead of ". For example, a legacy function: func MyFunc(text) " function body endfunc Becomes: def MyFunc(text: string): number # function body enddef - Remove "a:" used for arguments. E.g.: return len(a:text) Becomes: return len(text) - Change let used to declare a variable to var. - Remove let used to assign a value to a variable. This is for local variables already declared and b: w: g: and t: variables. For example, legacy function: let lnum = 1 let lnum += 3 let b:result = 42 Becomes: var lnum = 1 lnum += 3 b:result = 42 - Insert white space in expressions where needed. - Change "." used for concatenation to "..". For example, legacy function: echo line(1).line(2) Becomes: echo line(1) .. line(2) - line continuation does not always require a backslash: echo ['one', \ 'two', \ 'three' \ ] Becomes: echo ['one', 'two', 'three' ] Calling a function in an expr option expr-option-function The value of a few options, such as 'foldexpr', is an expression that is evaluated to get a value. The evaluation can have quite a bit of overhead. One way to minimize the overhead, and also to keep the option value very simple, is to define a compiled function and set the option to call it without arguments. Example: vim9script def MyFoldFunc(): any ... compute fold level for line v:lnum return level enddef set foldexpr=s:MyFoldFunc() ============================================================================== 4. Types vim9-types E1008 E1009 E1010 E1012 E1013 E1029 E1030 The following builtin types are supported: bool number float string blob list<{type}> dict<{type}> job channel func func: {type} func({type}, ...) func({type}, ...): {type} void Not supported yet: tuple<a: {type}, b: {type}, ...> These types can be used in declarations, but no simple value will actually have the "void" type. Trying to use a void (e.g. a function without a return value) results in error E1031 E1186 . There is no array type, use list<{type}> instead. For a list constant an efficient implementation is used that avoids allocating a lot of small pieces of memory. vim9-func-declaration E1005 E1007 A partial and function can be declared in more or less specific ways: func any kind of function reference, no type checking for arguments or return value func: void any number and type of arguments, no return value func: {type} any number and type of arguments with specific return type func() function with no argument, does not return a value func(): void same func(): {type} function with no argument and return type func({type}) function with argument type, does not return a value func({type}): {type} function with argument type and return type func(?{type}) function with type of optional argument, does not return a value func(...list<{type}>) function with type of list for variable number of arguments, does not return a value func({type}, ?{type}, ...list<{type}>): {type} function with: - type of mandatory argument - type of optional argument - type of list for variable number of arguments - return type If the return type is "void" the function does not return a value. The reference can also be a Partial, in which case it stores extra arguments and/or a dictionary, which are not visible to the caller. Since they are called in the same way the declaration is the same. Custom types can be defined with :type: :type MyList list<string> Custom types must start with a capital letter, to avoid name clashes with builtin types added later, similarly to user functions. And classes and interfaces can be used as types: :class MyClass :var mine: MyClass :interface MyInterface :var mine: MyInterface :class MyTemplate<Targ> :var mine: MyTemplate<number> :var mine: MyTemplate<string> :class MyInterface<Targ> :var mine: MyInterface<number> :var mine: MyInterface<string> {not implemented yet} Variable types and type casting variable-types Variables declared in Vim9 script or in a :def function have a type, either specified explicitly or inferred from the initialization. Global, buffer, window and tab page variables do not have a specific type, the value can be changed at any time, possibly changing the type. Therefore, in compiled code the "any" type is assumed. This can be a problem when the "any" type is undesired and the actual type is expected to always be the same. For example, when declaring a list: var l: list<number> = [1, g:two] At compile time Vim doesn't know the type of "g:two" and the expression type becomes list<any>. An instruction is generated to check the list type before doing the assignment, which is a bit inefficient. type-casting E1104 To avoid this, use a type cast: var l: list<number> = [1, <number>g:two] The compiled code will then only check that "g:two" is a number and give an error if it isn't. This is called type casting. The syntax of a type cast is: "<" {type} ">". There cannot be white space after the "<" or before the ">" (to avoid them being confused with smaller-than and bigger-than operators). The semantics is that, if needed, a runtime type check is performed. The value is not actually changed. If you need to change the type, e.g. to change it to a string, use the string() function. Or use str2nr() to convert a string to a number. If a type is given where it is not expected you can get E1272 . If a type is incomplete you get E1363 , e.g. when you have an object for which the class is not known (usually that is a null object). Type inference type-inference In general: Whenever the type is clear it can be omitted. For example, when declaring a variable and giving it a value: var name = 0 # infers number type var name = 'hello' # infers string type The type of a list and dictionary comes from the common type of the values. If the values all have the same type, that type is used for the list or dictionary. If there is a mix of types, the "any" type is used. [1, 2, 3] list<number> ['a', 'b', 'c'] list<string> [1, 'x', 3] list<any> The common type of function references, if they do not all have the same number of arguments, uses "(...)" to indicate the number of arguments is not specified. For example: def Foo(x: bool) enddef def Bar(x: bool, y: bool) enddef var funclist = [Foo, Bar] echo funclist->typename() Results in: list<func(...)> For script-local variables in Vim9 script the type is checked, also when the variable was declared in a legacy function. When a type has been declared this is attached to a List or Dictionary. When later some expression attempts to change the type an error will be given: var ll: list<number> = [1, 2, 3] ll->extend(['x']) # Error, 'x' is not a number If the type is not declared then it is allowed to change: [1, 2, 3]->extend(['x']) # result: [1, 2, 3, 'x'] For a variable declaration an inferred type matters: var ll = [1, 2, 3] ll->extend(['x']) # Error, 'x' is not a number That is because the declaration looks like a list of numbers, thus is equivalent to: var ll: list<number> = [1, 2, 3] If you do want a more permissive list you need to declare the type: var ll: list<any> = [1, 2, 3] ll->extend(['x']) # OK Stricter type checking type-checking In legacy Vim script, where a number was expected, a string would be automatically converted to a number. This was convenient for an actual number such as "123", but leads to unexpected problems (and no error message) if the string doesn't start with a number. Quite often this leads to hard-to-find bugs. e.g.: echo 123 == '123' 1 With an accidental space: echo 123 == ' 123' 0 E1206 E1210 E1212 In Vim9 script this has been made stricter. In most places it works just as before if the value used matches the expected type. There will sometimes be an error, thus breaking backwards compatibility. For example: - Using a number other than 0 or 1 where a boolean is expected. E1023 - Using a string value when setting a number option. - Using a number where a string is expected. E1024 E1105 One consequence is that the item type of a list or dict given to map() must not change, if the type was declared. This will give an error in Vim9 script: var mylist: list<number> = [1, 2, 3] echo map(mylist, (i, v) => 'item ' .. i) E1012: Type mismatch; expected number but got string in map() Instead use mapnew(), it creates a new list: var mylist: list<number> = [1, 2, 3] echo mapnew(mylist, (i, v) => 'item ' .. i) ['item 0', 'item 1', 'item 2'] If the item type was not declared or determined to be "any" it can change to a more specific type. E.g. when a list of mixed types gets changed to a list of strings: var mylist = [1, 2.0, '3'] # typename(mylist) == "list<any>" map(mylist, (i, v) => 'item ' .. i) # typename(mylist) == "list<string>", no error There is a subtle difference between using a list constant directly and through a variable declaration. Because of type inference, when using a list constant to initialize a variable, this also sets the declared type: var mylist = [1, 2, 3] # typename(mylist) == "list<number>" echo map(mylist, (i, v) => 'item ' .. i) # Error! When using the list constant directly, the type is not declared and is allowed to change: echo map([1, 2, 3], (i, v) => 'item ' .. i) # OK The reasoning behind this is that when a type is declared and the list is passed around and changed, the declaration must always hold. So that you can rely on the type to match the declared type. For a constant this is not needed. E1158 Same for extend(), use extendnew() instead, and for flatten(), use flattennew() instead. Since flatten() is intended to always change the type, it can not be used in Vim9 script. Assigning to a funcref with specified arguments (see vim9-func-declaration) does strict type checking of the arguments. For variable number of arguments the type must match: var FuncRef: func(string, number, bool): number FuncRef = (v1: string, v2: number, v3: bool) => 777 # OK FuncRef = (v1: string, v2: number, v3: number) => 777 # Error! # variable number of arguments must have same type var FuncVA: func(...list<string>): number FuncVA = (...v: list<number>): number => v # Error! FuncVA = (...v: list<any>): number => v # OK, `any` runtime check FuncVA = (v1: string, v: string2): number => 333 # Error! FuncVA = (v: list<string>): number => 3 # Error! If the destination funcref has no specified arguments, then there is no argument type checking: var FuncUnknownArgs: func: number FuncUnknownArgs = (v): number => v # OK FuncUnknownArgs = (v1: string, v2: string): number => 3 # OK FuncUnknownArgs = (...v1: list<string>): number => 333 # OK E1211 E1217 E1218 E1219 E1220 E1221 E1222 E1223 E1224 E1225 E1226 E1227 E1228 E1238 E1250 E1251 E1252 E1256 E1297 E1298 E1301 Types are checked for most builtin functions to make it easier to spot mistakes. Categories of variables, defaults and null handling variable-categories null-variables There are categories of variables: primitive number, float, boolean container string, blob, list, dict specialized function, job, channel, user-defined-object When declaring a variable without an initializer, an explicit type must be provided. Each category has different default initialization semantics. Here's an example for each category: var num: number # primitives default to a 0 equivalent var cont: list<string> # containers default to an empty container var spec: job # specialized variables default to null Vim does not have a familiar null value; it has various null_<type> predefined values, for example null_string, null_list, null_job. Primitives do not have a null_<type>. The typical use cases for null_<type> are: - to clear a variable and release its resources; - as a default for a parameter in a function definition, see null-compare. For a specialized variable, like job, null_<type> is used to clear the resources. For a container variable, resources can also be cleared by assigning an empty container to the variable. For example: var j: job = job_start(...) # ... job does its work j = null_job # clear the variable and release the job's resources var l: list<any> # ... add lots of stuff to list l = [] # clear the variable and release container resources Using the empty container, rather than null_<type>, to clear a container variable may avoid null complications as described in null-anomalies. The initialization semantics of container variables and specialized variables differ. An uninitialized container defaults to an empty container: var l1: list<string> # empty container var l2: list<string> = [] # empty container var l3: list<string> = null_list # null container "l1" and "l2" are equivalent and indistinguishable initializations; but "l3" is a null container. A null container is similar to, but different from, an empty container, see null-anomalies. Specialized variables default to null. These job initializations are equivalent and indistinguishable: var j1: job var j2: job = null_job var j3 = null_job When a list or dict is declared, if the item type is not specified and can not be inferred, then the type is "any": var d1 = {} # type is "dict<any>" var d2 = null_dict # type is "dict<any>" Declaring a function, see vim9-func-declaration, is particularly unique. null-compare For familiar null compare semantics, where a null container is not equal to an empty container, do not use null_<type> in a comparison: vim9script def F(arg: list<string> = null_list) if arg == null echo "null" else echo printf("not null, %sempty", empty(arg) ? '' : 'not ') endif enddef F() # output: "null" F(null_list) # output: "null" F([]) # output: "not null, empty" F(['']) # output: "not null, not empty" The above function takes a list of strings and reports on it. Change the above function signature to accept different types of arguments: def F(arg: list<any> = null_list) # any type of list def F(arg: any = null) # any type In the above example, where the goal is to distinguish a null list from an empty list, comparing against null instead of null_list is the correct choice. The basic reason is because "null_list == null" and "[] != null". Comparing to null_list fails since "[] == null_list". In the following section there are details about comparison results. null-details null-anomalies This section describes issues about using null and null_<type>; included below are the enumerated results of null comparisons. In some cases, if familiar with vim9 null semantics, the programmer may chose to use null_<type> in comparisons and/or other situations. Elsewhere in the documentation it says: Quite often a null value is handled the same as an empty value, but not always Here's an example: vim9script var s1: list<string> var s2: list<string> = null_list echo s1 # output: "[]" echo s2 # output: "[]" echo s1 + ['a'] # output: "['a']" echo s2 + ['a'] # output: "['a']" echo s1->add('a') # output: "['a']" echo s2->add('a') # E1130: Can not add to null list Two values equal to a null_<type> are not necessarily equal to each other: vim9script echo {} == null_dict # true echo null_dict == null # true echo {} == null # false Unlike the other containers, an uninitialized string is equal to null. The 'is' operator can be used to determine if it is a null_string: vim9script var s1: string var s2 = null_string echo s1 == null # true - this is unexpected echo s2 == null # true echo s2 is null_string # true var b1: blob var b2 = null_blob echo b1 == null # false echo b2 == null # true Any variable initialized to the null_<type> is equal to the null_<type> and is also equal to null. For example: vim9script var x = null_blob echo x == null_blob # true echo x == null # true An uninitialized variable is usually equal to null; it depends on its type: var s: string s == null var b: blob b != null *** var l: list<any> l != null *** var d: dict<any> d != null *** var f: func f == null var j: job j == null var c: channel c == null var o: Class o == null A variable initialized to empty equals null_<type>; but not null: var s2: string = "" == null_string != null var b2: blob = 0z == null_blob != null var l2: list<any> = [] == null_list != null var d2: dict<any> = {} == null_dict != null NOTE: the specialized variables, like job, default to null value and have no corresponding empty value. ============================================================================== 5. Namespace, Import and Export vim9script vim9-export vim9-import A Vim9 script can be written to be imported. This means that some items are intentionally exported, made available to other scripts. When the exporting script is imported in another script, these exported items can then be used in that script. All the other items remain script-local in the exporting script and cannot be accessed by the importing script. This mechanism exists for writing a script that can be sourced (imported) by other scripts, while making sure these other scripts only have access to what you want them to. This also avoids using the global namespace, which has a risk of name collisions. For example when you have two plugins with similar functionality. You can cheat by using the global namespace explicitly. That should be done only for things that really are global. Namespace vim9-namespace To recognize a file that can be imported the vim9script statement must appear as the first statement in the file (see vim9-mix for an exception). It tells Vim to interpret the script in its own namespace, instead of the global namespace. If a file starts with: vim9script var myvar = 'yes' Then "myvar" will only exist in this file. While without vim9script it would be available as g:myvar from any other script and function. E1101 The variables at the file level are very much like the script-local "s:" variables in legacy Vim script, but the "s:" is omitted. And they cannot be deleted. In Vim9 script the global "g:" namespace can still be used as before. And the "w:", "b:" and "t:" namespaces. These have in common that variables are not declared, have no specific type and they can be deleted. E1304 A side effect of :vim9script is that the 'cpoptions' option is set to the Vim default value, like with: :set cpo&vim One of the effects is that line-continuation is always enabled. The original value of 'cpoptions' is restored at the end of the script, while flags added or removed in the script are also added to or removed from the original value to get the same effect. The order of flags may change. In the vimrc file sourced on startup this does not happen. vim9-mix There is one way to use both legacy and Vim9 syntax in one script file: " comments may go here if !has('vim9script') " legacy script commands go here finish endif vim9script # Vim9 script commands go here This allows for writing a script that takes advantage of the Vim9 script syntax if possible, but will also work on a Vim version without it. This can only work in two ways: 1. The "if" statement evaluates to false, the commands up to endif are skipped and vim9script is then the first command actually executed. 2. The "if" statement evaluates to true, the commands up to endif are executed and finish bails out before reaching vim9script. Export :export :exp Exporting an item can be written as: export const EXPORTED_CONST = 1234 export var someValue = ... export final someValue = ... export const someValue = ... export def MyFunc() ... export class MyClass ... export interface MyClass ... E1043 E1044 As this suggests, only constants, variables, :def functions and classes can be exported. E1042 :export can only be used in Vim9 script, at the script level. Import :import :imp E1094 E1047 E1262 E1048 E1049 E1053 E1071 E1088 E1236 The exported items can be imported in another script. The import syntax has two forms. The simple form: import {filename} Where {filename} is an expression that must evaluate to a string. In this form the filename should end in ".vim" and the portion before ".vim" will become the script local name of the namespace. For example: import "myscript.vim" This makes each exported item in "myscript.vim" available as "myscript.item". :import-as E1257 E1261 In case the name is long or ambiguous, this form can be used to specify another name: import {longfilename} as {name} In this form {name} becomes a specific script local name for the imported namespace. Therefore {name} must consist of letters, digits and '_', like internal-variables. The {longfilename} expression must evaluate to any filename. For example: import "thatscript.vim.v2" as that E1060 E1258 E1259 E1260 Then you can use "that.item", etc. You are free to choose the name "that". Use something that will be recognized as referring to the imported script. Avoid command names, command modifiers and builtin function names, because the name will shadow them. It's better not to start the name with a capital letter, since it can then also shadow global user commands and functions. Also, you cannot use the name for something else in the script, such as a function or variable name. In case the dot in the name is undesired, a local reference can be made for a function: var LongFunc = that.LongFuncName This also works for constants: const MAXLEN = that.MAX_LEN_OF_NAME This does not work for variables, since the value would be copied once and when changing the variable the copy will change, not the original variable. You will need to use the full name, with the dot. :import can not be used in a function. Imported items are intended to exist at the script level and only imported once. The script name after import can be: - A relative path, starting "." or "..". This finds a file relative to the location of the script file itself. This is useful to split up a large plugin into several files. - An absolute path, starting with "/" on Unix or "D:/" on MS-Windows. This will rarely be used. - A path not being relative or absolute. This will be found in the "import" subdirectories of 'runtimepath' entries. The name will usually be longer and unique, to avoid loading the wrong file. Note that "after/import" is not used. If the name does not end in ".vim" then the use of "as name" is required. Once a vim9 script file has been imported, the result is cached and used the next time the same script is imported. It will not be read again. It is not allowed to import the same script twice, also when using two different "as" names. When using the imported name the dot and the item name must be in the same line, there can be no line break: echo that. name # Error! echo that .name # Error! import-map When you've imported a function from one script into a vim9 script you can refer to the imported function in a mapping by prefixing it with <SID>: noremap <silent> ,a :call <SID>name.Function()<CR> When the mapping is defined "<SID>name." will be replaced with <SNR> and the script ID of the imported script. An even simpler solution is using <ScriptCmd>: noremap ,a <ScriptCmd>name.Function()<CR> Note that this does not work for variables, only for functions. import-legacy legacy-import :import can also be used in legacy Vim script. The imported namespace still becomes script-local, even when the "s:" prefix is not given. For example: import "myfile.vim" call s:myfile.MyFunc() And using the "as name" form: import "otherfile.vim9script" as that call s:that.OtherFunc() However, the namespace cannot be resolved on its own: import "that.vim" echo s:that " ERROR: E1060: Expected dot after name: s:that This also affects the use of <SID> in the legacy mapping context. Since <SID> is only a valid prefix for a function and NOT for a namespace, you cannot use it to scope a function in a script local namespace. Instead of prefixing the function with <SID> you should use<ScriptCmd>. For example: noremap ,a <ScriptCmd>:call s:that.OtherFunc()<CR> :import-cycle The import commands are executed when encountered. If script A imports script B, and B (directly or indirectly) imports A, this will be skipped over. At this point items in A after "import B" will not have been processed and defined yet. Therefore cyclic imports can exist and not result in an error directly, but may result in an error for items in A after "import B" not being defined. This does not apply to autoload imports, see the next section. Importing an autoload script vim9-autoload import-autoload For optimal startup speed, loading scripts should be postponed until they are actually needed. Using the autoload mechanism is recommended: E1264 1. In the plugin define user commands, functions and/or mappings that refer to items imported from an autoload script. import autoload 'for/search.vim' command -nargs=1 SearchForStuff search.Stuff(<f-args>) This goes in .../plugin/anyname.vim. "anyname.vim" can be freely chosen. The "SearchForStuff" command is now available to the user. The "autoload" argument to :import means that the script is not loaded until one of the items is actually used. The script will be found under the "autoload" directory in 'runtimepath' instead of the "import" directory. Alternatively a relative or absolute name can be used, see below. 2. In the autoload script put the bulk of the code. vim9script export def Stuff(arg: string) ... This goes in .../autoload/for/search.vim. Putting the "search.vim" script under the "/autoload/for/" directory has the effect that "for#search#" will be prefixed to every exported item. The prefix is obtained from the file name, as you would to manually in a legacy autoload script. Thus the exported function can be found with "for#search#Stuff", but you would normally use `import autoload` and not use the prefix (which has the side effect of loading the autoload script when compiling a function that encounters this name). You can split up the functionality and import other scripts from the autoload script as you like. This way you can share code between plugins. Searching for the autoload script in all entries in 'runtimepath' can be a bit slow. If the plugin knows where the script is located, quite often a relative path can be used. This avoids the search and should be quite a bit faster. Another advantage is that the script name does not need to be unique. An absolute path is also possible. Examples: import autoload '../lib/implement.vim' import autoload MyScriptsDir .. '/lib/implement.vim' For defining a mapping that uses the imported autoload script the special key <ScriptCmd> is useful. It allows for a command in a mapping to use the script context of where the mapping was defined. When compiling a :def function and a function in an autoload script is encountered, the script is not loaded until the :def function is called. This also means you get any errors only at runtime, since the argument and return types are not known yet. If you would use the name with '#' characters then the autoload script IS loaded. Be careful to not refer to an item in an autoload script that does trigger loading it unintentionally. For example, when setting an option that takes a function name, make sure to use a string, not a function reference: import autoload 'qftf.vim' &quickfixtextfunc = 'qftf.Func' # autoload script NOT loaded &quickfixtextfunc = qftf.Func # autoload script IS loaded On the other hand, it can be useful to load the script early, at a time when any errors should be given. For testing the test_override() function can be used to have the `import autoload` load the script right away, so that the items and types can be checked without waiting for them to be actually used: test_override('autoload', 1) Reset it later with: test_override('autoload', 0) Or: test_override('ALL', 0) ============================================================================== 6. Classes and interfaces vim9-classes In legacy script a Dictionary could be used as a kind-of object, by adding members that are functions. However, this is quite inefficient and requires the writer to do the work of making sure all the objects have the right members. See Dictionary-function. In Vim9 script you can have classes, objects and interfaces like in most popular object-oriented programming languages. Since this is a lot of functionality it is located in a separate help file: vim9class.txt. ============================================================================== 9. Rationale vim9-rationale The :def command Plugin writers have asked for much faster Vim script. Investigations have shown that keeping the existing semantics of function calls make this close to impossible, because of the overhead involved with calling a function, setting up the local function scope and executing lines. There are many details that need to be handled, such as error messages and exceptions. The need to create a dictionary for a: and l: scopes, the a:000 list and several others add too much overhead that cannot be avoided. Therefore the :def method to define a new-style function had to be added, which allows for a function with different semantics. Most things still work as before, but some parts do not. A new way to define a function was considered the best way to separate the legacy style code from Vim9 style code. Using "def" to define a function comes from Python. Other languages use "function" which clashes with legacy Vim script. Type checking When compiling lines of Vim commands into instructions as much as possible should be done at compile time. Postponing it to runtime makes the execution slower and means mistakes are found only later. For example, when encountering the "+" character and compiling this into a generic add instruction, at runtime the instruction would have to inspect the type of the arguments and decide what kind of addition to do. And when the type is dictionary throw an error. If the types are known to be numbers then an "add number" instruction can be used, which is faster. The error can be given at compile time, no error handling is needed at runtime, since adding two numbers cannot fail. The syntax for types, using <type> for compound types, is similar to Java. It is easy to understand and widely used. The type names are what were used in Vim before, with some additions such as "void" and "bool". Removing clutter and weirdness Once decided that :def functions have different syntax than legacy functions, we are free to add improvements to make the code more familiar for users who know popular programming languages. In other words: remove weird things that only Vim does. We can also remove clutter, mainly things that were done to make Vim script backwards compatible with the good old Vi commands. Examples: - Drop :call for calling a function and :eval for evaluating an expression. - Drop using a leading backslash for line continuation, automatically figure out where an expression ends. However, this does require that some things need to change: - Comments start with # instead of ", to avoid confusing them with strings. This is good anyway, it is also used by several popular languages. - Ex command ranges need to be prefixed with a colon, to avoid confusion with expressions (single quote can be a string or a mark, "/" can be divide or a search command, etc.). Goal is to limit the differences. A good criteria is that when the old syntax is accidentally used you are very likely to get an error message. Syntax and semantics from popular languages Script writers have complained that the Vim script syntax is unexpectedly different from what they are used to. To reduce this complaint popular languages are used as an example. At the same time, we do not want to abandon the well-known parts of legacy Vim script. For many things TypeScript is followed. It's a recent language that is gaining popularity and has similarities with Vim script. It also has a mix of static typing (a variable always has a known value type) and dynamic typing (a variable can have different types, this changes at runtime). Since legacy Vim script is dynamically typed and a lot of existing functionality (esp. builtin functions) depends on that, while static typing allows for much faster execution, we need to have this mix in Vim9 script. There is no intention to completely match TypeScript syntax and semantics. We just want to take those parts that we can use for Vim and we expect Vim users will be happy with. TypeScript is a complex language with its own history, advantages and disadvantages. To get an idea of the disadvantages read the book: "JavaScript: The Good Parts". Or find the article "TypeScript: the good parts" and read the "Things to avoid" section. People familiar with other languages (Java, Python, etc.) will also find things in TypeScript that they do not like or do not understand. We'll try to avoid those things. Specific items from TypeScript we avoid: - Overloading "+", using it both for addition and string concatenation. This goes against legacy Vim script and often leads to mistakes. For that reason we will keep using ".." for string concatenation. Lua also uses ".." this way. And it allows for conversion to string for more values. - TypeScript can use an expression like "99 || 'yes'" in a condition, but cannot assign the value to a boolean. That is inconsistent and can be annoying. Vim recognizes an expression with && or || and allows using the result as a bool. The falsy-operator was added for the mechanism to use a default value. - TypeScript considers an empty string as Falsy, but an empty list or dict as Truthy. That is inconsistent. In Vim an empty list and dict are also Falsy. - TypeScript has various "Readonly" types, which have limited usefulness, since a type cast can remove the immutable nature. Vim locks the value, which is more flexible, but is only checked at runtime. - TypeScript has a complicated "import" statement that does not match how the Vim import mechanism works. A much simpler mechanism is used instead, which matches that the imported script is only sourced once. Declarations Legacy Vim script uses :let for every assignment, while in Vim9 declarations are used. That is different, thus it's good to use a different command: :var. This is used in many languages. The semantics might be slightly different, but it's easily recognized as a declaration. Using :const for constants is common, but the semantics varies. Some languages only make the variable immutable, others also make the value immutable. Since "final" is well known from Java for only making the variable immutable we decided to use that. And then :const can be used for making both immutable. This was also used in legacy Vim script and the meaning is almost the same. What we end up with is very similar to Dart: :var name # mutable variable and value :final name # immutable variable, mutable value :const name # immutable variable and value Since legacy and Vim9 script will be mixed and global variables will be shared, optional type checking is desirable. Also, type inference will avoid the need for specifying the type in many cases. The TypeScript syntax fits best for adding types to declarations: var name: string # string type is specified ... name = 'John' const greeting = 'hello' # string type is inferred This is how we put types in a declaration: var mylist: list<string> final mylist: list<string> = ['foo'] def Func(arg1: number, arg2: string): bool Two alternatives were considered: 1. Put the type before the name, like Dart: var list<string> mylist final list<string> mylist = ['foo'] def Func(number arg1, string arg2) bool 2. Put the type after the variable name, but do not use a colon, like Go: var mylist list<string> final mylist list<string> = ['foo'] def Func(arg1 number, arg2 string) bool The first is more familiar for anyone used to C or Java. The second one doesn't really have an advantage over the first, so let's discard the second. Since we use type inference the type can be left out when it can be inferred from the value. This means that after var we don't know if a type or a name follows. That makes parsing harder, not only for Vim but also for humans. Also, it will not be allowed to use a variable name that could be a type name, using `var string string` is too confusing. The chosen syntax, using a colon to separate the name from the type, adds punctuation, but it actually makes it easier to recognize the parts of a declaration. Expressions Expression evaluation was already close to what other languages are doing. Some details are unexpected and can be improved. For example a boolean condition would accept a string, convert it to a number and check if the number is non-zero. This is unexpected and often leads to mistakes, since text not starting with a number would be converted to zero, which is considered false. Thus using a string for a condition would often not give an error and be considered false. That is confusing. In Vim9 type checking is stricter to avoid mistakes. Where a condition is used, e.g. with the :if command and the || operator, only boolean-like values are accepted: true: true, v:true, 1, `0 < 9` false: false, v:false, 0, `0 > 9` Note that the number zero is false and the number one is true. This is more permissive than most other languages. It was done because many builtin functions return these values, and changing that causes more problems than it solves. After using this for a while it turned out to work well. If you have any type of value and want to use it as a boolean, use the !! operator: true: !!'text' !![99] `!!{'x': 1}` !!99 false: !!'' !![] !!{} From a language like JavaScript we have this handy construct: GetName() || 'unknown' However, this conflicts with only allowing a boolean for a condition. Therefore the "??" operator was added: GetName() ?? 'unknown' Here you can explicitly express your intention to use the value as-is and not result in a boolean. This is called the falsy-operator. Import and Export A problem of legacy Vim script is that by default all functions and variables are global. It is possible to make them script-local, but then they are not available in other scripts. This defies the concept of a package that only exports selected items and keeps the rest local. In Vim9 script a mechanism very similar to the JavaScript import and export mechanism is supported. It is a variant to the existing :source command that works like one would expect: - Instead of making everything global by default, everything is script-local, some of these are exported. - When importing a script the symbols that are imported are explicitly listed, avoiding name conflicts and failures if functionality is added later. - The mechanism allows for writing a big, long script with a very clear API: the exported functions, variables and classes. - By using relative paths loading can be much faster for an import inside of a package, no need to search many directories. - Once an import has been used, its items are cached and loading it again is not needed. - The Vim-specific use of "s:" to make things script-local can be dropped. When sourcing a Vim9 script (from a Vim9 or legacy script), only the items defined globally can be used, not the exported items. Alternatives considered: - All the exported items become available as script-local items. This makes it uncontrollable what items get defined and likely soon leads to trouble. - Use the exported items and make them global. Disadvantage is that it's then not possible to avoid name clashes in the global namespace. - Completely disallow sourcing a Vim9 script, require using :import. That makes it difficult to use scripts for testing, or sourcing them from the command line to try them out. Note that you CAN also use :import in legacy Vim script, see above. Compiling functions early Functions are compiled when called or when :defcompile is used. Why not compile them early, so that syntax and type errors are reported early? The functions can't be compiled right away when encountered, because there may be forward references to functions defined later. Consider defining functions A, B and C, where A calls B, B calls C, and C calls A again. It's impossible to reorder the functions to avoid forward references. An alternative would be to first scan through the file to locate items and figure out their type, so that forward references are found, and only then execute the script and compile the functions. This means the script has to be parsed twice, which is slower, and some conditions at the script level, such as checking if a feature is supported, are hard to use. An attempt was made to see if it works, but it turned out to be impossible to make work well. It would be possible to compile all the functions at the end of the script. The drawback is that if a function never gets called, the overhead of compiling it counts anyway. Since startup speed is very important, in most cases it's better to do it later and accept that syntax and type errors are only reported then. In case these errors should be found early, e.g. when testing, a :defcompile command at the end of the script will help out. Why not use an existing embedded language? Vim supports interfaces to Perl, Python, Lua, Tcl and a few others. But these interfaces have never become widely used, for various reasons. When Vim9 was designed a decision was made to make these interfaces lower priority and concentrate on Vim script. Still, plugin writers may find other languages more familiar, want to use existing libraries or see a performance benefit. We encourage plugin authors to write code in any language and run it as an external process, using jobs and channels. We can try to make this easier somehow. Using an external tool also has disadvantages. An alternative is to convert the tool into Vim script. For that to be possible without too much translation, and keeping the code fast at the same time, the constructs of the tool need to be supported. Since most languages support classes the lack of support for classes in Vim is then a problem. vim:tw=78:ts=8:noet:ft=help:norl: