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*vim9.txt* For Vim version 8.2. Last change: 2021 Dec 27 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. Future work: classes |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* 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* 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. ============================================================================== 2. Differences from legacy Vim script *vim9-differences* Overview ~ 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`, declare variables with `:var`: > var count = 0 count += 3 - Constants can be declared with `:final` and `:const`: > final matches = [] # add matches 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 `:xit`, `:t`, `:k`, `:append`, `:change`, `:insert`, `:open`, and `:s` or `:d` with only flags. - 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. 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! 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. 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 ~ 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* 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* Later classes will be added, which replaces the "dict function" mechanism. For now you will need to pass the dictionary explicitly: > def DictFunc(d: dict<any>, arg: string) echo d[arg] enddef var d = {item: 'value', func: DictFunc} d.func(d, '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 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. Arguments are accessed by name, without "a:", just like any other language. There is no "a:" dictionary or "a:000" list. *vim9-variable-arguments* 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* 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(myList, (_, 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, as if "s:" was prefixed. Using the "s:" prefix is optional. To define a global function or variable the "g:" prefix must be used. For functions in an autoload script the "name#" prefix is sufficient. > def ThisFunction() # script-local def s:ThisFunction() # script-local def g:ThatFunction() # global def scriptname#function() # autoload 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. In a `:def` function it is not possible to define a script-local function. 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, possibly imported - in the list of global functions However, it is recommended to always use "g:" to refer to a global function for clarity. 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. 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!). 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. 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* When loading a legacy Vim script a second time nothing is removed, the commands will replace existing variables and functions and create new ones. 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: > vim9script noclear setlocal completefunc=SomeFunc if exists('*g:SomeFunc') | finish | endif def g:SomeFunc() .... Variable declarations with :var, :final and :const ~ *vim9-declaration* *:var* 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 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. 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. 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`. `: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. 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:" when defining them, but can be called without "g:". > vim9script def g:GlobalFunc(): string return 'text' enddef echo 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 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. `: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* `: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 ~ 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 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* 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* 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* The block can also be used for defining a user command. Inside the block Vim9 syntax will be used. 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* 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 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. 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 ~ 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 ) 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. Dictionary literals ~ *vim9-literal-dict* 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 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. A number can be given with and without the []: > var dict = {123: 'without', [456]: 'with'} echo dict {'456': 'with', '123': 'without'} No :xit, :t, :k, :append, :change or :insert ~ 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. 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 ~ 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. 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* In Vim9 script one can use "true" for v:true, "false" for v:false and "null" for v:null. When converting a boolean to a string "false" and "true" are used, not "v:false" and "v:true" like in legacy script. "v:none" is not changed, it is only used in JSON and has no equivalent in other languages. 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* :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`. When {return-type} is omitted or is "void" the function is not expected to return anything. {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. [!] 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* :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. *:defc* *:defcompile* :defc[ompile] Compile functions defined in the current script that were not compiled yet. This will report errors found during the compilation. *:disa* *:disassemble* :disa[ssemble] {func} Show the instructions generated for {func}. This is for debugging and testing. 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] 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] ============================================================================== 4. Types *vim9-types* 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} 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. There is no array type, use list<{type}> instead. For a list constant an efficient implementation is used that avoids allocating lot of small pieces of memory. 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(...{type}) function with type of variable number of arguments, does not return a value func({type}, ?{type}, ...{type}): {type} function with: - type of mandatory argument - type of optional argument - type of 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. {not implemented yet} 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* 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. 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. 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. 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* One consequence is that the item type of a list or dict given to |map()| must not change. This will give an error in Vim9 script: > echo map([1, 2, 3], (i, v) => 'item ' .. i) E1012: Type mismatch; expected number but got string Instead use |mapnew()|: > echo mapnew([1, 2, 3], (i, v) => 'item ' .. i) ['item 0', 'item 1', 'item 2'] If the item type was 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 Same for |extend()|, use |extendnew()| instead, and for |flatten()|, use |flattennew()| instead. ============================================================================== 5. Namespace, Import and Export *vim9script* *vim9-export* *vim9-import* A Vim9 script can be written to be imported. This means that everything in the script is local, unless exported. Those exported items, and only those items, can then be imported in another script. You can cheat by using the global namespace explicitly. We will assume here that you don't do that. 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. 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 and they can be deleted. 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 ... As this suggests, only constants, variables, `:def` functions and classes can be exported. {not implemented yet: class, interface} *E1042* `:export` can only be used in Vim9 script, at the script level. Import ~ *:import* *:imp* *E1094* The exported items can be imported individually in another Vim9 script: > import EXPORTED_CONST from "thatscript.vim" import MyClass from "myclass.vim" To import multiple items at the same time: > import {someValue, MyClass} from "thatscript.vim" In case the name is ambiguous, another name can be specified: > import MyClass as ThatClass from "myclass.vim" import {someValue, MyClass as ThatClass} from "myclass.vim" To import all exported items under a specific identifier: > import * as That from 'thatscript.vim' Then you can use "That.EXPORTED_CONST", "That.someValue", etc. You are free to choose the name "That", but it is highly recommended to use the name of the script file to avoid confusion. Also avoid command names, because the name will shadow them. `:import` can also be used in legacy Vim script. The imported items still become script-local, even when the "s:" prefix is not given. `: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. 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. *:import-cycle* The `import` commands are executed when encountered. If that script (directly or indirectly) imports the current script, then items defined after the `import` won't be processed yet. Therefore cyclic imports can exist, but may result in undefined items. Import in an autoload script ~ For optimal startup speed, loading scripts should be postponed until they are actually needed. A recommended mechanism: 1. In the plugin define user commands, functions and/or mappings that refer to an autoload script. > command -nargs=1 SearchForStuff searchfor#Stuff(<f-args>) < This goes in .../plugin/anyname.vim. "anyname.vim" can be freely chosen. 2. In the autoload script do the actual work. You can import items from other files to split up functionality in appropriate pieces. > vim9script import FilterFunc from "../import/someother.vim" def searchfor#Stuff(arg: string) var filtered = FilterFunc(arg) ... < This goes in .../autoload/searchfor.vim. "searchfor" in the file name must be exactly the same as the prefix for the function name, that is how Vim finds the file. 3. Other functionality, possibly shared between plugins, contains the exported items and any private items. > vim9script var localVar = 'local' export def FilterFunc(arg: string): string ... < This goes in .../import/someother.vim. 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. Import in legacy Vim script ~ If an `import` statement is used in legacy Vim script, the script-local "s:" namespace will be used for the imported item, even when "s:" is not specified. ============================================================================== 6. Future work: classes *vim9-classes* Above "class" was mentioned a few times, but it has not been implemented yet. Most of Vim9 script can be created without this functionality, and since implementing classes is going to be a lot of work, it is left for the future. For now we'll just make sure classes can be added later. Thoughts: - `class` / `endclass`, the whole class must be in one file - Class names are always CamelCase (to avoid a name clash with builtin types) - A single constructor called "constructor" - Single inheritance with `class ThisClass extends BaseClass` - `abstract class` (class with incomplete implementation) - `interface` / `endinterface` (abstract class without any implementation) - `class SomeClass implements SomeInterface` - Generics for class: `class <Tkey, Tentry>` - Generics for function: `def <Tkey> GetLast(key: Tkey)` Again, much of this is from TypeScript with a slightly different syntax. Some things that look like good additions: - Use a class as an interface (like Dart) - Extend a class with methods, using an import (like Dart) - Mixins - For testing: Mock mechanism An important class that will be provided is "Promise". Since Vim is single threaded, connecting asynchronous operations is a natural way of allowing plugins to do their work without blocking the user. It's a uniform way to invoke callbacks and handle timeouts and errors. Some examples: > abstract class Person static const prefix = 'xxx' var name: string def constructor(name: string) this.name = name enddef def display(): void echo name enddef abstract def find(string): Person endclass ============================================================================== 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 manipulating data. - 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 known from 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. 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. 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, unless 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 function(s) and class(es). - 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, it can be cached and loading it again can be avoided. - The Vim-specific use of "s:" to make things script-local can be dropped. When sourcing a Vim9 script from a 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 nicely. 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, the `:defcompile` command will help out. Why not use an 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 tool, 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. Classes ~ Vim supports a kind-of object oriented programming by adding methods to a dictionary. With some care this can be made to work, but it does not look like real classes. On top of that, it's quite slow, because of the use of dictionaries. The support of classes in Vim9 script is a "minimal common functionality" of class support in most languages. It works much like Java, which is the most popular programming language. vim:tw=78:ts=8:noet:ft=help:norl: