runtime/doc/vim9.txt

*vim9.txt*	For Vim version 8.2.  Last change: 2020 Jul 25


		  VIM REFERENCE MANUAL	  by Bram Moolenaar


THIS IS STILL UNDER DEVELOPMENT - ANYTHING CAN BREAK - ANYTHING CAN CHANGE

Vim9 script commands and expressions.			*vim9*

Most expression help is in |eval.txt|.  This file is about the new syntax and
features in Vim9 script.

THIS IS STILL UNDER DEVELOPMENT - ANYTHING CAN BREAK - ANYTHING CAN CHANGE


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|

9.  Rationale				|vim9-rationale|

==============================================================================

1. What is Vim9 script?					*vim9-script*

THIS IS STILL UNDER DEVELOPMENT - ANYTHING CAN BREAK - ANYTHING CAN CHANGE

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`

When using `:function` in a Vim9 script file the legacy syntax is used.
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.

==============================================================================

2. Differences from legacy Vim script			*vim9-differences*

THIS IS STILL UNDER DEVELOPMENT - ANYTHING CAN BREAK - ANYTHING CAN CHANGE

Comments starting with # ~

In legacy Vim script comments start with double quote.  In Vim9 script
comments start with #. >
	# declarations
	let 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 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.  Note that #{ is the start of a dictionary, therefore
it does not start a comment.


Vim9 functions ~

A function defined with `:def` is compiled.  Execution is many times faster,
often 10x to 100x 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 the function is first called, or when the
`:defcompile` command is encountered in the script where the function was
defined. `:disassemble` also compiles the function.

`:def` has no options like `:function` does: "range", "abort", "dict" or
"closure".  A `:def` function always aborts on an error, does not get a range
passed and cannot be a "dict" function.

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:".  There is no "a:" dictionary or
"a:000" list.  Just like any other language.

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
	     ...


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 or use a global function or variable the "g:" prefix must be used.

When using `:function` or `:def` to specify a new function inside a function,
the function is local to the function.  It is not possible to define a
script-local function inside a function. It is possible to define a global
function, using the "g:" prefix.

When referring to a function and no "s:" or "g:" prefix is used, Vim will
search for the function in this order:
- Local to the current scope and outer scopes up to the function scope.
- Local to the current script file.
- Imported functions, see `:import`.
In all cases the function must be defined before used.  That is when it is
first called or when `:defcompile` causes the call to be compiled.

The result is that functions and variables without a namespace can always be
found in the script, either defined there or imported.  Global functions and
variables could be defined anywhere (good luck finding where!).

Global functions can be 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.


Variable declarations with :let and :const ~
							*vim9-declaration*
Local variables need to be declared with `:let`.  Local constants need to be
declared with `:const`.  We refer to both as "variables".

Variables can be local to a script, function or code block: >
	vim9script
	let script_var = 123
	def SomeFunc()
	  let func_var = script_var
	  if cond
	    let 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
	   let inner = 5
	else
	   let inner = 0
	endif
	echo inner  " Error!

The declaration must be done earlier: >
	let inner: number
	if cond
	   inner = 5
	else
	   inner = 0
	endif
	echo inner

To intentionally avoid a variable being available later, a block can be used:
>
	{
	   let temp = 'temp'
	   ...
	}
	echo temp  " Error!

An existing variable cannot be assigned to with `:let`, since that implies a
declaration.  Global, window, tab, buffer and Vim variables can only be used
without `:let`, because they are are not really declared, they can also be
deleted with `:unlet`.

Variables cannot shadow previously defined variables.
Variables may shadow Ex commands, rename the variable if needed.

Global variables and user defined functions must be prefixed with "g:", also
at the script level. >
	vim9script
	let script_local = 'text'
	g:global = 'value'
	let Funcref = g:ThatFunction

Since "&opt = value" is now assigning a value to option "opt", ":&" cannot be
used to repeat a `:substitute` command.

							*E1092*
Declaring more than one variable at a time, using the unpack notation, is
currently not supported: >
	let [v1, v2] = GetValues()  # Error!
That is because the type needs to be inferred from the list item type, which
isn't that easy.


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.  Examples: >
	myList->add(123)
	g:myList->add(123)
	[1, 2, 3]->Process()
	#{a: 1, b: 2}->Process()
	{'a': 1, 'b': 2}->Process()
	"foobar"->Process()
	("foobar")->Process()
	'foobar'->Process()
	('foobar')->Process()

In rare case there is ambiguity between a function name and an Ex command, use
":" 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 (

Note that while variables need to be defined before they can be used,
functions can be called before being defined.  This is required to be able
have 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. >

	let Funcref = MyFunction

When using `function()` the resulting type is "func", a function with any
number of arguments and any return type.  The function can be defined later.


Automatic 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.  For
example, when a list spans multiple lines: >
	let mylist = [
		'one',
		'two',
		]
And when a dict spans multiple lines: >
	let mydict = #{
		one: 1,
		two: 2,
		}
Function call: >
	let 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: >
	let text = lead
		   .. middle
		   .. end
	let total = start +
	            end -
		    correction
	let result = positive
			? PosFunc(arg)
			: NegFunc(arg)

For a method call using "->" and a member using a dot, a line break is allowed
before it: >
	let result = GetBuilder()
			->BuilderSetWidth(333)
			->BuilderSetHeight(777)
			->BuilderBuild()
	let result = MyDict
			.member

<							*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 will add
"start" and print: >
	let result = start
	+ print
Like this: >
	let result = start + print

This will assign "start" and print a line: >
	let result = start
	:+ print

It is also possible to split a function header over multiple lines, in between
arguments: >
	def MyFunc(
		text: string,
		separator = '-'
		): string

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]
- No line break is allowed in the arguments of a lambda, between the "{" and
  "->".  This is OK: >
  	filter(list, {k, v ->
			v > 0})
<  This does not work: >
  	filter(list, {k,
			v -> v > 0})


No curly braces expansion ~

|curly-braces-names| cannot be used.


No :xit, :append, :change or :insert ~

These commands are too easily confused with local variable names.  Instead of
`:x` or `:xit` you can use `:exit`.


Comparators ~

The 'ignorecase' option is not used for comparators that use strings.


White space ~

Vim9 script enforces proper use of white space.  This is no longer allowed: >
	let var=234	" Error!
	let var= 234	" Error!
	let var =234	" Error!
There must be white space before and after the "=": >
	let var = 234	" OK
White space must also be put before the # that starts a comment after a
command: >
	let var = 234# Error!
	let var = 234 # OK

White space is required around most operators.

White space is not allowed:
- Between a function name and the "(": >
  	call Func (arg)	   " Error!
  	call Func
	     \ (arg)	   " Error!
  	call Func(arg)	   " OK
  	call Func(
	     \ arg)	   " OK
  	call Func(
	     \ arg	   " OK
	     \ )


Conditions and expressions ~

Conditions and expression are mostly working like they do in JavaScript.  A
difference is made where JavaScript does not work like most people expect.
Specifically, an empty list is falsey.

Any type of variable can be used as a condition, there is no error, not even
for using a list or job.  This is very much like JavaScript, but there are a
few exceptions.

	type		TRUE when ~
	bool		v:true
	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		v:true
	job		when not NULL
	channel		when not NULL
	class		when not NULL
	object		when not NULL (TODO: when isTrue() returns v:true)

The boolean operators "||" and "&&" do not change the value: >
	8 || 2   == 8
	0 || 2   == 2
	0 || ''  == ''
	8 && 2   == 2
	0 && 2   == 0
	2 && 0   == 0
	[] && 2  == []

When using `..` for string concatenation the arguments are always converted to
string. >
	'hello ' .. 123  == 'hello 123'
	'hello ' .. v:true  == 'hello true'

In Vim9 script one can use "true" for v:true and "false" for v:false.


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 continuation line
	:->		" Vim9: shifts the previous line to the right

	%s/a/b		" legacy Vim: substitute on all lines
	x = alongname
	     % another	" Vim9: line continuation without a backslash
	:%s/a/b		" Vim9: substitute on all lines
	'text'->func()	" Vim9: method call
	:'t		" legacy Vim: jump to mark m

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 compilation error
	enddef
For a workaround, split it in two functions: >
	func Maybe()
	  if has('feature')
	    call MaybyInner()
	  endif
	endfunc
	if has('feature')
	  def MaybeInner()
	    use-feature
	  enddef
	endif

==============================================================================

3. New style functions					*fast-functions*

THIS IS STILL UNDER DEVELOPMENT - ANYTHING CAN BREAK - ANYTHING CAN CHANGE

							*: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 in Vim9
			script script-local functions cannot be deleted or
			redefined later in the same script.

							*:enddef*
:enddef			End of a function defined with `:def`. It should be on
			a line by its own.


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.

						*: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.

Limitations ~

Local variables will not be visible to string evaluation.  For example: >
	def EvalString(): list<string>
	  let 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 EvalString(): list<string>
	  let list = ['aa', 'bb', 'cc', 'dd']
	  return range(1, 2)->map({ _, v -> list[v] })
	enddef


==============================================================================

4. Types					*vim9-types*

THIS IS STILL UNDER DEVELOPMENT - ANYTHING CAN BREAK - ANYTHING CAN CHANGE

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 value will have this type:
	{type}|{type}  {not implemented yet}
	void
	any

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: {type}			any number and type of arguments with specific
				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>
{not implemented yet}

And classes and interfaces can be used as types: >
	:class MyClass
	:let mine: MyClass

	:interface MyInterface
	:let mine: MyInterface

	:class MyTemplate<Targ>
	:let mine: MyTemplate<number>
	:let mine: MyTemplate<string>

	:class MyInterface<Targ>
	:let mine: MyInterface<number>
	:let mine: MyInterface<string>
{not implemented yet}


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: >
	let var = 0		" infers number type
	let var = 'hello'	" infers string type


==============================================================================

5.  Namespace, Import and Export
					*vim9script* *vim9-export* *vim9-import*

THIS IS STILL UNDER DEVELOPMENT - ANYTHING CAN BREAK - ANYTHING CAN CHANGE

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.


Namespace ~
							*:vim9script* *:vim9*
To recognize a file that can be imported the `vim9script` statement must
appear as the first statement in the file.  It tells Vim to interpret the
script in its own namespace, instead of the global namespace.  If a file
starts with: >
	vim9script
	let 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.


Export ~
							*:export* *:exp*
Exporting an item can be written as: >
	export const EXPORTED_CONST = 1234
	export let someValue = ...
	export def MyFunc() ...
	export class MyClass ...

As this suggests, only constants, variables, `:def` functions and classes can
be exported. {classes are not implemented yet}

							*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.

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 be rarely 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.

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 call searchfor#Stuff(<f-args>)

<   This goes in .../plugin/anyname.vim.  "anyname.vim" can be freely chosen.

2. In the autocommand 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)
	  let 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
	let localVar = 'local'
   	export def FilterFunc(arg: string): string
	   ...
<   This goes in .../import/someother.vim.


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.


==============================================================================

9. Rationale						*vim9-rationale*

The :def command ~

Plugin writers have asked for a much faster Vim script.  Investigation 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 old-style code from Vim9 script 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 execution time 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.

The syntax for types is similar to Java, since it is easy to understand and
widely used.  The type names are what was used in Vim before, with some
additions such as "void" and "bool".


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.


TypeScript syntax and semantics ~

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.

Since Vim already uses `:let` and `:const` and optional type checking is
desirable, the JavaScript/TypeScript syntax fits best for variable
declarations. >
	const greeting = 'hello'  " string type is inferred
	let name: string
	...
	name = 'John'

Expression evaluation was already close to what JavaScript and other languages
are doing.  Some details are unexpected and can be fixed.  For example how the
|| and && operators work.  Legacy Vim script: >
	let result = 44
	...
	return result || 0	" returns 1

Vim9 script works like JavaScript/Typescript, keep the value: >
	let result = 44
	...
	return result || 0	" returns 44

On the other hand, overloading "+" to use both for addition and string
concatenation 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.


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.

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 listed, avoiding
  name conflicts and failures if later functionality is added.
- 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.
- 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.


Classes ~

Vim supports interfaces to Perl, Python, Lua, Tcl and a few others.  But
these have never become widespread.  When Vim 9 was designed a decision was
made to phase out these interfaces and concentrate on Vim script, while
encouraging plugin authors to write code in any language and run it as an
external tool, using jobs and channels.

Still, using an external tool 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
class support in Vim is then a problem.

Previously Vim supported a kind-of object oriented programming by adding
methods to a dictionary.  With some care this could be made to work, but it
does not look like real classes.  On top of that, it's very 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 mostly like Java, which is the most
popular programming language.



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