*vim9.txt*	For Vim version 8.2.  Last change: 2019 Dec 06


		  VIM REFERENCE MANUAL	  by Bram Moolenaar


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

Vim9 script commands and expressions.

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 keeping backwards compatibility.
That means bad choices from the past often can't be changed.  Execution is
quite slow, every line is parsed every time it is executed.

The main goal of Vim9 script is to drastically improve performance.  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, in a function the arguments are not available in the
"a:" dictionary, as creating that dictionary adds quite a lot of overhead.
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 is discouraged.

Vim9 script and legacy Vim script can be mixed.  There is no need 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

Vim9 functions ~

`:def` has no extra arguments 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.

In the function body:
- Arguments are accessed by name, without "a:".
- There is no "a:" dictionary or "a:000" list.  Variable arguments are defined
  with a name and have a list type: >
  	def MyFunc(...itemlist: list<type>)
	   for item in itemlist
	     ...


Variable declarations with :let and :const ~

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 use a variable that won't be available later, a block can be
used: >
	{
	   let temp = 'temp'
	   ...
	}
	echo temp  " Error!

An existing variable cannot be assigend to with `:let`, since that implies a
declaration.  An exception is global variables: these can be both used with
and without `:let`, because there is no rule about where they are declared.

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

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


Omitting :call and :eval ~

Functions can be called without `:call`: >
  	writefile(lines, 'file')
Using `:call` is still posible, 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.  It does not work for string constants: >
	myList->add(123)		" works
	g:myList->add(123)		" works
	[1, 2, 3]->Process()		" works
	#{a: 1, b: 2}->Process()	" works
	{'a': 1, 'b': 2}->Process()	" works
	"foobar"->Process()		" does NOT work
	eval "foobar"->Process()	" works


No curly braces expansion ~

|curly-braces-names| cannot be used.


Comperators ~

The 'ignorecase' option is not used for comperators 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 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


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)
	funcref		when not NULL
	partial		when not NULL
	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  == []

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.


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

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 the return type will be
			decided upon by the first encountered `return`
			statement in the function.  E.g., for: >
				return 'message'
<			The return type will be "string".
			
			{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.

			[!] is used as with `:function`.

							*:enddef*
:enddef			End of a function defined with `:def`.


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

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>
	(a: type, b: type): type
	job
	channel

Not supported yet:
	tuple<a: type, b: type, ...>

These types can be used in declarations, but no variable will have this type:
	type|type
	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 function defined with `:def` must declare the return type.  If there is no
type then the function doesn't return anything.  "void" is used in type
declarations.

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 an 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.

In Vim9 script the global "g:" namespace can still be used as before.

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 one 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.

Alternatively, an export statement can be used to export several already
defined (otherwise script-local) items: >
	export {EXPORTED_CONST, someValue, MyFunc, MyClass}


Import ~
							*:import* *:imp*
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 ambigiuous, 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 mechamism:

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, for identifier the
script-local "s:" namespace will be used, 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 funtion 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".


JavaScript/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 will be used as an example.  At the same time, we do not want to
abondon 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, 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.


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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