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usr_46.txt    For Vim version 8.2.  Last change: 2020 Jun 14

                     VIM USER MANUAL - by Bram Moolenaar

                       Write plugins using Vim9 script

The Vim9 script language is used for writing plugins, especially larger ones
that use multiple files.  This chapter explains how to split up a plugin into
modules, import and export items and keep the rest local.

46.1  Introduction
46.2  Variable declarations
46.3  Functions and types
46.?  Using a Vim9 script from legacy script

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46.1  Introduction                            vim9-script-intro

Vim9 script was designed to make it easier to write large Vim scripts.  It
looks more like other script languages, especially Typescript.  Also,
functions are compiled into instructions that can be executed quickly.  This
makes Vim9 script a lot faster, up to a 100 times.

The basic idea is that a script file has items that are private, only used
inside the script file, and items that are exported, used outside of the
script file.  The exported items can then be used by scripts that import them.
That makes very clear what is defined where.

Let's start with an example, a script that exports one function and has one
private function: 

        vim9script  " This indicates a Vim9 script file, 

        export def GetMessage(): string
           let result = ''
           result = GetPart(count)
           return result

        def GetPart(nr: number): string
          if nr == 4
             return 'yes'
             return 'no'

The vim9script command must be the very first command in the file.  Without
it Vim will assume legacy script syntax.

The `export def GetMessage(): string` line starts with export, meaning that
this function can be imported and called by other scripts. The line
`def GetPart(...` does not start with export, this is a script-local
function, it can only be used inside this script file.

In the `export def GetMessage(): string` line you will notice the colon and
the return type.  Vim9 functions, defined with def, require specifying the
type of arguments and the return type.  That way Vim can compile the code
efficiently.  The GetPart function defines an argument "nr" of type "number".

Notice that the assignment `result = GetPart(count)` does not use the let
command.  That is explained in the next section.

46.2  Variable declarations                           vim9-declarations

In Vim9 script variables are declared once with a :let or :const command.
Assigning a value is done without :let and it is not possible to :unlet
the variable.

In most cases you will want to declare the variable and initialize it at the
same time: 
        let myText = 'some text'
        myText = 'other text'

The type of the variable will be inferred from the expression.  In this case
it is a string.  If you initialize with a number, then the type is number: 
        let myNumber = 1234
        myNumber = 0

If you try to assign a string to this variable, you will get an error: 
        let myNumber = 'this fails!'

In the rare case you want a variable that can take values of any type, you
have to specify the type: 
        let myVar: any = 1234
        myVar = 'text also works'

You can also declare a variable without assigning a value.  In that case Vim
will initialize it to zero or empty: 
        let word: string
        if condition
          word = 'yes'
          word = 'no'

Although it's shorter to do: 
        let word = condition ? 'yes' : 'no'

46.3  Functions and types

Legacy Vim script does have type checking, but this happens at runtime, when
the code is executed.  And it's permissive, often a computation gives an
unexpected value instead of reporting an error.  Thus you can define a
function and think it's fine, but see a problem only later when it is called: 
        let s:collected = ''
        func ExtendAndReturn(add)
           let s:collected += a:add
           return s:collected

Can you spot the error?  Try this: 
        echo ExtendAndReturn('text')
And you'll see zero.  Why?  Because in legacy Vim script "+=" will convert the
arguments to numbers, and any string without a number results in zero!

With :def the type checking happens when compiling the function.  For that
you need to specify the argument types and the return type.  Also notice that
the argument is used without the "a:" prefix: 
        let s:collected = ''
        def ExtendAndReturn(add: string): string
           s:collected += add
           return s:collected

Here we use :defcompile to do the compilation right away, without it the
compilation would happen when the function is called.  Vim will tell you what
you did wrong: 
        E1013: type mismatch, expected number but got string

Vim9 script is strict, it uses the "+" operator only for numbers and floats.
For string concatenation ".." must be used.  This avoids mistakes and avoids
the automatic conversion that gave a surprising result above.  So you change
the first line of the function to: 
           s:collected ..= add
And now it works.

If the function does not return anything, just leave out the return type: 
        def ReportResult(result: string)
           echo 'The result is: ' .. result

This is also checked, if you try to return a value you'll get an error.

In case you don't care about types or have a function that does work with
multiple types, you can use the "any" type: 
        def Store(key: string, value: any)
          resultDict[key] = value

46.?  Using a Vim9 script from legacy script          source-vim9-script

In some cases you have a legacy Vim script where you want to use items from a
Vim9 script.  For example in your .vimrc you want to initialize a plugin.  The
best way to do this is to use :import.  For example: 

        import Init as NiceInit from 'myNicePlugin.vim'
        call NiceInit('today')

This finds the exported function "Init" in the Vim9 script file and makes it
available as script-local item "NiceInit". :import always uses the script
namespace, even when "s:" is not given.  If "myNicePlugin.vim" was already
sourced it is not sourced again.

Besides avoiding putting any items in the global namespace (where name clashes
can cause unexpected errors), this also means the script is sourced only once,
no matter how many times items from it are imported.

In some cases, e.g. for testing, you may just want to source the Vim9 script.
That is OK, but then only global items will be available.  The Vim9 script
will have to make sure to use a unique name for these global items. Example: 
        source ~/.vim/extra/myNicePlugin.vim
        call g:NicePluginTest()


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