vim9 - Vim Documentation

vim9.txt      For Vim version 9.1.  Last change: 2024 May 31


                  VIM REFERENCE MANUAL    by Bram Moolenaar


Vim9 script commands and expressions.                   Vim9 vim9

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



1.  What is Vim9 script?                Vim9-script
2.  Differences                         vim9-differences
3.  New style functions                 fast-functions
4.  Types                               vim9-types
5.  Namespace, Import and Export        vim9script
6.  Classes and interfaces              vim9-classes

9.  Rationale                           vim9-rationale

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

1. What is Vim9 script?                                 Vim9-script

Vim script has been growing over time, while preserving backwards
compatibility.  That means bad choices from the past often can't be changed
and compatibility with Vi restricts possible solutions.  Execution is quite
slow, each line is parsed every time it is executed.

The main goal of Vim9 script is to drastically improve performance.  This is
accomplished by compiling commands into instructions that can be efficiently
executed.  An increase in execution speed of 10 to 100 times can be expected.

A secondary goal is to avoid Vim-specific constructs and get closer to
commonly used programming languages, such as JavaScript, TypeScript and Java.

The performance improvements can only be achieved by not being 100% backwards
compatible.  For example, making function arguments available in the "a:"
dictionary adds quite a lot of overhead.  In a Vim9 function this dictionary
is not available.  Other differences are more subtle, such as how errors are
handled.

The Vim9 script syntax and semantics are used in:
- a function defined with the :def command
- a script file where the first command is vim9script
- an autocommand defined in the context of the above
- a command prefixed with the vim9cmd command modifier

When using :function in a Vim9 script file the legacy syntax is used, with
the highest scriptversion.  However, this can be confusing and is therefore
discouraged.

Vim9 script and legacy Vim script can be mixed.  There is no requirement to
rewrite old scripts, they keep working as before.  You may want to use a few
:def functions for code that needs to be fast.

:vim9[cmd] {cmd}                                :vim9 :vim9cmd E1164
                Evaluate and execute {cmd} using Vim9 script syntax and
                semantics.  Useful when typing a command and in a legacy
                script or function.

:leg[acy] {cmd}                                 :leg :legacy E1189 E1234
                Evaluate and execute {cmd} using legacy script syntax and
                semantics.  Only useful in a Vim9 script or a :def function.
                Note that {cmd} cannot use local variables, since it is parsed
                with legacy expression syntax.

See some examples of Vim9 script at 52.6.
==============================================================================

2. Differences from legacy Vim script                   vim9-differences

Overview
                                                        E1146
Brief summary of the differences you will most often encounter when using Vim9
script and :def functions; details are below:
- Comments start with #, not ":
- Using a backslash for line continuation is hardly ever needed:
- White space is required in many places to improve readability.
- Assign values without :let E1126 , declare variables with :var:
- Constants can be declared with :final and :const:
- :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:
- Call functions without :call:
- You cannot use old Ex commands:
        :Print
        :append
        :change
        :d  directly followed by 'd' or 'p'.
        :insert
        :k
        :mode
        :open
        :s  with only flags
        :t
        :xit
- Some commands, especially those used for flow control, cannot be shortened.
  E.g., :throw cannot be written as :th.  vim9-no-shorten
- You cannot use curly-braces names.
- A range before a command must be prefixed with a colon:
- Executing a register with "@r" does not work, you can prepend a colon or use
  :exe:
- Unless mentioned specifically, the highest scriptversion is used.
- When defining an expression mapping, the expression will be evaluated in the
  context of the script where it was defined.
- When indexing a string the index is counted in characters, not bytes:
  vim9-string-index
- Some possibly unexpected differences: vim9-gotchas.


Comments starting with #

In legacy Vim script comments start with double quote.  In Vim9 script
comments start with #.

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.

To improve readability there must be a space between a command and the #
that starts a comment:
                                                        E1170
Do not start a comment with #{, it looks like the legacy dictionary literal
and produces an error where this might be confusing.  #{{ or #{{{ are OK,
these can be used to start a fold.

When starting to read a script file Vim doesn't know it is Vim9 script until
the vim9script command is found.  Until that point you would need to use
legacy comments:

That looks ugly, better put vim9script in the very first line:

In legacy Vim script # is also used for the alternate file name.  In Vim9
script you need to use %% instead.  Instead of ## use %%% (stands for all
arguments).


Vim9 functions
                                                        E1099
A function defined with :def is compiled.  Execution is many times faster,
often 10 to 100 times.

Many errors are already found when compiling, before the function is executed.
The syntax is strict, to enforce code that is easy to read and understand.

Compilation is done when any of these is encountered:
- the first time the function is called
- when the :defcompile command is encountered in the script after the
  function was defined
- :disassemble is used for the function.
- a function that is compiled calls the function or uses it as a function
  reference (so that the argument and return types can be checked)
                                                E1091 E1191
If compilation fails it is not tried again on the next call, instead this
error is given: "E1091: Function is not compiled: {name}".
Compilation will fail when encountering a user command that has not been
created yet.  In this case you can call execute() to invoke it at runtime.

:def has no options like :function does: "range", "abort", "dict" or
"closure".  A :def function always aborts on an error (unless :silent! was
used for the command or the error was caught a :try block), does not get a
range passed, cannot be a "dict" function, and can always be a closure.
                                                vim9-no-dict-function
You can use a Vim9 Class (Vim9-class) instead of a "dict function".
You can also pass the dictionary explicitly:

You can call a legacy dict function though:
                                                E1096 E1174 E1175
The argument types and return type need to be specified.  The "any" type can
be used, type checking will then be done at runtime, like with legacy
functions.
                                                        E1106
Arguments are accessed by name, without "a:", just like any other language.
There is no "a:" dictionary or "a:000" list.
                        vim9-variable-arguments E1055 E1160 E1180
Variable arguments are defined as the last argument, with a name and have a
list type, similar to TypeScript.  For example, a list of numbers:

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:

                                        vim9-ignored-argument E1181
The argument "_" (an underscore) can be used to ignore the argument.  This is
most useful in callbacks where you don't need it, but do need to give an
argument to match the call.  E.g. when using map() two arguments are passed,
the key and the value, to ignore the key:
There is no error for using the "_" argument multiple times.  No type needs to
be given.


Functions and variables are script-local by default
                                                        vim9-scopes
When using :function or :def to specify a new function at the script level
in a Vim9 script, the function is local to the script.  Like prefixing "s:" in
legacy script.  To define a global function or variable the "g:" prefix must
be used.  For functions in a script that is to be imported and in an autoload
script "export" needs to be used for those to be used elsewhere.
                                                E1058 E1075
When using :function or :def to specify a nested function inside a :def
function and no namespace was given, this nested function is local to the code
block it is defined in.  It cannot be used in function() with a string
argument, pass the function reference itself:

Detail: this is because "Inner" will actually become a function reference to a
function with a generated name.

It is not possible to define a script-local function in a function.  You can
define a local function and assign it to a script-local Funcref (it must have
been declared at the script level).  It is possible to define a global
function by using the "g:" prefix.

When referring to a function and no "s:" or "g:" prefix is used, Vim will
search for the function:
- in the function scope, in block scopes
- in the script scope

Imported functions are found with the prefix from the :import command.

Since a script-local function reference can be used without "s:" the name must
start with an upper case letter even when using the "s:" prefix.  In legacy
script "s:funcref" could be used, because it could not be referred to with
"funcref".  In Vim9 script it can, therefore "s:Funcref" must be used to avoid
that the name interferes with builtin functions.
                                                vim9-s-namespace E1268
The use of the "s:" prefix is not supported at the Vim9 script level.  All
functions and variables without a prefix are script-local.

In :def functions the use of "s:" depends on the script: Script-local
variables and functions in a legacy script do use "s:", while in a Vim9 script
they do not use "s:".  This matches what you see in the rest of the file.

In legacy functions the use of "s:" for script items is required, as before.
No matter if the script is Vim9 or legacy.

In all cases the function must be defined before used.  That is when it is
called, when :defcompile causes it to be compiled, or when code that calls
it is being compiled (to figure out the return type).

The result is that functions and variables without a namespace can usually be
found in the script, either defined there or imported.  Global functions and
variables could be defined anywhere (good luck finding out where!  You can
often see where it was last set using :verbose).
                                                        E1102
Global functions can still be defined and deleted at nearly any time.  In
Vim9 script script-local functions are defined once when the script is sourced
and cannot be deleted or replaced by itself (it can be by reloading the
script).

When compiling a function and a function call is encountered for a function
that is not (yet) defined, the FuncUndefined autocommand is not triggered.
You can use an autoload function if needed, or call a legacy function and have
FuncUndefined triggered there.


Reloading a Vim9 script clears functions and variables by default
                                                vim9-reload E1149 E1150
When loading a legacy Vim script a second time nothing is removed, the
commands will replace existing variables and functions, create new ones, and
leave removed things hanging around.

When loading a Vim9 script a second time all existing script-local functions
and variables are deleted, thus you start with a clean slate.  This is useful
if you are developing a plugin and want to try a new version.  If you renamed
something you don't have to worry about the old name still hanging around.

If you do want to keep items, use:

You want to use this in scripts that use a finish command to bail out at
some point when loaded again.  E.g. when a buffer local option is set to a
function, the function does not need to be defined more than once:


Variable declarations with :var, :final and :const
                                vim9-declaration :var E1079
                                E1017 E1020 E1054 E1087 E1124
Local variables need to be declared with :var.  Local constants need to be
declared with :final or :const.  We refer to both as "variables" in this
section.

Variables can be local to a script, function or code block:

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:

The declaration must be done earlier:

Although this is shorter and faster for simple values:
                                                        E1025 E1128
To intentionally hide a variable from code that follows, a block can be
used:

This is especially useful in a user command:

And with autocommands:

Although using a :def function probably works better.

                                E1022 E1103 E1130 E1131 E1133
                                E1134
Declaring a variable with a type but without an initializer will initialize to
false (for bool), empty (for string, list, dict, etc.) or zero (for number,
any, etc.).  This matters especially when using the "any" type, the value will
default to the number zero.  For example, when declaring a list, items can be
added:

Initializing a variable to a null value, e.g. null_list, differs from not
initializing the variable.  This throws an error:

                                                E1016 E1052 E1066
In Vim9 script :let cannot be used.  An existing variable is assigned to
without any command.  The same for global, window, tab, buffer and Vim
variables, because they are not really declared.  Those can also be deleted
with :unlet.
                                                        E1065
You cannot use :va to declare a variable, it must be written with the full
name :var.  Just to make sure it is easy to read.
                                                        E1178
:lockvar does not work on local variables.  Use :const and :final
instead.

The exists() and exists_compiled() functions do not work on local variables
or arguments.
                                E1006 E1041 E1167 E1168 E1213
Variables, functions and function arguments cannot shadow previously defined
or imported variables and functions in the same script file.
Variables may shadow Ex commands, rename the variable if needed.

Global variables must be prefixed with "g:", also at the script level.

Global functions must be prefixed with "g:":
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:

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:

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:

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:
To ignore any remaining items:
                                                        E1163 E1080
Declaring more than one variable at a time, using the unpack notation, is
possible.  Each variable can have a type or infer it from the value:
Use this only when there is a list with values, declaring one variable per
line is much easier to read and change later.


Constants
                                                vim9-const vim9-final
How constants work varies between languages.  Some consider a variable that
can't be assigned another value a constant.  JavaScript is an example.  Others
also make the value immutable, thus when a constant uses a list, the list
cannot be changed.  In Vim9 we can use both.
                                                        E1021 E1307
:const is used for making both the variable and the value a constant.  Use
this for composite structures that you want to make sure will not be modified.
Example:
                                                        :final E1125
:final is used for making only the variable a constant, the value can be
changed.  This is well known from Java.  Example:

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.


Omitting :call and :eval
                                                        E1190
Functions can be called without :call:
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:

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:

If the expression starts with "!" this is interpreted as a shell command, not
negation of a condition.  Thus this is a shell command:
Put the expression in parentheses to use the "!" for negation:

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.


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:
                                                        E1157
No line break is allowed in the arguments of a lambda up to and including the
"=>" (so that Vim can tell the difference between an expression in parentheses
and lambda arguments).  This is OK:
This does not work:
This also does not work:
But you can use a backslash to concatenate the lines before parsing:
                                        vim9-lambda-arguments E1172
In legacy script a lambda could be called with any number of extra arguments,
there was no way to warn for not using them.  In Vim9 script the number of
arguments must match.  If you do want to accept any arguments, or any further
arguments, use "..._", which makes the function accept
vim9-variable-arguments.  Example:

                                                inline-function E1171
Additionally, a lambda can contain statements in {}:
This can be useful for a timer, for example:

The ending "}" must be at the start of a line.  It can be followed by other
characters, e.g.:
No command can follow the "{", only a comment can be used there.

                                                command-block E1026
The block can also be used for defining a user command.  Inside the block Vim9
syntax will be used.

This is an example of using here-docs:

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:
Put the '}' after the last item to avoid this:

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:

Also when confused with the start of a command block:


Automatic line continuation
                                        vim9-line-continuation E1097
In many cases it is obvious that an expression continues on the next line.  In
those cases there is no need to prefix the line with a backslash (see
line-continuation).  For example, when a list spans multiple lines:
And when a dict spans multiple lines:
With a function call:

For binary operators in expressions not in [], {} or () a line break is
possible just before or after the operator.  For example:

For a method call using "->" and a member using a dot, a line break is allowed
before it:

For commands that have an argument that is a list of commands, the | character
at the start of the line indicates line continuation:

Note that this means that in heredoc the first line cannot start with a bar:
Either use an empty line at the start or do not use heredoc.  Or temporarily
add the "C" flag to 'cpoptions':
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 '#\ ':
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:

                                                        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":
Like this:

This will assign "start" and print a line:

After the range an Ex command must follow.  Without the colon you can call a
function without :call, but after a range you do need it:

Note that the colon is not required for the +cmd argument:

It is also possible to split a function header over multiple lines, in between
arguments:

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:
Now "exit_cb: Func})" is actually a valid command: save any changes to the
file "_cb: Func})" and exit.  To avoid this kind of mistake in Vim9 script
there must be white space between most command names and the argument.
E1144

However, the argument of a command that is a command won't be recognized.  For
example, after "windo echo expr" a line break inside "expr" will not be seen.


Notes:
- "enddef" cannot be used at the start of a continuation line, it ends the
  current function.
- No line break is allowed in the LHS of an assignment.  Specifically when
  unpacking a list :let-unpack. This is OK:
   This does not work:
- No line break is allowed in between arguments of an :echo, :execute and
  similar commands.  This is OK:
   This does not work:
- In some cases it is difficult for Vim to parse a command, especially when
  commands are used as an argument to another command, such as :windo.  In
  those cases the line continuation with a backslash has to be used.


White space
                        E1004 E1068 E1069 E1074 E1127 E1202
Vim9 script enforces proper use of white space.  This is no longer allowed:
There must be white space before and after the "=":
White space must also be put before the # that starts a comment after a
command:

White space is required around most operators.

White space is required in a sublist (list slice) around the ":", except at
the start and end:

White space is not allowed:
- Between a function name and the "(":
                                                        E1205
White space is not allowed in a :set command between the option name and a
following "&", "!", "<", "=", "+=", "-=" or "^=".


No curly braces expansion

curly-braces-names cannot be used.


Command modifiers are not ignored
                                                                E1176
Using a command modifier for a command that does not use it gives an error.
                                                                E1082
Also, using a command modifier without a following command is now an error.


Dictionary literals
                                                vim9-literal-dict E1014
Traditionally Vim has supported dictionary literals with a {} syntax:

Later it became clear that using a simple text key is very common, thus
literal dictionaries were introduced in a backwards compatible way:

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:

This works for alphanumeric characters, underscore and dash.  If you want to
use another character, use a single or double quoted string:
                                                        E1139
In case the key needs to be an expression, square brackets can be used, just
like in JavaScript:

The key type can be string, number, bool or float.  Other types result in an
error.  Without using [] the value is used as a string, keeping leading zeros.
An expression given with [] is evaluated and then converted to a string.
Leading zeros will then be dropped:
A float only works inside [] because the dot is not accepted otherwise:


No :xit, :t, :k, :append, :change or :insert
                                                        E1100
These commands are too easily confused with local variable names.
Instead of :x or :xit you can use :exit.
Instead of :t you can use :copy.
Instead of :k you can use :mark.


Comparators

The 'ignorecase' option is not used for comparators that use strings.
Thus "=~" works like "=~#".

"is" and "isnot" (expr-is and expr-isnot) when used on strings now return
false.  In legacy script they just compare the strings, in Vim9 script they
check identity, and strings are copied when used, thus two strings are never
the same (this might change someday if strings are not copied but reference
counted).


Abort after error

In legacy script, when an error is encountered, Vim continues to execute
following lines.  This can lead to a long sequence of errors and need to type
CTRL-C to stop it.  In Vim9 script execution of commands stops at the first
error.  Example:


For loop
                                                        E1254
The loop variable must not be declared yet:

It is possible to use a global variable though:

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:
Would echo:
        1
        2
        3
        4
In compiled Vim9 script you get:
        1
        3
Generally, you should not change the list that is iterated over.  Make a copy
first if needed.
When looping over a list of lists, the nested lists can be changed.  The loop
variable is "final", it cannot be changed but what its value can be changed.
                                                        E1306
The depth of loops, :for and :while loops added together, cannot exceed 10.


Conditions and expressions
                                                vim9-boolean
Conditions and expressions are mostly working like they do in other languages.
Some values are different from legacy Vim script:
        value           legacy Vim script       Vim9 script
        0               falsy                   falsy
        1               truthy                  truthy
        99              truthy                  Error!
        "0"             falsy                   Error!
        "99"            truthy                  Error!
        "text"          falsy                   Error!

For the "??" operator and when using "!" then there is no error, every value
is either falsy or truthy.  This is mostly like JavaScript, except that an
empty list and dict is falsy:

        type            truthy when
        bool            true, v:true or 1
        number          non-zero
        float           non-zero
        string          non-empty
        blob            non-empty
        list            non-empty (different from JavaScript)
        dictionary      non-empty (different from JavaScript)
        func            when there is a function name
        special         true or v:true
        job             when not NULL
        channel         when not NULL
        class           when not NULL
        object          when not NULL (TODO: when isTrue() returns true)

The boolean operators "||" and "&&" expect the values to be boolean, zero or
one:

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:

When using ".." for string concatenation arguments of simple types are
always converted to string:

Simple types are Number, Float, Special and Bool.  For other types string()
should be used.
                        false true null null_blob null_channel
                        null_class null_dict null_function null_job
                        null_list null_object null_partial null_string
                        E1034
In Vim9 script one can use the following predefined values:
true is the same as v:true, false the same as v:false, null the same
as v:null.

While null has the type "special", the other "null_" values have the type
indicated by their name.  Quite often a null value is handled the same as an
empty value, but not always.  The values can be useful to clear a script-local
variable, since they cannot be deleted with :unlet.  E.g.:

The values can also be useful as the default value for an argument:
See null-compare for more information about testing against null.

It is possible to compare null  with any value, this will not give a type
error.  However, comparing null with a number, float or bool will always
result in false.  This is different from legacy script, where comparing
null with zero or false would return true.
                                                        vim9-false-true
When converting a boolean to a string false and true are used, not
v:false and v:true like in legacy script.  v:none has no none
replacement, it has no equivalent in other languages.
                                                        vim9-string-index
Indexing a string with [idx] or taking a slice with [idx : idx] uses character
indexes instead of byte indexes.  Composing characters are included.
Example:
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.


Some Ex commands can be confused with assignments in Vim9 script:

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 '.'.

Also, there cannot be a space between the command and the separator:

Functions defined with :def compile the whole function.  Legacy functions
can bail out, and the following lines are not parsed:
Vim9 functions are compiled as a whole:
For a workaround, split it in two functions:
Or put the unsupported code inside an if with a constant expression that
evaluates to false:
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:
This will give an error for "MyCommand" not being defined:
A workaround is to invoke the command indirectly with :execute:

Note that for unrecognized commands there is no check for "|" and a following
command.  This will give an error for missing endif:

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:

Using ++var or --var in an expression is not supported yet.

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

3. New style functions                                  fast-functions

                                                        :def E1028
:def[!] {name}([arguments])[: {return-type}]
                        Define a new function by the name {name}.  The body of
                        the function follows in the next lines, until the
                        matching :enddef. E1073
                                                        E1011
                        The {name} must be less than 100 bytes long.
                                        E1003 E1027 E1056 E1059
                        The type of value used with :return must match
                        {return-type}.  When {return-type} is omitted or is
                        "void" the function is not expected to return
                        anything.
                                                        E1077 E1123
                        {arguments} is a sequence of zero or more argument
                        declarations.  There are three forms:
                                {name}: {type}
                                {name} = {value}
                                {name}: {type} = {value}
                        The first form is a mandatory argument, the caller
                        must always provide them.
                        The second and third form are optional arguments.
                        When the caller omits an argument the {value} is used.

                        The function will be compiled into instructions when
                        called, or when :disassemble or :defcompile is
                        used.  Syntax and type errors will be produced at that
                        time.

                        It is possible to nest :def inside another :def or
                        :function up to about 50 levels deep.
                                                        E1117
                        [!] is used as with :function.  Note that
                        script-local functions cannot be deleted or redefined
                        later in Vim9 script.  They can only be removed by
                        reloading the same script.

                                        :enddef E1057 E1152 E1173
:enddef                 End of a function defined with :def. It should be on
                        a line by its own.

You may also find this wiki useful.  It was written by an early adopter of
Vim9 script: https://github.com/lacygoill/wiki/blob/master/vim/vim9.md

If the script the function is defined in is Vim9 script, then script-local
variables can be accessed without the "s:" prefix.  They must be defined
before the function is compiled.  If the script the function is defined in is
legacy script, then script-local variables must be accessed with the "s:"
prefix if they do not exist at the time of compiling.
                                                        E1269
Script-local variables in a Vim9 script must be declared at the script
level.  They cannot be created in a function, also not in a legacy function.

                                                :defc :defcompile
:defc[ompile]           Compile functions and classes (class-compile)
                        defined in the current script that were not compiled
                        yet.  This will report any errors found during
                        compilation.

:defc[ompile] MyClass   Compile all methods in a class. class-compile

:defc[ompile] {func}
:defc[ompile] debug {func}
:defc[ompile] profile {func}
                        Compile function {func}, if needed.  Use "debug" and
                        "profile" to specify the compilation mode.
                        This will report any errors found during compilation.
                        {func} call also be "ClassName.functionName" to
                        compile a function or method in a class.
                        {func} call also be "ClassName" to compile all
                        functions and methods in a class.

                                                :disa :disassemble
:disa[ssemble] {func}   Show the instructions generated for {func}.
                        This is for debugging and testing. E1061
                        Note that for command line completion of {func} you
                        can prepend "s:" to find script-local functions.

:disa[ssemble] profile {func}
                        Like :disassemble but with the instructions used for
                        profiling.

:disa[ssemble] debug {func}
                        Like :disassemble but with the instructions used for
                        debugging.

Limitations

Local variables will not be visible to string evaluation.  For example:

The map argument is a string expression, which is evaluated without the
function scope.  Instead, use a lambda:

For commands that are not compiled, such as :edit, backtick expansion can be
used and it can use the local scope.  Example:

Closures defined in a loop will share the same context.  For example:
                                                        E1271
A closure must be compiled in the context that it is defined in, so that
variables in that context can be found.  This mostly happens correctly, except
when a function is marked for debugging with :breakadd after it was compiled.
Make sure to define the breakpoint before compiling the outer function.

The "inloop" variable will exist only once, all closures put in the list refer
to the same instance, which in the end will have the value 4.  This is
efficient, also when looping many times.  If you do want a separate context
for each closure, call a function to define it:


In some situations, especially when calling a Vim9 closure from legacy
context, the evaluation will fail.  E1248

Note that at the script level the loop variable will be invalid after the
loop, also when used in a closure that is called later, e.g. with a timer.
This will generate error E1302:

You need to use a block and define a variable there, and use that one in the
closure:

Using :echowindow is useful in a timer, the messages go into a popup and will
not interfere with what the user is doing when it triggers.


Converting a function from legacy to Vim9
                                        convert_legacy_function_to_vim9
These are the most changes that need to be made to convert a legacy function
to a Vim9 function:

- Change func or function to def.
- Change endfunc or endfunction to enddef.
- Add types to the function arguments.
- If the function returns something, add the return type.
- Change comments to start with # instead of ".

  For example, a legacy function:
   Becomes:

- Remove "a:" used for arguments. E.g.:
   Becomes:

- Change let used to declare a variable to var.
- Remove let used to assign a value to a variable.  This is for local
  variables already declared and b: w: g: and t: variables.

  For example, legacy function:
   Becomes:

- Insert white space in expressions where needed.
- Change "." used for concatenation to "..".

  For example, legacy function:
   Becomes:

- line continuation does not always require a backslash:
   Becomes:


Calling a function in an expr option
                                                        expr-option-function
The value of a few options, such as 'foldexpr', is an expression that is
evaluated to get a value.  The evaluation can have quite a bit of overhead.
One way to minimize the overhead, and also to keep the option value very
simple, is to define a compiled function and set the option to call it
without arguments.  Example:

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

4. Types                                        vim9-types
                                        E1008 E1009 E1010 E1012
                                        E1013 E1029 E1030
The following builtin types are supported:
        bool
        number
        float
        string
        blob
        list<{type}>
        dict<{type}>
        job
        channel
        func
        func: {type}
        func({type}, ...)
        func({type}, ...): {type}
        void

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

These types can be used in declarations, but no simple value will actually
have the "void" type.  Trying to use a void (e.g. a function without a
return value) results in error E1031  E1186 .

There is no array type, use list<{type}> instead.  For a list constant an
efficient implementation is used that avoids allocating a lot of small pieces
of memory.
                                    vim9-func-declaration E1005 E1007
A partial and function can be declared in more or less specific ways:
func                            any kind of function reference, no type
                                checking for arguments or return value
func: void                      any number and type of arguments, no return
                                value
func: {type}                    any number and type of arguments with specific
                                return type

func()                          function with no argument, does not return a
                                value
func(): void                    same
func(): {type}                  function with no argument and return type

func({type})                    function with argument type, does not return
                                a value
func({type}): {type}            function with argument type and return type
func(?{type})                   function with type of optional argument, does
                                not return a value
func(...list<{type}>)           function with type of list for variable number
                                of arguments, does not return a value
func({type}, ?{type}, ...list<{type}>): {type}
                                function with:
                                - type of mandatory argument
                                - type of optional argument
                                - type of list for variable number of
                                  arguments
                                - return type

If the return type is "void" the function does not return a value.

The reference can also be a Partial, in which case it stores extra arguments
and/or a dictionary, which are not visible to the caller.  Since they are
called in the same way the declaration is the same.

Custom types can be defined with :type:
Custom types must start with a capital letter, to avoid name clashes with
builtin types added later, similarly to user functions.

And classes and interfaces can be used as types:



{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:
At compile time Vim doesn't know the type of "g:two" and the expression type
becomes list<any>.  An instruction is generated to check the list type before
doing the assignment, which is a bit inefficient.
                                                type-casting E1104
To avoid this, use a type cast:
The compiled code will then only check that "g:two" is a number and give an
error if it isn't.  This is called type casting.

The syntax of a type cast is:  "<" {type} ">".  There cannot be white space
after the "<" or before the ">" (to avoid them being confused with
smaller-than and bigger-than operators).

The semantics is that, if needed, a runtime type check is performed.  The
value is not actually changed.  If you need to change the type, e.g. to change
it to a string, use the string() function.  Or use str2nr() to convert a
string to a number.

If a type is given where it is not expected you can get E1272 .

If a type is incomplete you get E1363 , e.g. when you have an object for
which the class is not known (usually that is a null object).

Type inference
                                                        type-inference
In general: Whenever the type is clear it can be omitted.  For example, when
declaring a variable and giving it a value:

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.

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:
Results in:
        list<func(...)>

For script-local variables in Vim9 script the type is checked, also when the
variable was declared in a legacy function.

When a type has been declared this is attached to a List or Dictionary.  When
later some expression attempts to change the type an error will be given:

If the type is not declared then it is allowed to change:

For a variable declaration an inferred type matters:
That is because the declaration looks like a list of numbers, thus is
equivalent to:
If you do want a more permissive list you need to declare the type:


Stricter type checking
                                                        type-checking
In legacy Vim script, where a number was expected, a string would be
automatically converted to a number.  This was convenient for an actual number
such as "123", but leads to unexpected problems (and no error message) if the
string doesn't start with a number.  Quite often this leads to hard-to-find
bugs. e.g.:
        1
With an accidental space:
        0
                                                        E1206 E1210 E1212
In Vim9 script this has been made stricter.  In most places it works just as
before if the value used matches the expected type.  There will sometimes be
an error, thus breaking backwards compatibility.  For example:
- Using a number other than 0 or 1 where a boolean is expected.  E1023
- Using a string value when setting a number option.
- Using a number where a string is expected.   E1024 E1105

One consequence is that the item type of a list or dict given to map() must
not change, if the type was declared.  This will give an error in Vim9
script:
        E1012: Type mismatch; expected number but got string in map()

Instead use mapnew(), it creates a new list:
        ['item 0', 'item 1', 'item 2']

If the item type was not declared or determined to be "any" it can change to a
more specific type.  E.g. when a list of mixed types gets changed to a list of
strings:

There is a subtle difference between using a list constant directly and
through a variable declaration.  Because of type inference, when using a list
constant to initialize a variable, this also sets the declared type:

When using the list constant directly, the type is not declared and is allowed
to change:

The reasoning behind this is that when a type is declared and the list is
passed around and changed, the declaration must always hold.  So that you can
rely on the type to match the declared type.  For a constant this is not
needed.

                                                                E1158
Same for extend(), use extendnew() instead, and for flatten(), use
flattennew() instead.  Since flatten() is intended to always change the
type, it can not be used in Vim9 script.

Assigning to a funcref with specified arguments (see vim9-func-declaration)
does strict type checking of the arguments. For variable number of arguments
the type must match:

If the destination funcref has no specified arguments, then there is no
argument type checking:

                         E1211 E1217 E1218 E1219 E1220 E1221
                         E1222 E1223 E1224 E1225 E1226 E1227
                         E1228 E1238 E1250 E1251 E1252 E1256
                         E1297 E1298 E1301
Types are checked for most builtin functions to make it easier to spot
mistakes.

Categories of variables, defaults and null handling
                                variable-categories null-variables
There are categories of variables:
        primitive       number, float, boolean
        container       string, blob, list, dict
        specialized     function, job, channel, user-defined-object

When declaring a variable without an initializer, an explicit type must be
provided. Each category has different default initialization semantics. Here's
an example for each category:

Vim does not have a familiar null value; it has various null_<type> predefined
values, for example null_string, null_list, null_job. Primitives do not
have a null_<type>. The typical use cases for null_<type> are:
- to clear a variable and release its resources;
- as a default for a parameter in a function definition, see null-compare.

For a specialized variable, like job, null_<type> is used to clear the
resources. For a container variable, resources can also be cleared by
assigning an empty container to the variable. For example:

Using the empty container, rather than null_<type>, to clear a container
variable may avoid null complications as described in null-anomalies.

The initialization semantics of container variables and specialized variables
differ. An uninitialized container defaults to an empty container:
"l1" and "l2" are equivalent and indistinguishable initializations; but "l3"
is a null container. A null container is similar to, but different from, an
empty container, see null-anomalies.

Specialized variables default to null. These job initializations are
equivalent and indistinguishable:

When a list or dict is declared, if the item type is not specified and can not
be inferred, then the type is "any":

Declaring a function, see vim9-func-declaration, is particularly unique.

                                                null-compare
For familiar null compare semantics, where a null container is not equal to
an empty container, do not use null_<type> in a comparison:
The above function takes a list of strings and reports on it.
Change the above function signature to accept different types of arguments:

In the above example, where the goal is to distinguish a null list from an
empty list, comparing against null instead of null_list is the correct
choice. The basic reason is because "null_list == null" and "[] != null".
Comparing to null_list fails since "[] == null_list". In the following section
there are details about comparison results.

                                        null-details null-anomalies
This section describes issues about using null and null_<type>; included below
are the enumerated results of null comparisons. In some cases, if familiar
with vim9 null semantics, the programmer may chose to use null_<type> in
comparisons and/or other situations.

Elsewhere in the documentation it says:
        Quite often a null value is handled the same as an empty value, but
        not always
Here's an example:



Two values equal to a null_<type> are not necessarily equal to each other:

Unlike the other containers, an uninitialized string is equal to null. The
'is' operator can be used to determine if it is a null_string:


Any variable initialized to the null_<type> is equal to the null_<type> and is
also equal to null. For example:

An uninitialized variable is usually equal to null; it depends on its type:
        var s: string           s == null
        var b: blob             b != null   ***
        var l: list<any>        l != null   ***
        var d: dict<any>        d != null   ***
        var f: func             f == null
        var j: job              j == null
        var c: channel          c == null
        var o: Class            o == null

A variable initialized to empty equals null_<type>; but not null:
        var s2: string = ""       == null_string        != null
        var b2: blob = 0z         == null_blob          != null
        var l2: list<any> = []    == null_list          != null
        var d2: dict<any> = {}    == null_dict          != null

NOTE: the specialized variables, like job, default to null value and have no
corresponding empty value.

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

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

A Vim9 script can be written to be imported.  This means that some items are
intentionally exported, made available to other scripts.  When the exporting
script is imported in another script, these exported items can then be used in
that script.  All the other items remain script-local in the exporting script
and cannot be accessed by the importing script.

This mechanism exists for writing a script that can be sourced (imported) by
other scripts, while making sure these other scripts only have access to what
you want them to.  This also avoids using the global namespace, which has a
risk of name collisions.  For example when you have two plugins with similar
functionality.

You can cheat by using the global namespace explicitly.  That should be done
only for things that really are global.


Namespace
                                                        vim9-namespace
To recognize a file that can be imported the vim9script statement must
appear as the first statement in the file (see vim9-mix for an exception).
It tells Vim to interpret the script in its own namespace, instead of the
global namespace.  If a file starts with:
Then "myvar" will only exist in this file.  While without vim9script it would
be available as g:myvar from any other script and function.
                                                        E1101
The variables at the file level are very much like the script-local "s:"
variables in legacy Vim script, but the "s:" is omitted.  And they cannot be
deleted.

In Vim9 script the global "g:" namespace can still be used as before.  And the
"w:", "b:" and "t:" namespaces.  These have in common that variables are not
declared, have no specific type and they can be deleted.  E1304

A side effect of :vim9script is that the 'cpoptions' option is set to the
Vim default value, like with:
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:
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:
                                                        E1043 E1044
As this suggests, only constants, variables, :def functions and classes can
be exported.

                                                        E1042
:export can only be used in Vim9 script, at the script level.


Import
                                :import :imp E1094 E1047 E1262
                                E1048 E1049 E1053 E1071 E1088 E1236
The exported items can be imported in another script. The import syntax has
two forms. The simple form:

Where {filename} is an expression that must evaluate to a string.  In this
form the filename should end in ".vim" and the portion before ".vim" will
become the script local name of the namespace. For example:

This makes each exported item in "myscript.vim" available as "myscript.item".
                                                :import-as E1257 E1261
In case the name is long or ambiguous, this form can be used to specify
another name:

In this form {name} becomes a specific script local name for the imported
namespace.  Therefore {name} must consist of letters, digits and '_', like
internal-variables.  The {longfilename} expression must evaluate to any
filename.  For example:
                                                E1060 E1258 E1259 E1260
Then you can use "that.item", etc.  You are free to choose the name "that".
Use something that will be recognized as referring to the imported script.
Avoid command names, command modifiers and builtin function names, because the
name will shadow them.  It's better not to start the name with a capital
letter, since it can then also shadow global user commands and functions.
Also, you cannot use the name for something else in the script, such as a
function or variable name.

In case the dot in the name is undesired, a local reference can be made for a
function:

This also works for constants:

This does not work for variables, since the value would be copied once and
when changing the variable the copy will change, not the original variable.
You will need to use the full name, with the dot.

:import can not be used in a function.  Imported items are intended to exist
at the script level and only imported once.

The script name after import can be:
- A relative path, starting "." or "..".  This finds a file relative to the
  location of the script file itself.  This is useful to split up a large
  plugin into several files.
- An absolute path, starting with "/" on Unix or "D:/" on MS-Windows.  This
  will rarely be used.
- A path not being relative or absolute.  This will be found in the
  "import" subdirectories of 'runtimepath' entries.  The name will usually be
  longer and unique, to avoid loading the wrong file.
  Note that "after/import" is not used.

If the name does not end in ".vim" then the use of "as name" is required.

Once a vim9 script file has been imported, the result is cached and used the
next time the same script is imported.  It will not be read again.

It is not allowed to import the same script twice, also when using two
different "as" names.

When using the imported name the dot and the item name must be in the same
line, there can be no line break:
                                                import-map
When you've imported a function from one script into a vim9 script you can
refer to the imported function in a mapping by prefixing it with <SID>:

When the mapping is defined "<SID>name." will be replaced with <SNR> and the
script ID of the imported script.
An even simpler solution is using <ScriptCmd>:

Note that this does not work for variables, only for functions.

                                            import-legacy legacy-import
:import can also be used in legacy Vim script.  The imported namespace still
becomes script-local, even when the "s:" prefix is not given. For example:

And using the "as name" form:

However, the namespace cannot be resolved on its own:

This also affects the use of <SID> in the legacy mapping context.  Since
<SID> is only a valid prefix for a function and NOT for a namespace, you
cannot use it to scope a function in a script local namespace. Instead of
prefixing the function with <SID> you should use<ScriptCmd>. For example:


                                                        :import-cycle
The import commands are executed when encountered.  If script A imports
script B, and B (directly or indirectly) imports A, this will be skipped over.
At this point items in A after "import B" will not have been processed and
defined yet.  Therefore cyclic imports can exist and not result in an error
directly, but may result in an error for items in A after "import B" not being
defined.  This does not apply to autoload imports, see the next section.


Importing an autoload script
                                        vim9-autoload import-autoload
For optimal startup speed, loading scripts should be postponed until they are
actually needed.  Using the autoload mechanism is recommended:
                                                        E1264
1. In the plugin define user commands, functions and/or mappings that refer to
   items imported from an autoload script.

    This goes in .../plugin/anyname.vim.  "anyname.vim" can be freely chosen.
   The "SearchForStuff" command is now available to the user.

   The "autoload" argument to :import means that the script is not loaded
   until one of the items is actually used.  The script will be found under
   the "autoload" directory in 'runtimepath' instead of the "import"
   directory.  Alternatively a relative or absolute name can be used, see
   below.

2. In the autoload script put the bulk of the code.

    This goes in .../autoload/for/search.vim.

   Putting the "search.vim" script under the "/autoload/for/" directory has
   the effect that "for#search#" will be prefixed to every exported item.  The
   prefix is obtained from the file name, as you would to manually in a
   legacy autoload script.  Thus the exported function can be found with
   "for#search#Stuff", but you would normally use import autoload and not
   use the prefix (which has the side effect of loading the autoload script
   when compiling a function that encounters this name).

   You can split up the functionality and import other scripts from the
   autoload script as you like.  This way you can share code between plugins.

Searching for the autoload script in all entries in 'runtimepath' can be a bit
slow.  If the plugin knows where the script is located, quite often a relative
path can be used.  This avoids the search and should be quite a bit faster.
Another advantage is that the script name does not need to be unique.  An
absolute path is also possible.  Examples:

For defining a mapping that uses the imported autoload script the special key
<ScriptCmd> is useful.  It allows for a command in a mapping to use the
script context of where the mapping was defined.

When compiling a :def function and a function in an autoload script is
encountered, the script is not loaded until the :def function is called.
This also means you get any errors only at runtime, since the argument and
return types are not known yet.  If you would use the name with '#' characters
then the autoload script IS loaded.

Be careful to not refer to an item in an autoload script that does trigger
loading it unintentionally.  For example, when setting an option that takes a
function name, make sure to use a string, not a function reference:
On the other hand, it can be useful to load the script early, at a time when
any errors should be given.

For testing the test_override() function can be used to have the
import autoload load the script right away, so that the items and types can
be checked without waiting for them to be actually used:
Reset it later with:
Or:


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

6. Classes and interfaces                               vim9-classes

In legacy script a Dictionary could be used as a kind-of object, by adding
members that are functions.  However, this is quite inefficient and requires
the writer to do the work of making sure all the objects have the right
members.  See Dictionary-function.

In Vim9 script you can have classes, objects and interfaces like in most
popular object-oriented programming languages.  Since this is a lot of
functionality it is located in a separate help file: vim9class.txt.


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

9. Rationale                                            vim9-rationale

The :def command

Plugin writers have asked for much faster Vim script.  Investigations have
shown that keeping the existing semantics of function calls make this close to
impossible, because of the overhead involved with calling a function, setting
up the local function scope and executing lines.  There are many details that
need to be handled, such as error messages and exceptions.  The need to create
a dictionary for a: and l: scopes, the a:000 list and several others add too
much overhead that cannot be avoided.

Therefore the :def method to define a new-style function had to be added,
which allows for a function with different semantics.  Most things still work
as before, but some parts do not.  A new way to define a function was
considered the best way to separate the legacy style code from Vim9 style code.

Using "def" to define a function comes from Python. Other languages use
"function" which clashes with legacy Vim script.


Type checking

When compiling lines of Vim commands into instructions as much as possible
should be done at compile time.  Postponing it to runtime makes the execution
slower and means mistakes are found only later.  For example, when
encountering the "+" character and compiling this into a generic add
instruction, at runtime the instruction would have to inspect the type of the
arguments and decide what kind of addition to do.  And when the type is
dictionary throw an error.  If the types are known to be numbers then an "add
number" instruction can be used, which is faster.  The error can be given at
compile time, no error handling is needed at runtime, since adding two numbers
cannot fail.

The syntax for types, using <type> for compound types, is similar to Java.  It
is easy to understand and widely used.  The type names are what were used in
Vim before, with some additions such as "void" and "bool".


Removing clutter and weirdness

Once decided that :def functions have different syntax than legacy functions,
we are free to add improvements to make the code more familiar for users who
know popular programming languages.  In other words: remove weird things that
only Vim does.

We can also remove clutter, mainly things that were done to make Vim script
backwards compatible with the good old Vi commands.

Examples:
- Drop :call for calling a function and :eval for evaluating an
  expression.
- Drop using a leading backslash for line continuation, automatically figure
  out where an expression ends.

However, this does require that some things need to change:
- Comments start with # instead of ", to avoid confusing them with strings.
  This is good anyway, it is also used by several popular languages.
- Ex command ranges need to be prefixed with a colon, to avoid confusion with
  expressions (single quote can be a string or a mark, "/" can be divide or a
  search command, etc.).

Goal is to limit the differences.  A good criteria is that when the old syntax
is accidentally used you are very likely to get an error message.


Syntax and semantics from popular languages

Script writers have complained that the Vim script syntax is unexpectedly
different from what they are used to.  To reduce this complaint popular
languages are used as an example.  At the same time, we do not want to abandon
the well-known parts of legacy Vim script.

For many things TypeScript is followed.  It's a recent language that is
gaining popularity and has similarities with Vim script.  It also has a
mix of static typing (a variable always has a known value type) and dynamic
typing (a variable can have different types, this changes at runtime).  Since
legacy Vim script is dynamically typed and a lot of existing functionality
(esp. builtin functions) depends on that, while static typing allows for much
faster execution, we need to have this mix in Vim9 script.

There is no intention to completely match TypeScript syntax and semantics.  We
just want to take those parts that we can use for Vim and we expect Vim users
will be happy with.  TypeScript is a complex language with its own history,
advantages and disadvantages.  To get an idea of the disadvantages read the
book: "JavaScript: The Good Parts".  Or find the article "TypeScript: the good
parts" and read the "Things to avoid" section.

People familiar with other languages (Java, Python, etc.) will also find
things in TypeScript that they do not like or do not understand.  We'll try to
avoid those things.

Specific items from TypeScript we avoid:
- Overloading "+", using it both for addition and string concatenation.  This
  goes against legacy Vim script and often leads to mistakes.  For that reason
  we will keep using ".." for string concatenation.  Lua also uses ".." this
  way.  And it allows for conversion to string for more values.
- TypeScript can use an expression like "99 || 'yes'" in a condition, but
  cannot assign the value to a boolean.  That is inconsistent and can be
  annoying.  Vim recognizes an expression with && or || and allows using the
  result as a bool.  The falsy-operator was added for the mechanism to use a
  default value.
- TypeScript considers an empty string as Falsy, but an empty list or dict as
  Truthy.  That is inconsistent.  In Vim an empty list and dict are also
  Falsy.
- TypeScript has various "Readonly" types, which have limited usefulness,
  since a type cast can remove the immutable nature.  Vim locks the value,
  which is more flexible, but is only checked at runtime.
- TypeScript has a complicated "import" statement that does not match how the
  Vim import mechanism works.  A much simpler mechanism is used instead, which
  matches that the imported script is only sourced once.


Declarations

Legacy Vim script uses :let for every assignment, while in Vim9 declarations
are used.  That is different, thus it's good to use a different command:
:var.  This is used in many languages.  The semantics might be slightly
different, but it's easily recognized as a declaration.

Using :const  for constants is common, but the semantics varies.  Some
languages only make the variable immutable, others also make the value
immutable.  Since "final" is well known from Java for only making the variable
immutable we decided to use that.  And then :const can be used for making
both immutable.  This was also used in legacy Vim script and the meaning is
almost the same.

What we end up with is very similar to Dart:

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:

This is how we put types in a declaration:

Two alternatives were considered:
1. Put the type before the name, like Dart:
2. Put the type after the variable name, but do not use a colon, like Go:

The first is more familiar for anyone used to C or Java.  The second one
doesn't really have an advantage over the first, so let's discard the second.

Since we use type inference the type can be left out when it can be inferred
from the value.  This means that after var we don't know if a type or a name
follows.  That makes parsing harder, not only for Vim but also for humans.
Also, it will not be allowed to use a variable name that could be a type name,
using var string string is too confusing.

The chosen syntax, using a colon to separate the name from the type, adds
punctuation, but it actually makes it easier to recognize the parts of a
declaration.


Expressions

Expression evaluation was already close to what other languages are doing.
Some details are unexpected and can be improved.  For example a boolean
condition would accept a string, convert it to a number and check if the
number is non-zero.  This is unexpected and often leads to mistakes, since
text not starting with a number would be converted to zero, which is
considered false.  Thus using a string for a condition would often not give an
error and be considered false.  That is confusing.

In Vim9 type checking is stricter to avoid mistakes.  Where a condition is
used, e.g. with the :if command and the || operator, only boolean-like
values are accepted:
        true:  true, v:true, 1, 0 < 9
        false: false, v:false, 0, 0 > 9
Note that the number zero is false and the number one is true.  This is more
permissive than most other languages.  It was done because many builtin
functions return these values, and changing that causes more problems than it
solves.  After using this for a while it turned out to work well.

If you have any type of value and want to use it as a boolean, use the !!
operator:
        true: !!'text'   !![99]   !!{'x': 1}   !!99
        false: !!''   !![]   !!{}

From a language like JavaScript we have this handy construct:
However, this conflicts with only allowing a boolean for a condition.
Therefore the "??" operator was added:
Here you can explicitly express your intention to use the value as-is and not
result in a boolean. This is called the falsy-operator.


Import and Export

A problem of legacy Vim script is that by default all functions and variables
are global.  It is possible to make them script-local, but then they are not
available in other scripts.  This defies the concept of a package that only
exports selected items and keeps the rest local.

In Vim9 script a mechanism very similar to the JavaScript import and export
mechanism is supported.  It is a variant to the existing :source command
that works like one would expect:
- Instead of making everything global by default, everything is script-local,
  some of these are exported.
- When importing a script the symbols that are imported are explicitly listed,
  avoiding name conflicts and failures if functionality is added later.
- The mechanism allows for writing a big, long script with a very clear API:
  the exported functions, variables and classes.
- By using relative paths loading can be much faster for an import inside of a
  package, no need to search many directories.
- Once an import has been used, its items are cached and loading it again is
  not needed.
- The Vim-specific use of "s:" to make things script-local can be dropped.

When sourcing a Vim9 script (from a Vim9 or legacy script), only the items
defined globally can be used, not the exported items.  Alternatives
considered:
- All the exported items become available as script-local items.  This makes
  it uncontrollable what items get defined and likely soon leads to trouble.
- Use the exported items and make them global.  Disadvantage is that it's then
  not possible to avoid name clashes in the global namespace.
- Completely disallow sourcing a Vim9 script, require using :import.  That
  makes it difficult to use scripts for testing, or sourcing them from the
  command line to try them out.
Note that you CAN also use :import in legacy Vim script, see above.


Compiling functions early

Functions are compiled when called or when :defcompile is used.  Why not
compile them early, so that syntax and type errors are reported early?

The functions can't be compiled right away when encountered, because there may
be forward references to functions defined later.  Consider defining functions
A, B and C, where A calls B, B calls C, and C calls A again.  It's impossible
to reorder the functions to avoid forward references.

An alternative would be to first scan through the file to locate items and
figure out their type, so that forward references are found, and only then
execute the script and compile the functions.  This means the script has to be
parsed twice, which is slower, and some conditions at the script level, such
as checking if a feature is supported, are hard to use.  An attempt was made
to see if it works, but it turned out to be impossible to make work well.

It would be possible to compile all the functions at the end of the script.
The drawback is that if a function never gets called, the overhead of
compiling it counts anyway.  Since startup speed is very important, in most
cases it's better to do it later and accept that syntax and type errors are
only reported then.  In case these errors should be found early, e.g. when
testing, a :defcompile command at the end of the script will help out.


Why not use an existing embedded language?

Vim supports interfaces to Perl, Python, Lua, Tcl and a few others.  But
these interfaces have never become widely used, for various reasons.  When
Vim9 was designed a decision was made to make these interfaces lower priority
and concentrate on Vim script.

Still, plugin writers may find other languages more familiar, want to use
existing libraries or see a performance benefit.  We encourage plugin authors
to write code in any language and run it as an external process, using jobs
and channels.  We can try to make this easier somehow.

Using an external tool also has disadvantages.  An alternative is to convert
the tool into Vim script.  For that to be possible without too much
translation, and keeping the code fast at the same time, the constructs of the
tool need to be supported.  Since most languages support classes the lack of
support for classes in Vim is then a problem.



 vim:tw=78:ts=8:noet:ft=help:norl: