Vim documentation: usr_41

*usr_41.txt*	For Vim version 8.2.  Last change: 2021 Jan 13

		     VIM USER MANUAL - by Bram Moolenaar

			      Write a Vim script


The Vim script language is used for the startup vimrc file, syntax files, and
many other things.  This chapter explains the items that can be used in a Vim
script.  There are a lot of them, thus this is a long chapter.

|41.1|	Introduction
|41.2|	Variables
|41.3|	Expressions
|41.4|	Conditionals
|41.5|	Executing an expression
|41.6|	Using functions
|41.7|	Defining a function
|41.8|	Lists and Dictionaries
|41.9|	Exceptions
|41.10|	Various remarks
|41.11|	Writing a plugin
|41.12|	Writing a filetype plugin
|41.13|	Writing a compiler plugin
|41.14|	Writing a plugin that loads quickly
|41.15|	Writing library scripts
|41.16|	Distributing Vim scripts

     Next chapter: |usr_42.txt|  Add new menus
 Previous chapter: |usr_40.txt|  Make new commands
Table of contents: |usr_toc.txt|

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

*41.1*	Introduction				*vim-script-intro* *script*

Your first experience with Vim scripts is the vimrc file.  Vim reads it when
it starts up and executes the commands.  You can set options to values you
prefer.  And you can use any colon command in it (commands that start with a
":"; these are sometimes referred to as Ex commands or command-line commands).
   Syntax files are also Vim scripts.  As are files that set options for a
specific file type.  A complicated macro can be defined by a separate Vim
script file.  You can think of other uses yourself.

	If you are familiar with Python, you can find a comparison between
	Python and Vim script here, with pointers to other documents:
	   https://gist.github.com/yegappan/16d964a37ead0979b05e655aa036cad0
	And if you are familiar with JavaScript:
	   https://w0rp.com/blog/post/vim-script-for-the-javascripter/

Let's start with a simple example:

	:let i = 1
	:while i < 5
	:  echo "count is" i
	:  let i += 1
	:endwhile
 
	Note:
	The ":" characters are not really needed here.  You only need to use
	them when you type a command.  In a Vim script file they can be left
	out.  We will use them here anyway to make clear these are colon
	commands and make them stand out from Normal mode commands.
	Note:
	You can try out the examples by yanking the lines from the text here
	and executing them with :@"

The output of the example code is:

	count is 1 
	count is 2 
	count is 3 
	count is 4 

In the first line the ":let" command assigns a value to a variable.  The
generic form is:

	:let {variable} = {expression}

In this case the variable name is "i" and the expression is a simple value,
the number one.
   The ":while" command starts a loop.  The generic form is:

	:while {condition}
	:  {statements}
	:endwhile

The statements until the matching ":endwhile" are executed for as long as the
condition is true.  The condition used here is the expression "i < 5".  This
is true when the variable i is smaller than five.
	Note:
	If you happen to write a while loop that keeps on running, you can
	interrupt it by pressing CTRL-C (CTRL-Break on MS-Windows).

The ":echo" command prints its arguments.  In this case the string "count is"
and the value of the variable i.  Since i is one, this will print:

	count is 1 

Then there is the ":let i += 1" command.  This does the same thing as
":let i = i + 1".  This adds one to the variable i and assigns the new value
to the same variable.
Note: this is how it works in legacy Vim script, which is what we discuss in
this file.  In Vim9 script it's a bit different, see |usr_46.txt|.

The example was given to explain the commands, but would you really want to
make such a loop, it can be written much more compact:

	:for i in range(1, 4)
	:  echo "count is" i
	:endfor

We won't explain how |:for| and |range()| work until later.  Follow the links
if you are impatient.


FOUR KINDS OF NUMBERS

Numbers can be decimal, hexadecimal, octal or binary.  A hexadecimal number
starts with "0x" or "0X".  For example "0x1f" is decimal 31.  An octal number
starts with a zero.  "017" is decimal 15.  A binary number starts with "0b" or
"0B".  For example "0b101" is decimal 5.  Careful: don't put a zero before a
decimal number, it will be interpreted as an octal number!
   The ":echo" command always prints decimal numbers.  Example:

	:echo 0x7f 0o36
 	127 30 

A number is made negative with a minus sign.  This also works for hexadecimal,
octal and binary numbers.  A minus sign is also used for subtraction.  Compare
this with the previous example:

	:echo 0x7f -0o36
 	97 

White space in an expression is ignored.  However, it's recommended to use it
for separating items, to make the expression easier to read.  For example, to
avoid the confusion with a negative number above, put a space between the
minus sign and the following number:

	:echo 0x7f - 0o36

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

*41.2*	Variables

A variable name consists of ASCII letters, digits and the underscore.  It
cannot start with a digit.  Valid variable names are:

	counter
	_aap3
	very_long_variable_name_with_underscores
	FuncLength
	LENGTH

Invalid names are "foo+bar" and "6var".
   These variables are global.  To see a list of currently defined variables
use this command:

	:let

You can use global variables everywhere.  This also means that when the
variable "count" is used in one script file, it might also be used in another
file.  This leads to confusion at least, and real problems at worst.  To avoid
this, you can use a variable local to a script file by prepending "s:".  For
example, one script contains this code:

	:let s:count = 1
	:while s:count < 5
	:  source other.vim
	:  let s:count += 1
	:endwhile

Since "s:count" is local to this script, you can be sure that sourcing the
"other.vim" script will not change this variable.  If "other.vim" also uses an
"s:count" variable, it will be a different copy, local to that script.  More
about script-local variables here: |script-variable|.

There are more kinds of variables, see |internal-variables|.  The most often
used ones are:

	b:name		variable local to a buffer
	w:name		variable local to a window
	g:name		global variable (also in a function)
	v:name		variable predefined by Vim


DELETING VARIABLES

Variables take up memory and show up in the output of the ":let" command.  To
delete a variable use the ":unlet" command.  Example:

	:unlet s:count

This deletes the script-local variable "s:count" to free up the memory it
uses.  If you are not sure if the variable exists, and don't want an error
message when it doesn't, append !:

	:unlet! s:count

When a script finishes, the local variables used there will not be
automatically freed.  The next time the script executes, it can still use the
old value.  Example:

	:if !exists("s:call_count")
	:  let s:call_count = 0
	:endif
	:let s:call_count = s:call_count + 1
	:echo "called" s:call_count "times"

The "exists()" function checks if a variable has already been defined.  Its
argument is the name of the variable you want to check.  Not the variable
itself!  If you would do this:

	:if !exists(s:call_count)

Then the value of s:call_count will be used as the name of the variable that
exists() checks.  That's not what you want.
   The exclamation mark ! negates a value.  When the value was true, it
becomes false.  When it was false, it becomes true.  You can read it as "not".
Thus "if !exists()" can be read as "if not exists()".
   What Vim calls true is anything that is not zero.  Zero is false.
	Note:
	Vim automatically converts a string to a number when it is looking for
	a number.  When using a string that doesn't start with a digit the
	resulting number is zero.  Thus look out for this:
		:if "true"
 	The "true" will be interpreted as a zero, thus as false!


STRING VARIABLES AND CONSTANTS

So far only numbers were used for the variable value.  Strings can be used as
well.  Numbers and strings are the basic types of variables that Vim supports.
The type is dynamic, it is set each time when assigning a value to the
variable with ":let".  More about types in |41.8|.
   To assign a string value to a variable, you need to use a string constant.
There are two types of these.  First the string in double quotes:

	:let name = "peter"
	:echo name
 	peter 

If you want to include a double quote inside the string, put a backslash in
front of it:

	:let name = "\"peter\""
	:echo name
 	"peter" 

To avoid the need for a backslash, you can use a string in single quotes:

	:let name = '"peter"'
	:echo name
 	"peter" 

Inside a single-quote string all the characters are as they are.  Only the
single quote itself is special: you need to use two to get one.  A backslash
is taken literally, thus you can't use it to change the meaning of the
character after it.
   In double-quote strings it is possible to use special characters.  Here are
a few useful ones:

	\t		<Tab>
	\n		<NL>, line break
	\r		<CR>, <Enter>
	\e		<Esc>
	\b		<BS>, backspace
	\"		"
	\\		\, backslash
	\<Esc>		<Esc>
	\<C-W>		CTRL-W

The last two are just examples.  The  "\<name>" form can be used to include
the special key "name".
   See |expr-quote| for the full list of special items in a string.

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

*41.3*	Expressions

Vim has a rich, yet simple way to handle expressions.  You can read the
definition here: |expression-syntax|.  Here we will show the most common
items.
   The numbers, strings and variables mentioned above are expressions by
themselves.  Thus everywhere an expression is expected, you can use a number,
string or variable.  Other basic items in an expression are:

	$NAME		environment variable
	&name		option
	@r		register

Examples:

	:echo "The value of 'tabstop' is" &ts
	:echo "Your home directory is" $HOME
	:if @a > 5

The &name form can be used to save an option value, set it to a new value,
do something and restore the old value.  Example:

	:let save_ic = &ic
	:set noic
	:/The Start/,$delete
	:let &ic = save_ic

This makes sure the "The Start" pattern is used with the 'ignorecase' option
off.  Still, it keeps the value that the user had set.  (Another way to do
this would be to add "\C" to the pattern, see |/\C|.)


MATHEMATICS

It becomes more interesting if we combine these basic items.  Let's start with
mathematics on numbers:

	a + b		add
	a - b		subtract
	a * b		multiply
	a / b		divide
	a % b		modulo

The usual precedence is used.  Example:

	:echo 10 + 5 * 2
 	20 

Grouping is done with parentheses.  No surprises here.  Example:

	:echo (10 + 5) * 2
 	30 

Strings can be concatenated with ".." (see |expr6|).  Example:

	:echo "foo" .. "bar"
 	foobar 

When the ":echo" command gets multiple arguments, it separates them with a
space.  In the example the argument is a single expression, thus no space is
inserted.

Borrowed from the C language is the conditional expression:

	a ? b : c

If "a" evaluates to true "b" is used, otherwise "c" is used.  Example:

	:let i = 4
	:echo i > 5 ? "i is big" : "i is small"
 	i is small 

The three parts of the constructs are always evaluated first, thus you could
see it work as:

	(a) ? (b) : (c)

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

*41.4*	Conditionals

The ":if" commands executes the following statements, until the matching
":endif", only when a condition is met.  The generic form is:

	:if {condition}
	   {statements}
	:endif

Only when the expression {condition} evaluates to true (non-zero) will the
{statements} be executed.  These must still be valid commands.  If they
contain garbage, Vim won't be able to find the ":endif".
   You can also use ":else".  The generic form for this is:

	:if {condition}
	   {statements}
	:else
	   {statements}
	:endif

The second {statements} is only executed if the first one isn't.
   Finally, there is ":elseif":

	:if {condition}
	   {statements}
	:elseif {condition}
	   {statements}
	:endif

This works just like using ":else" and then "if", but without the need for an
extra ":endif".
   A useful example for your vimrc file is checking the 'term' option and
doing something depending upon its value:

	:if &term == "xterm"
	:  " Do stuff for xterm
	:elseif &term == "vt100"
	:  " Do stuff for a vt100 terminal
	:else
	:  " Do something for other terminals
	:endif


LOGIC OPERATIONS

We already used some of them in the examples.  These are the most often used
ones:

	a == b		equal to
	a != b		not equal to
	a >  b		greater than
	a >= b		greater than or equal to
	a <  b		less than
	a <= b		less than or equal to

The result is one if the condition is met and zero otherwise.  An example:

	:if v:version >= 700
	:  echo "congratulations"
	:else
	:  echo "you are using an old version, upgrade!"
	:endif

Here "v:version" is a variable defined by Vim, which has the value of the Vim
version.  600 is for version 6.0.  Version 6.1 has the value 601.  This is
very useful to write a script that works with multiple versions of Vim.
|v:version|

The logic operators work both for numbers and strings.  When comparing two
strings, the mathematical difference is used.  This compares byte values,
which may not be right for some languages.
   When comparing a string with a number, the string is first converted to a
number.  This is a bit tricky, because when a string doesn't look like a
number, the number zero is used.  Example:

	:if 0 == "one"
	:  echo "yes"
	:endif

This will echo "yes", because "one" doesn't look like a number, thus it is
converted to the number zero.

For strings there are two more items:

	a =~ b		matches with
	a !~ b		does not match with

The left item "a" is used as a string.  The right item "b" is used as a
pattern, like what's used for searching.  Example:

	:if str =~ " "
	:  echo "str contains a space"
	:endif
	:if str !~ '\.$'
	:  echo "str does not end in a full stop"
	:endif

Notice the use of a single-quote string for the pattern.  This is useful,
because backslashes would need to be doubled in a double-quote string and
patterns tend to contain many backslashes.

The 'ignorecase' option is used when comparing strings.  When you don't want
that, append "#" to match case and "?" to ignore case.  Thus "==?" compares
two strings to be equal while ignoring case.  And "!~#" checks if a pattern
doesn't match, also checking the case of letters.  For the full table see
|expr-==|.


MORE LOOPING

The ":while" command was already mentioned.  Two more statements can be used
in between the ":while" and the ":endwhile":

	:continue		Jump back to the start of the while loop; the
				loop continues.
	:break			Jump forward to the ":endwhile"; the loop is
				discontinued.

Example:

	:while counter < 40
	:  call do_something()
	:  if skip_flag
	:    continue
	:  endif
	:  if finished_flag
	:    break
	:  endif
	:  sleep 50m
	:endwhile

The ":sleep" command makes Vim take a nap.  The "50m" specifies fifty
milliseconds.  Another example is ":sleep 4", which sleeps for four seconds.

Even more looping can be done with the ":for" command, see below in |41.8|.

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

*41.5*	Executing an expression

So far the commands in the script were executed by Vim directly.  The
":execute" command allows executing the result of an expression.  This is a
very powerful way to build commands and execute them.
   An example is to jump to a tag, which is contained in a variable:

	:execute "tag " .. tag_name

The ".." is used to concatenate the string "tag " with the value of variable
"tag_name".  Suppose "tag_name" has the value "get_cmd", then the command that
will be executed is:

	:tag get_cmd

The ":execute" command can only execute colon commands.  The ":normal" command
executes Normal mode commands.  However, its argument is not an expression but
the literal command characters.  Example:

	:normal gg=G

This jumps to the first line and formats all lines with the "=" operator.
   To make ":normal" work with an expression, combine ":execute" with it.
Example:

	:execute "normal " .. normal_commands

The variable "normal_commands" must contain the Normal mode commands.
   Make sure that the argument for ":normal" is a complete command.  Otherwise
Vim will run into the end of the argument and abort the command.  For example,
if you start Insert mode, you must leave Insert mode as well.  This works:

	:execute "normal Inew text \<Esc>"

This inserts "new text " in the current line.  Notice the use of the special
key "\<Esc>".  This avoids having to enter a real <Esc> character in your
script.

If you don't want to execute a string but evaluate it to get its expression
value, you can use the eval() function:

	:let optname = "path"
	:let optval = eval('&' .. optname)

A "&" character is prepended to "path", thus the argument to eval() is
"&path".  The result will then be the value of the 'path' option.
   The same thing can be done with:
	:exe 'let optval = &' .. optname

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

*41.6*	Using functions

Vim defines many functions and provides a large amount of functionality that
way.  A few examples will be given in this section.  You can find the whole
list here: |functions|.

A function is called with the ":call" command.  The parameters are passed in
between parentheses separated by commas.  Example:

	:call search("Date: ", "W")

This calls the search() function, with arguments "Date: " and "W".  The
search() function uses its first argument as a search pattern and the second
one as flags.  The "W" flag means the search doesn't wrap around the end of
the file.

A function can be called in an expression.  Example:

	:let line = getline(".")
	:let repl = substitute(line, '\a', "*", "g")
	:call setline(".", repl)

The getline() function obtains a line from the current buffer.  Its argument
is a specification of the line number.  In this case "." is used, which means
the line where the cursor is.
   The substitute() function does something similar to the ":substitute"
command.  The first argument is the string on which to perform the
substitution.  The second argument is the pattern, the third the replacement
string.  Finally, the last arguments are the flags.
   The setline() function sets the line, specified by the first argument, to a
new string, the second argument.  In this example the line under the cursor is
replaced with the result of the substitute().  Thus the effect of the three
statements is equal to:

	:substitute/\a/*/g

Using the functions becomes more interesting when you do more work before and
after the substitute() call.



FUNCTIONS						*function-list*

There are many functions.  We will mention them here, grouped by what they are
used for.  You can find an alphabetical list here: |functions|.  Use CTRL-] on
the function name to jump to detailed help on it.


String manipulation:					*string-functions*
	nr2char()		get a character by its number value
	list2str()		get a character string from a list of numbers
	char2nr()		get number value of a character
	str2list()		get list of numbers from a string
	str2nr()		convert a string to a Number
	str2float()		convert a string to a Float
	printf()		format a string according to % items
	escape()		escape characters in a string with a '\'
	shellescape()		escape a string for use with a shell command
	fnameescape()		escape a file name for use with a Vim command
	tr()			translate characters from one set to another
	strtrans()		translate a string to make it printable
	tolower()		turn a string to lowercase
	toupper()		turn a string to uppercase
	charclass()		class of a character
	match()			position where a pattern matches in a string
	matchend()		position where a pattern match ends in a string
	matchfuzzy()		fuzzy matches a string in a list of strings
	matchfuzzypos()		fuzzy matches a string in a list of strings
	matchstr()		match of a pattern in a string
	matchstrpos()		match and positions of a pattern in a string
	matchlist()		like matchstr() and also return submatches
	stridx()		first index of a short string in a long string
	strridx()		last index of a short string in a long string
	strlen()		length of a string in bytes
	strchars()		length of a string in characters
	strwidth()		size of string when displayed
	strdisplaywidth()	size of string when displayed, deals with tabs
	setcellwidths()		set character cell width overrides
	substitute()		substitute a pattern match with a string
	submatch()		get a specific match in ":s" and substitute()
	strpart()		get part of a string using byte index
	strcharpart()		get part of a string using char index
	slice()			take a slice of a string, using char index in
				Vim9 script
	strgetchar()		get character from a string using char index
	expand()		expand special keywords
	expandcmd()		expand a command like done for `:edit`
	iconv()			convert text from one encoding to another
	byteidx()		byte index of a character in a string
	byteidxcomp()		like byteidx() but count composing characters
	charidx()		character index of a byte in a string
	repeat()		repeat a string multiple times
	eval()			evaluate a string expression
	execute()		execute an Ex command and get the output
	win_execute()		like execute() but in a specified window
	trim()			trim characters from a string
	gettext()		lookup message translation


List manipulation:					*list-functions*
	get()			get an item without error for wrong index
	len()			number of items in a List
	empty()			check if List is empty
	insert()		insert an item somewhere in a List
	add()			append an item to a List
	extend()		append a List to a List
	extendnew()		make a new List and append items
	remove()		remove one or more items from a List
	copy()			make a shallow copy of a List
	deepcopy()		make a full copy of a List
	filter()		remove selected items from a Lis