*usr_50.txt*	For Vim version 8.2.  Last change: 2022 Jun 03

		     VIM USER MANUAL - by Bram Moolenaar

			 Advanced Vim script writing


|50.1|	Exceptions
|50.2|    Function with variable number of arguments
|50.3|	Restoring the view

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*50.1*	Exceptions

Let's start with an example: >

	try
	   read ~/templates/pascal.tmpl
	catch /E484:/
	   echo "Sorry, the Pascal template file cannot be found."
	endtry

The `read` command will fail if the file does not exist.  Instead of
generating an error message, this code catches the error and gives the user a
message with more information.

For the commands in between `try` and `endtry` errors are turned into
exceptions.  An exception is a string.  In the case of an error the string
contains the error message.  And every error message has a number.  In this
case, the error we catch contains "E484:".  This number is guaranteed to stay
the same (the text may change, e.g., it may be translated).

Besides being able to give a nice error message, Vim will also continue
executing commands after the `:endtry`.  Otherwise, once an uncaught error is
encountered, execution of the script/function/mapping will be aborted.

When the `read` command causes another error, the pattern "E484:" will not
match in it.  Thus this exception will not be caught and result in the usual
error message and execution is aborted.

You might be tempted to do this: >

	try
	   read ~/templates/pascal.tmpl
	catch
	   echo "Sorry, the Pascal template file cannot be found."
	endtry

This means all errors are caught.  But then you will not see an error that
would indicate a completely different problem, such as "E21: Cannot make
changes, 'modifiable' is off".  Think twice before you catch any error!

Another useful mechanism is the `finally` command: >

	var tmp = tempname()
	try
	   exe ":.,$write " .. tmp
	   exe "!filter " .. tmp
	   :.,$delete
	   exe ":$read " .. tmp
	finally
	   delete(tmp)
	endtry

This filters the lines from the cursor until the end of the file through the
"filter" command, which takes a file name argument.  No matter if the
filtering works, if something goes wrong in between `try` and `finally` or the
user cancels the filtering by pressing CTRL-C, the `delete(tmp)` call is
always executed.  This makes sure you don't leave the temporary file behind.

The `finally` does not catch the exception, the error will still abort
further execution.

More information about exception handling can be found in the reference
manual: |exception-handling|.

==============================================================================
*50.2*	Function with variable number of arguments

Vim enables you to define functions that have a variable number of arguments.
The following command, for instance, defines a function that must have 1
argument (start) and can have up to 20 additional arguments: >

	def Show(start: string, ...items: list<string>)

The variable "items" will be a list in the function containing the extra
arguments.  You can use it like any list, for example: >

	def Show(start: string, ...items: list<string>)
	  echohl Title
	  echo "start is " .. start
	  echohl None
	  for index in range(len(items))
	    echon $"  Arg {index} is {items[index]}"
	  endfor
	  echo
	enddef

You can call it like this: >

	Show('Title', 'one', 'two', 'three')
<	start is Title  Arg 0 is one  Arg 1 is two  Arg 2 is three ~
	
This uses the `echohl` command to specify the highlighting used for the
following `echo` command.  `echohl None` stops it again.  The `echon` command
works like `echo`, but doesn't output a line break.

If you call it with one argument the "items" list will be empty.
`range(len(items))` returns a list with the indexes, what `for` loops over,
we'll explain that further down.

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*50.3*	Restoring the view

Sometimes you want to make a change and go back to where the cursor was.
Restoring the relative position would also be nice, so that the same line
appears at the top of the window.

This example yanks the current line, puts it above the first line in the file
and then restores the view: >

	map ,p ma"aYHmbgg"aP`bzt`a

What this does: >
	ma"aYHmbgg"aP`bzt`a
<	ma			set mark a at cursor position
	  "aY			yank current line into register a
	     Hmb		go to top line in window and set mark b there
		gg		go to first line in file
		  "aP		put the yanked line above it
		     `b		go back to top line in display
		       zt	position the text in the window as before
			 `a	go back to saved cursor position


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Next chapter: |usr_51.txt|  Create a plugin

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