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usr_50.txt    For Vim version 9.0.  Last change: 2022 Jun 20

                     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:

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

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:

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

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()
           exe ":.,$write " .. tmp
           exe "!filter " .. tmp
           exe ":$read " .. tmp

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]}"

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.

50.3  Restoring the view

Sometimes you want to jump around, make a change and then go back to the same
position and view.  For example to change something in the file header.  This
can be done with two functions:

        var view = winsaveview()
        # Move around, make changes


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