*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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Table of contents: |usr_toc|
*50.1* Exceptions Let's start with an example:tryread ~/templates/pascal.tmplcatch /E484:/echo "Sorry, the Pascal template file cannot be found."endtryThe `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:tryread ~/templates/pascal.tmplcatchecho "Sorry, the Pascal template file cannot be found."endtryThis 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()tryexe ":.,$write " .. tmpexe "!filter " .. tmp:.,$deleteexe ":$read " .. tmpfinallydelete(tmp)endtryThis 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 pressingCTRL-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 Titleecho "start is " .. startechohl Nonefor index in range(len(items))echon $" Arg{index}is{items[index]}"endforechoenddefYou can call it like this:Show('Title', 'one', 'two', 'three')start is Title Arg 0 is one Arg 1 is two Arg 2 is threeThis 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 changeswinrestview(view)
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