Function Recursiveness, Cleaning Up with `:defer`

function is executed for each line in the range, with the cursor in the first column of that line. The cursor is left at the last line (possibly moved by the last function call). The arguments are re-evaluated for each line. Thus this works: *function-range-example* > :function Mynumber(arg) : echo line(".") .. " " .. a:arg :endfunction :1,5call Mynumber(getline(".")) < The "a:firstline" and "a:lastline" are defined anyway, they can be used to do something different at the start or end of the range. Example of a function that handles the range itself: > :function Cont() range : execute (a:firstline + 1) .. "," .. a:lastline .. 's/^/\t\\ ' :endfunction :4,8call Cont() < This function inserts the continuation character "\" in front of all the lines in the range, except the first one. When the function returns a composite value it can be further dereferenced, but the range will not be used then. Example: > :4,8call GetDict().method() < Here GetDict() gets the range but method() does not. *E132* The recursiveness of user functions is restricted with the |'maxfuncdepth'| option. It is also possible to use `:eval`. It does not support a range, but does allow for method chaining, e.g.: > eval GetList()->Filter()->append('$') A function can also be called as part of evaluating an expression or when it is used as a method: > let x = GetList() let y = GetList()->Filter() < ============================================================================== 3. Cleaning up in a function ~ *:defer* :defer {func}({args}) Call {func} when the current function is done. {args} are evaluated here. Quite often a command in a function has a global effect, which must be undone when the function finishes. Handling this in all kinds of situations can be a hassle. Especially when an unexpected error is encountered. This can be done with `try` / `finally` blocks, but this gets complicated when there is more than one. A much simpler solution is using `defer`. It schedules a function call when the function is returning, no matter if there is an error. Example: > func Filter(text) abort call writefile(a:text, 'Tempfile') call system('filter < Tempfile > Outfile') call Handle('Outfile') call delete('Tempfile') call delete('Outfile') endfunc Here 'Tempfile' and 'Outfile' will not be deleted if something causes the function to abort. `:defer` can be used to avoid that: > func Filter(text) abort call writefile(a:text, 'Tempfile') defer delete('Tempfile') defer delete('Outfile') call system('filter < Tempfile > Outfile') call Handle('Outfile') endfunc Note that deleting "Outfile" is scheduled before calling `system()`, since it can be created even when `system()` fails. The deferred functions are called in reverse order, the last one added is executed first. A useless example: > func Useless() abort for s in range(3) defer execute('echomsg "number ' .. s .. '"') endfor endfunc Now `:messages` shows: number 2 number 1 number 0 Any return value of the deferred function is discarded. The function cannot be followed by anything, such as "->func" or ".member". Currently `:defer GetArg()->TheFunc()` does not work, it may work in a later version. Errors are reported but do not cause aborting execution of deferred functions or altering execution outside

This section explains how functions are executed within a range, with the cursor moving to each line. It presents examples of functions using `a:firstline` and `a:lastline` to handle ranges and a function that inserts continuation characters. The passage also discusses the restriction of user function recursiveness using the `'maxfuncdepth'` option and alternative methods like `:eval` and method chaining for calling functions. Additionally, it introduces the `:defer` command for scheduling function calls upon function return to ensure cleanup, even in case of errors, with examples of deleting temporary files.