Mercurial > vim
comparison runtime/doc/eval.txt @ 30065:6cf788ab844c v9.0.0370
patch 9.0.0370: cleaning up afterwards can make a function messy
Commit: https://github.com/vim/vim/commit/1d84f7608f1e41dad03b8cc7925895437775f7c0
Author: Bram Moolenaar <Bram@vim.org>
Date: Sat Sep 3 21:35:53 2022 +0100
patch 9.0.0370: cleaning up afterwards can make a function messy
Problem: Cleaning up afterwards can make a function messy.
Solution: Add the :defer command.
| author | Bram Moolenaar <Bram@vim.org> |
|---|---|
| date | Sat, 03 Sep 2022 22:45:03 +0200 |
| parents | e37754a13778 |
| children | ebed259f919f |
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| 30064:a8f1fbaa43c8 | 30065:6cf788ab844c |
|---|---|
| 2975 | 2975 |
| 2976 A function can also be called as part of evaluating an expression or when it | 2976 A function can also be called as part of evaluating an expression or when it |
| 2977 is used as a method: > | 2977 is used as a method: > |
| 2978 let x = GetList() | 2978 let x = GetList() |
| 2979 let y = GetList()->Filter() | 2979 let y = GetList()->Filter() |
| 2980 | |
| 2981 | |
| 2982 CLEANING UP IN A FUNCTION ~ | |
| 2983 *:defer* | |
| 2984 :defer {func}({args}) Call {func} when the current function is done. | |
| 2985 {args} are evaluated here. | |
| 2986 | |
| 2987 Quite often a command in a function has a global effect, which must be undone | |
| 2988 when the function finishes. Handling this in all kinds of situations can be a | |
| 2989 hassle. Especially when an unexpected error is encountered. This can be done | |
| 2990 with `try` / `finally` blocks, but this gets complicated when there is more | |
| 2991 than one. | |
| 2992 | |
| 2993 A much simpler solution is using `defer`. It schedules a function call when | |
| 2994 the function is returning, no matter if there is an error. Example: > | |
| 2995 func Filter(text) | |
| 2996 call writefile(a:text, 'Tempfile') | |
| 2997 call system('filter < Tempfile > Outfile') | |
| 2998 call Handle('Outfile') | |
| 2999 call delete('Tempfile') | |
| 3000 call delete('Outfile') | |
| 3001 endfunc | |
| 3002 | |
| 3003 Here 'Tempfile' and 'Outfile' will not be deleted if something causes the | |
| 3004 function to abort. `:defer` can be used to avoid that: > | |
| 3005 func Filter(text) | |
| 3006 call writefile(a:text, 'Tempfile') | |
| 3007 defer delete('Tempfile') | |
| 3008 defer delete('Outfile') | |
| 3009 call system('filter < Tempfile > Outfile') | |
| 3010 call Handle('Outfile') | |
| 3011 endfunc | |
| 3012 | |
| 3013 Note that deleting "Outfile" is scheduled before calling system(), since it | |
| 3014 can be created even when `system()` fails. | |
| 3015 | |
| 3016 The defered functions are called in reverse order, the last one added is | |
| 3017 executed first. A useless example: > | |
| 3018 func Useless() | |
| 3019 for s in range(3) | |
| 3020 defer execute('echomsg "number ' .. s .. '"') | |
| 3021 endfor | |
| 3022 endfunc | |
| 3023 | |
| 3024 Now `:messages` shows: | |
| 3025 number 2 | |
| 3026 number 1 | |
| 3027 number 0 | |
| 3028 | |
| 3029 Any return value of the deferred function is discarded. The function cannot | |
| 3030 be followed by anything, such as "->func" or ".member". Currently `:defer | |
| 3031 GetArg()->TheFunc()` does not work, it may work in a later version. | |
| 3032 | |
| 3033 Errors are reported but do not cause aborting execution of deferred functions. | |
| 3034 | |
| 3035 No range is accepted. | |
| 2980 | 3036 |
| 2981 | 3037 |
| 2982 AUTOMATICALLY LOADING FUNCTIONS ~ | 3038 AUTOMATICALLY LOADING FUNCTIONS ~ |
| 2983 *autoload-functions* | 3039 *autoload-functions* |
| 2984 When using many or large functions, it's possible to automatically define them | 3040 When using many or large functions, it's possible to automatically define them |