44969: completion-style-guide: Mention defaults and superfluous descriptions

2 files changed, 27 insertions, 1 deletions

diff --git a/ChangeLog b/ChangeLog
index 348cd58da..35674511c 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,8 @@
 2019-12-02  dana  <dana@dana.is>
 
+	* 44969: Etc/completion-style-guide: Mention defaults and
+	superfluous descriptions
+
 	* unposted (cf. 44967): Completion/Linux/Command/_alsa-utils:
 	Fix minor syntax error in arg spec
 
diff --git a/Etc/completion-style-guide b/Etc/completion-style-guide
index af7c46bfd..53d522764 100644
--- a/Etc/completion-style-guide
+++ b/Etc/completion-style-guide
@@ -69,12 +69,35 @@ for example, do not use:
   '--timeout[specify connection timeout in milliseconds]:timeout'
 but use:
   '--timeout[specify connection timeout]:timeout (ms)'
-  
+To indicate a default value, use square brackets:
+  '--timeout[specify connection timeout]:timeout (ms) [5000]'
+These two conventions can be used together or individually as appropriate.
+
 Group descriptions should be singular because only one thing is being
 completed even though many may be listed. This applies even where you
 complete a list of the things. Tags, functions for completing types of
 things (such as _files), and states should have plural names.
 
+Group descriptions can be omitted where they are handled by a helper/type
+function. For example, the `file' description in the following line is
+unnecessary, as `_files' provides it by default:
+  '--import=[import specified file]:file:_files'
+In this case, the following syntax can be used instead:
+  '--import=[import specified file]: :_files'
+Of course, it may make sense to add an explicit description which is
+more specific than the default:
+  '--config=[use specified config file]:config file:_files'
+
+Similarly, group descriptions can and should be omitted where a `->state'
+is involved, as the description in the argument spec is always ignored
+in these cases. For example, instead of:
+  '--config=[use specified config file]:config file:->config-files'
+use:
+  '--config=[use specified config file]: :->config-files'
+Setting an explicit description here constitutes (a small amount of)
+unnecessary noise, and may be misleading to other developers who update
+the function.
+
 In a function, allow any descriptions passed as an argument to override
 the default you define. For example:
   _wanted directories expl directory _files -/ "$@" -