On Fri, Nov 12, 2021 at 10:32:56PM +0300, Arseny Maslennikov wrote:
> Hi!
> 
> The doc page for _alternative (citing `info zsh`, menu item `Completion
> Functions') states:
> _alternative [ -O NAME ] [ -C NAME ] SPEC ...
>      <...>
>      The tags to use and the action to perform if a tag is requested are
>      described using the SPECs which are of the form:
>      'TAG:DESCR:ACTION'.  The TAGs are offered using _tags and if the
>      tag is requested, the ACTION is executed with the given description
>      DESCR.  The ACTIONs are those accepted by the _arguments function
>      (described below), excluding the '->STATE' and '=...' forms.
>      <...>
> 
> Later on, the text for _arguments advises to escape the separating
> colons between the match candidate and its description in a (( ))
> action spec, due to more complex spec parsing and further optional
> fields. A thorough reader of the docs might think the same is
> required by _alternative as well.
> 
> I've been looking at the implementation of _alternative lately; the
> function splits its specs into exactly 3 fields, delimited by `:'. The
> following seems to be true:
> 
> * the first field is delimited by the first colon;
> * the second field spans from after the first field's delimiter to the
>   nearest colon;
> * the rest of the spec is interpreted as the action field, unlike
>   the specs for _arguments.
> 
On a related note: this looks so obvious with hindsight but I've found
that the following way to avoid escaping the `:' works for _arguments too:

  function _ooo {
    local __ndescs=(
        '0:0' '1:1' '2:2' '3:3'
        '4:4' '5:5' '6:6' '7:7'
        '8:8' '9:9' 'A:10' 'B:11'
        'C:12' 'D:13' 'E:14' 'F:15'
    )
    _arguments '1:letter:(a b c y z)' '2:digit:(( "${(@)__ndescs}" ))'
  }

The expansion of `__ndescs' happens during the action eval, and the
expression does not contain any colons.
This approach is surprisingly quite unpopular, especially in third-party
completions — instead, the authors prefer to use a function as the
action, which then calls _describe by itself (here goes another while
tags; do while _next_label; do done; done loop) and does not introduce
new tags.