Re: using dynamic patterns on the left hand side of case statement clauses

[Lawrence Velázquez <vq@xxxxxxxxx>]

> On Mar 18, 2020, at 7:54 PM, scowles@xxxxxxxx wrote:
> 
> 
> i would like to use dynamic patterns on the left hand side of clauses
> in case statements.
> 
> i'm working in zsh current as of 3.1-91-g7595b22e on ubu 19.10.
> 
> the options set for this example are:
>   setopt extended_glob
>   setopt glob
>   setopt no_no_match
>   setopt no_null_glob
> 
> the code is:
>   typeset -a a b c
>   a=( one two three four )
>   b=( 16 17 18 19 20 )
>   c=( two 20 )
>   vb=$'|'
>   for d in ${c}
>   do
>      case ${d} in
>          $( eval echo ${(j:${vb}:)a} ) ) echo "1 found it" ;;
>          $( eval echo ${(j:${vb}:)b} ) ) echo "2 found it" ;;
>          * )                             echo "did not find it" ;;
>      esac
>   done
> 
> 
> but, when i run the code, the interpreter escapes all the vbars and
> forces the entire lhs pattern to be a string.
> 
> does the case structure not allow this use case?  or am i just
> missing something from not reading docs carefully enough?

The result of an unquoted command substitution is not eligible for
interpretation as a pattern unless GLOB_SUBST is set. (See the
"COMMAND SUBSTITUTION" section of the zshexpn(1) man page, as well
as the "GLOB_SUBST" entry of the zshoptions(1) man page.) So to
make your code work, you could just add `setopt glob_subst`.

However, you can achieve the same result without the ugly
`$(eval echo BLAH)` contortions:

    % zsh --version
    zsh 5.8 (x86_64-apple-darwin18.7.0)
    % cat /tmp/zsh-case-test
    setopt extended_glob no_no_match

    a=(one two three four)
    b=(16 17 18 19 20)
    c=(two 20)

    for d in ${c}; do
        case ${d} in
            ${(j:|:)~a} ) echo "1 found it" ;;
            ${(j:|:)~b} ) echo "2 found it" ;;
            * ) echo "did not find it" ;;
        esac
    done
    % zsh -f /tmp/zsh-case-test
    1 found it
    2 found it
    %

Search the zshexpn(1) man page for "${~spec}" for a more thorough
explanation, but the '~' basically turns GLOB_SUBST on for just
that parameter expansion.

vq