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Date:         Mon, 8 Jul 2013 22:26:32 -0400
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From: Bruno Le Floch <blflatex@GMAIL.COM>
Subject: Re: Named options/enumerations (Was re: Defining \bool_case:nn)
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Status: R

Hello Joel,

Surely Joseph will have something to say about the topic (he is
currently doing some changes to l3keys), but let me try.

> This use of named options feels like it should be reasonably common in
> user-facing code, but Expl3 doesn=92t have an obvious way of managing t=
his.
>
>     \keys_define:nn { jcsfonts }
>       { font .choice_code:n =3D \tl_gset:NV \g_jcsfonts_option_tl
> \l_keys_choice_tl }
>
>     \str_case:Vnn \g_jcsfonts_option_tl
>       { { cmodern } {} % Computer Modern is LaTeX default
>         { kpfonts } { \RequirePackage{kpfonts} }
>       } { \msg_error=85 }

What about

    \keys_define:nn { jcsfonts }
      {
        font .choice: ,
        font / cmodern .code:n =3D { } ,
        font / kpfonts .code:n =3D { \RequirePackage{kpfonts} } ,
      }

This will trigger a "key-choice-unknown" error upon encountering an
unknown choice (well, right now the message is not defined... that'll
be fixed).  Maybe we could add a feature to allow arbitrary code for
unknown choices.

> I recently asked =93Can I refer to key value in general code?=94 (
> http://tex.stackexchange.com/q/120258/2966). My initial thought (somewh=
at
> bizarre, I=92ll admit) was to save the option is a quark, set equal to =
one of
> a group of quarks.

I think that code was missing at least one "_".  Does my answer there hel=
p you?

>     \enum_case:Vnn \g_jcsfonts_option_enum
>       { \c_jcsfonts_cmodern_enum {}
>         \c_jcsfonts_kpfonts_enum { \RequirePackage{kpfonts} }
>       } { \msg_error=85 }

I don't really get the issue with \str_case:Vnn, to be honest, and I
don't see the advantage of an enum type.  Given that it makes the
syntax much more verbose, I am so far not convinced.  Alternatively,
using a property list to hold the code corresponding to each value
could work (and be faster).

> It=92s too early for anything to be idiomatic LaTeX3, but what should
> *become* idiomatic?

Well, it's not clear yet if the enum type is that useful.  My second
language after TeX is Python, and the enum type was only added after
something like 20 years, so presumably people lived without it.  An
enum type seems to only be useful if it is significantly more
efficient than a property list ("associative array"/"dictionary") with
empty values.

Regards,
Bruno