From mboxrd@z Thu Jan  1 00:00:00 1970
From: ludo@gnu.org (Ludovic =?utf-8?Q?Court=C3=A8s?=)
Subject: Re: [PATCH 2/2] utils: Support defaults in
	substitute-keyword-arguments.
Date: Sat, 24 Sep 2016 11:21:40 +0900
Message-ID: <87mviyf363.fsf@gnu.org>
References: <20160920212916.18530-1-ericbavier@openmailbox.org>
	<20160920212916.18530-2-ericbavier@openmailbox.org>
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In-Reply-To: <20160920212916.18530-2-ericbavier@openmailbox.org> (Eric
	Bavier's message of "Tue, 20 Sep 2016 16:29:16 -0500")
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To: Eric Bavier <ericbavier@openmailbox.org>
Cc: guix-devel@gnu.org, Eric Bavier <bavier@member.fsf.org>

Eric Bavier <ericbavier@openmailbox.org> skribis:

> From: Eric Bavier <bavier@member.fsf.org>
>
> * guix/utils.scm (substitute-keyword-arguments): Allow default value
> declarations.
> * tests/utils.scm (substitute-keyword-arguments): New test.

Good idea!


[...]

> +replaced by EXP.  EXP is evaluated in a context where VAR is bound to the
> +previous value of the keyword argument, or DFLT if given."
> +    (syntax-case x ()
> +      ((_ original-args ((kw var dflt ...) exp) ...)
> +       #`(let loop ((args (default-keyword-arguments
> +                            original-args
> +                            (list #,@(append-map (match-lambda
> +                                                   ((k) '())
> +                                                   (x x))
> +                                                 #'((kw dflt ...) ...)))=
))
> +                    (before '()))

I would prefer to stick to =E2=80=98syntax-rules=E2=80=99 when matching the=
 clauses,
with a helper macro:

  (define-syntax expand-default-args
    (syntax-rules ()
      ((_ original args ((kw var) expr) rest ...)
       ...)
      ((_ original args ((kw var default) expr) rest ...)
       ...)))

  =E2=80=A6

    (syntax-rules ()
      ((_ original-args ((clause exp) ...)
       (let loop ((args (expand-default-args original-args clause ...)))
         =E2=80=A6))))

But this would be a bonus; the patch LGTM.

Thanks!

Ludo=E2=80=99.