From mboxrd@z Thu Jan  1 00:00:00 1970
From: ludo@gnu.org (Ludovic =?utf-8?Q?Court=C3=A8s?=)
Subject: Re: Some macros to make package definitions prettier
Date: Thu, 26 Feb 2015 00:32:14 +0100
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References: <874mqa6iz4.fsf@taylan.uni.cx>
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In-Reply-To: <874mqa6iz4.fsf@taylan.uni.cx> ("Taylan Ulrich
	\=\?utf-8\?Q\?\=5C\=22Bay\=C4\=B1rl\=C4\=B1\=2FKammer\=5C\=22\=22's\?\=
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To: Taylan Ulrich =?utf-8?Q?=22Bay=C4=B1rl=C4=B1?=
	=?utf-8?Q?=2FKammer=22?= <taylanbayirli@gmail.com>
Cc: guix-devel@gnu.org

taylanbayirli@gmail.com (Taylan Ulrich "Bay=C4=B1rl=C4=B1/Kammer") skribis:

> (modify-phases '((foo . 0) (bar . 1) (baz . 2))
>   (delete foo)
>   (replace bar 'x)
>   (add-before baz pre-baz 'y)) ;=3D> ((bar . x) (pre-baz . y) (baz . 2))

I like it!  Let=E2=80=99s put it in (guix utils)?

> ;;; phase-lambda
>
> (define-syntax phase-lambda
>   (syntax-rules ()
>     ((phase-lambda ((arg (alist-entry ...))
>                     ...)
>        body ...)
>      (lambda* (#:key arg ... #:allow-other-keys)
>        (let-values (((alist-entry ...)
>                      (let ((arg* arg))
>                        (values
>                         (assoc-ref arg* (symbol->string 'alist-entry))
>                         ...)))
>                     ...)
>          body ...)))))
>
> ;;; Usage example:
>
> (phase-lambda ((inputs (libx liby))
>                (outputs (out)))
>   ...)
>
> ;;; effectively equivalent to:
>
> (lambda* (#:key inputs outputs #:allow-other-keys)
>   (let ((libx (assoc-ref inputs "libx"))
>         (liby (assoc-ref inputs "liby"))
>         (out (assoc-ref outputs "out")))
>     ...))
>
>
> This saves the usual boilerplate of '(#:key inputs outputs
> #:allow-other-keys)' and the subsequent `assoc-ref' uses.

Clearly, this is something we must address.  However, I=E2=80=99m uneasy wi=
th
this solutions for two reasons:

  1. The embedded =E2=80=98symbol->string=E2=80=99, and the fact that the d=
eveloper no
     longer types in "foo" when the =E2=80=98inputs=E2=80=99 field has "foo=
" (as a
     string), making the connection non-obvious.

  2. In the grand scheme of things, gexps make the problem moot:

     (lambda _
       (frob-libs #$libx #$liby:include)
       (stuff-output #$output)
       ...)

However, there=E2=80=99s a complication: currently we can change the =E2=80=
=98inputs=E2=80=99
field of a package (like =E2=80=98package-with-explicit-inputs=E2=80=99 etc=
. do), and
its build script will get to see the new inputs, which is super
convenient.

Conversely, gexps currently end up hard-coding the output file name of
the package directly in the build script, which prevents this kind of
=E2=80=9Cinput rewriting.=E2=80=9D

I don=E2=80=99t yet have a nice idea on how to solve that, but that=E2=80=
=99s an
important thing to do.

WDYT?

Thank you,
Ludo=E2=80=99.