From mboxrd@z Thu Jan  1 00:00:00 1970
From: ludo@gnu.org (Ludovic =?utf-8?Q?Court=C3=A8s?=)
Subject: Re: How do I make a manifest file that installs a specific version of
	a dependency?
Date: Mon, 13 Mar 2017 10:20:03 +0100
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In-Reply-To: <87mvcpy4in.fsf@gmail.com> (Chris Marusich's message of "Mon, 13
	Mar 2017 01:17:36 -0700")
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To: Chris Marusich <cmmarusich@gmail.com>
Cc: help-guix@gnu.org, Zachary Kanfer <zkanfer@gmail.com>

Chris Marusich <cmmarusich@gmail.com> skribis:

> Just for fun, I tried running some commands in a REPL.  I wanted to find
> out why the "list" procedure was being used here.  What I saw was this:
>
> scheme@(guile-user)> (specification->package+output "icedtea@2.6.9:jdk")
> $6 =3D #<package icedtea@2.6.9 gnu/packages/java.scm:347 39a6000>
> $7 =3D "jdk"
> scheme@(guile-user)> (list (specification->package+output "icedtea@2.6.9:=
jdk"))
> $8 =3D (#<package icedtea@2.6.9 gnu/packages/java.scm:347 39a6000>)
> scheme@(guile-user)> (map (compose list specification->package+output) '(=
"icedtea@2.6.9:jdk"))
> $9 =3D ((#<package icedtea@2.6.9 gnu/packages/java.scm:347 39a6000> "jdk"=
))
> scheme@(guile-user)>
>
> Why does the string "jdk" appear in $9 but not in $8?  It looks like the
> list procedure ignored the second value (the "jdk" string) when
> producing $8, but not when producing $9.  Is that true?  Why?

=E2=80=98specification->package+output=E2=80=99 returns two values; in Sche=
me that
really means returning two values, and this is completely different from
returning a two-element list or anything like that (info "(guile)
Multiple Values").

Guile automatically truncates multiple-value returns: if a procedure
returns, say, two values but its continuation expects only one, then the
second return value is dismissed.  Like this:

--8<---------------cut here---------------start------------->8---
scheme@(guile-user)> (values 1 2)
$2 =3D 1
$3 =3D 2    ;the REPL lists all the return values
scheme@(guile-user)> (list (values 1 2))
$4 =3D (1)  ;continuation of the =E2=80=98values=E2=80=99 form expects one =
value
--8<---------------cut here---------------end--------------->8---

=E2=80=98call-with-values=E2=80=99 is the primitive that allows you to retr=
ieve all the
return values:

--8<---------------cut here---------------start------------->8---
scheme@(guile-user)> (call-with-values (lambda () (values 1 2))
		       list)  ;continuation expects any number of values
$5 =3D (1 2)
scheme@(guile-user)> (call-with-values (lambda () (values 1 2))
		       (lambda (a b)  ;continuation expects two values
			 (list b a)))
$6 =3D (2 1)
--8<---------------cut here---------------end--------------->8---

I must say that this is more complicated than I=E2=80=99d like in the case =
of
=E2=80=98specification->package+output=E2=80=99.

HTH!

Ludo=E2=80=99.