From: ludo@gnu.org (Ludovic Courtès)
To: Chris Marusich <cmmarusich@gmail.com>
Cc: help-guix@gnu.org, Zachary Kanfer <zkanfer@gmail.com>
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 [thread overview]
Message-ID: <87lgs9a5z0.fsf@gnu.org> (raw)
In-Reply-To: <87mvcpy4in.fsf@gmail.com> (Chris Marusich's message of "Mon, 13 Mar 2017 01:17:36 -0700")
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 = #<package icedtea@2.6.9 gnu/packages/java.scm:347 39a6000>
> $7 = "jdk"
> scheme@(guile-user)> (list (specification->package+output "icedtea@2.6.9:jdk"))
> $8 = (#<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 = ((#<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?
‘specification->package+output’ returns two values; in Scheme 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 = 1
$3 = 2 ;the REPL lists all the return values
scheme@(guile-user)> (list (values 1 2))
$4 = (1) ;continuation of the ‘values’ form expects one value
--8<---------------cut here---------------end--------------->8---
‘call-with-values’ is the primitive that allows you to retrieve 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 = (1 2)
scheme@(guile-user)> (call-with-values (lambda () (values 1 2))
(lambda (a b) ;continuation expects two values
(list b a)))
$6 = (2 1)
--8<---------------cut here---------------end--------------->8---
I must say that this is more complicated than I’d like in the case of
‘specification->package+output’.
HTH!
Ludo’.
prev parent reply other threads:[~2017-03-13 9:20 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-03-12 5:55 How do I make a manifest file that installs a specific version of a dependency? Zachary Kanfer
2017-03-12 21:50 ` Ludovic Courtès
2017-03-12 23:50 ` Zachary Kanfer
2017-03-13 8:24 ` Ludovic Courtès
2017-03-13 8:17 ` Chris Marusich
2017-03-13 9:20 ` Ludovic Courtès [this message]
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