Chris Marusich <cmmarus...@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’.
