bug#68413: Ungexp doesn't work on deep lists

[Josselin Poiret via Bug reports for GNU Guix <bug-guix@gnu.org>]

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Hi Ludo,

Ludovic Courtès <ludo@gnu.org> writes:

> Actually, what bug are we talking about?  It seems to work for me with
> this example:
>
> --8<---------------cut here---------------start------------->8---
> scheme@(guile-user)> (define packages `(("coreutils" ,coreutils) ("make" ,gnu-make)))
> scheme@(guile-user)> ,build (scheme-file "foo" #~(begin '#$packages))
> building /gnu/store/lq9gvbilv0y2nph00zxk6bn3lvcgdxqq-foo.drv...
> $7 = "/gnu/store/9ryh6v80jvjv3kwx0q782h26h9gbaclj-foo"
> scheme@(guile-user)> (call-with-input-file $7 get-string-all)
> $8 = "(begin (quote ((\"coreutils\" \"/gnu/store/mppp9hwxizx9g9pikwcvvshb2ffxyq7p-coreutils-9.1\") (\"make\" \"/gnu/store/9fadhs5qmwl5x7f669a0v39b3ryrmmf1-make-4.3\"))))"
> --8<---------------cut here---------------end--------------->8---
>
> One of the design decisions for gexps was to ensure that substituting a
> file-like object by its file name would be O(1) in most cases.
>
> Substitution in lists as in the example above is supported, but
> primarily for backward compatibility.  It should be avoided when
> possible because it’s inefficient: ‘gexp->sexp’ needs to traverse the
> whole list/tree in search of potential candidates.

Notice that OP's example uses `(("thing" . ,package)), ie. lists of
pairs and not lists of lists, as in your case!

Best,
-- 
Josselin Poiret

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