bug#71941: Broken `map-derivation' procedure

["Sergio Pastor Pérez" <sergio.pastorperez@outlook.es>]

Hello.

The procedure `map-derivation` from `(guix derivations)` seems broken.

Evaluating this yields an error, it probably shouldn't:
--8<---------------cut here---------------start------------->8---
scheme@(guix-user)> (use-modules (guix)
                                 (guix derivations)
                                 (gnu packages)
                                 (gnu packages perl)
                                 (gnu packages games))
scheme@(guix-user)> (with-store store
                      (let ((cowsay-drv (package-derivation store cowsay))
                            (perl-drv (package-derivation store perl))
                            (perl-5.6-drv (package-derivation store perl-5.6)))
                        (map-derivation store
                                        cowsay-drv
                                        `((,perl-drv . ,perl-5.6-drv)))))
ice-9/boot-9.scm:1685:16: In procedure raise-exception:
In procedure fport_read: Is a directory

Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
scheme@(guix-user) [1]> 
--8<---------------cut here---------------end--------------->8---

If you inspect the `cowsay` derivation, you will see that the mapping
should be possible since it contains the `perl` derivation.

Does anyone have an idea on what could be the issue or how to investigate
further?

Thanks,
Sergio.