Subject: Re: [bug#34137] [PATCH 1/2] gnu: Add ghc-mtl.

From: Timothy Sample <[email-masked]>
Date: Sun, 20 Jan 2019 11:26:47 -0500

Hi Gabriel,

Gabriel Hondet <[email-masked]> writes:

> * gnu/packages/haskell.scm (ghc-mtl): New variable.

Thank you very much for the patch!

GHC 8.4.3 has ghc-mtl built-in.  This means that we don’t need to have a
separate package for it.  We used to have a package for it, but it was
removed in commit f54f04756f6d90c9fb3b5e45f3947329287f9b05 as a part of
upgrading GHC.  We try to avoid having separate packages for Haskell
libraries that are built-in to GHC because they cause problems
(incompatibilities in the dependency graph).

You should be able to remove the “ghc-mtl” input from the MPD package
without any issue.

(Sadly, the importer is not clever enough to filter out built-in
libraries, so it has to be done manually.)

> [...]

-- Tim