gnu: Add ghc-mtl-compat.

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

Signed-off-by: Christopher Baines <mail@cbaines.net>

gnu/packages/haskell-xyz.scm

@@ -7631,6 +7631,41 @@ semi-direct products, \"deletable\" monoids, \"split\" monoids, and
 \"cut\" monoids.")
     (license license:bsd-3)))
 
+(define-public ghc-mtl-compat
+  (package
+    (name "ghc-mtl-compat")
+    (version "0.2.2")
+    (source
+     (origin
+       (method url-fetch)
+       (uri (string-append
+             "mirror://hackage/package/mtl-compat/mtl-compat-"
+             version
+             ".tar.gz"))
+       (sha256
+        (base32
+         "17iszr5yb4f17g8mq6i74hsamii8z6m2qfsmgzs78mhiwa7kjm8r"))))
+    (build-system haskell-build-system)
+    (home-page
+     "https://github.com/haskell-compat/mtl-compat")
+    (synopsis
+     "Backported Control.Monad.Except module from mtl")
+    (description
+     "This package backports the Control.Monad.Except module from mtl (if
+using mtl-2.2.0.1 or earlier), which reexports the ExceptT monad transformer
+and the MonadError class.
+
+This package should only be used if there is a need to use the
+Control.Monad.Except module specifically.  If you just want the mtl class
+instances for ExceptT, use transformers-compat instead, since mtl-compat does
+nothing but reexport the instances from that package.
+
+Note that unlike how mtl-2.2 or later works, the Control.Monad.Except
+module defined in this package exports all of ExceptT's monad class instances.
+Therefore, you may have to declare @code{import Control.Monad.Except ()} at
+the top of your file to get all of the ExceptT instances in scope.")
+    (license license:bsd-3)))
+
 (define-public ghc-murmur-hash
   (package
     (name "ghc-murmur-hash")