gnu: Add ghc-only.

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

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

gnu/packages/haskell-xyz.scm

@@ -8598,6 +8598,38 @@ old @code{time} library.  For new projects, the newer
 @uref{https://hackage.haskell.org/package/time, time library} is recommended.")
     (license license:bsd-3)))
 
+(define-public ghc-only
+  (package
+    (name "ghc-only")
+    (version "0.1")
+    (source
+     (origin
+       (method url-fetch)
+       (uri (string-append
+             "https://hackage.haskell.org/package/Only/Only-"
+             version
+             ".tar.gz"))
+       (sha256
+        (base32
+         "0rdj3a629fk2vp121jq8mf2smkblrz5w3cxhlsyx6my2x29s2ymb"))))
+    (build-system haskell-build-system)
+    (arguments
+     `(#:cabal-revision
+       ("1"
+        "1ahk7p34kmh041mz7lyc10nhcxgv2i4z8nvzxvqm2x34gslmsbzr")))
+    (home-page "https://hackage.haskell.org/package/Only")
+    (synopsis "The 1-tuple type or single-value collection")
+    (description
+     "This package provides a canonical anonymous 1-tuple type missing from
+Haskell for attaching typeclass instances.
+
+There is also the @url{https://hackage.haskell.org/package/OneTuple, OneTuple
+package} which by using a boxed @code{data}-type provides a 1-tuple type which
+has laziness properties which are more faithful to the ones of Haskell's
+native tuples; whereas the primary purpose of @code{Only} is to provide the
+traditionally so named type-wrapper for attaching typeclass instances.")
+    (license license:bsd-3)))
+
 (define-public ghc-opengl
   (package
     (name "ghc-opengl")