gnu: Add ghc-union-find.

* gnu/packages/haskell.scm (ghc-union-find): New variable.

gnu/packages/haskell.scm

@@ -6323,6 +6323,31 @@ (define-public ghc-pandoc
 provided for those who need a drop-in replacement for Markdown.pl.")
     (license license:gpl2+)))
 
+(define-public ghc-union-find
+  (package
+    (name "ghc-union-find")
+    (version "0.2")
+    (source (origin
+              (method url-fetch)
+              (uri (string-append
+                    "http://hackage.haskell.org/package/union-find/union-find-"
+                    version ".tar.gz"))
+              (sha256
+               (base32
+                "1v7hj42j9w6jlzi56jg8rh4p58gfs1c5dx30wd1qqvn0p0mnihp6"))))
+    (build-system haskell-build-system)
+    (home-page "http://github.com/nominolo/union-find")
+    (synopsis "Efficient union and equivalence testing of sets")
+    (description
+     "The Union/Find algorithm implements these operations in (effectively)
+constant-time:
+@enumerate
+@item Check whether two elements are in the same equivalence class.
+@item Create a union of two equivalence classes.
+@item Look up the descriptor of the equivalence class.
+@end enumerate\n")
+    (license license:bsd-3)))
+
 (define-public idris
   (package
     (name "idris")