gnu: Add ghc-equivalence.

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

Signed-off-by: Ricardo Wurmus <rekado@elephly.net>

gnu/packages/haskell.scm

@@ -4715,6 +4715,35 @@ (define-public ghc-adjunctions
 for Haskell.")
     (license license:bsd-3)))
 
+(define-public ghc-equivalence
+  (package
+    (name "ghc-equivalence")
+    (version "0.3.2")
+    (source
+     (origin
+       (method url-fetch)
+       (uri (string-append "https://hackage.haskell.org/package/equivalence"
+                           "/equivalence-" version ".tar.gz"))
+       (sha256
+        (base32 "0a85bdyyvjqs5z4kfhhf758210k9gi9dv42ik66a3jl0z7aix8kx"))))
+    (build-system haskell-build-system)
+    (inputs
+     `(("ghc-mtl" ,ghc-mtl)
+       ("ghc-stmonadtrans" ,ghc-stmonadtrans)
+       ("ghc-transformers-compat" ,ghc-transformers-compat)
+       ("ghc-quickcheck" ,ghc-quickcheck)
+       ("ghc-test-framework" ,ghc-test-framework)
+       ("ghc-test-framework-quickcheck2" ,ghc-test-framework-quickcheck2)))
+    (home-page "https://github.com/pa-ba/equivalence")
+    (synopsis "Maintaining an equivalence relation implemented as union-find")
+    (description
+     "This is an implementation of Tarjan's Union-Find algorithm (Robert E.@:
+Tarjan.  \"Efficiency of a Good But Not Linear Set Union Algorithm\",JACM
+22(2), 1975) in order to maintain an equivalence relation.  This
+implementation is a port of the @code{union-find} package using the @code{ST}
+monad transformer (instead of the IO monad).")
+    (license license:bsd-3)))
+
 (define-public ghc-fast-logger
   (package
     (name "ghc-fast-logger")