gnu: Add ghc-validation.

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

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

gnu/packages/haskell.scm

@@ -11608,6 +11608,60 @@ default)
 @end itemize")
     (license license:bsd-3)))
 
+(define-public ghc-validation
+  (package
+    (name "ghc-validation")
+    (version "1.1")
+    (source
+     (origin
+       (method url-fetch)
+       (uri (string-append
+             "mirror://hackage/package/validation/validation-"
+             version
+             ".tar.gz"))
+       (sha256
+        (base32
+         "1acj7mh3581ks405xswxw6667z7y1y0slisg6jvp6chc191ji9l5"))))
+    (build-system haskell-build-system)
+    (arguments
+     `(#:phases
+       (modify-phases %standard-phases
+         (add-after 'unpack 'add-setup-script
+           (lambda _
+             ;; The usual "Setup.hs" script is missing from the source.
+             (with-output-to-file "Setup.hs"
+               (lambda ()
+                 (format #t "import Distribution.Simple~%")
+                 (format #t "main = defaultMain~%"))))))))
+    (inputs
+     `(("ghc-semigroups" ,ghc-semigroups)
+       ("ghc-semigroupoids" ,ghc-semigroupoids)
+       ("ghc-bifunctors" ,ghc-bifunctors)
+       ("ghc-lens" ,ghc-lens)))
+    (native-inputs
+     `(("ghc-hedgehog" ,ghc-hedgehog)
+       ("ghc-hunit" ,ghc-hunit)))
+    (home-page "https://github.com/qfpl/validation")
+    (synopsis
+     "Data-type like Either but with an accumulating Applicative")
+    (description
+     "A data-type like Either but with differing properties and type-class
+instances.
+
+Library support is provided for this different representation, including
+@code{lens}-related functions for converting between each and abstracting over
+their similarities.
+
+The @code{Validation} data type is isomorphic to @code{Either}, but has an
+instance of @code{Applicative} that accumulates on the error side.  That is to
+say, if two (or more) errors are encountered, they are appended using a
+@{Semigroup} operation.
+
+As a consequence of this @code{Applicative} instance, there is no
+corresponding @code{Bind} or @code{Monad} instance.  @code{Validation} is an
+example of, \"An applicative functor that is not a monad.\"")
+    (license license:bsd-3)))
+
 (define-public ghc-concurrent-output
   (package
     (name "ghc-concurrent-output")