gnu: ghc-diff: Patch to work with newer QuickCheck.

* gnu/packages/patches/ghc-diff-swap-cover-args.patch: New file.
* gnu/local.mk: Add it.
* gnu/packages/haskell-xyz.scm (ghc-diff): Use it.

gnu/local.mk

@@ -878,6 +878,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/gd-freetype-test-failure.patch		\
   %D%/packages/patches/geoclue-config.patch			\
   %D%/packages/patches/ghc-8.0-fall-back-to-madv_dontneed.patch \
+  %D%/packages/patches/ghc-diff-swap-cover-args.patch           \
   %D%/packages/patches/ghc-dont-pass-linker-flags-via-response-files.patch	\
   %D%/packages/patches/ghc-haddock-library-unbundle.patch		\
   %D%/packages/patches/ghostscript-no-header-id.patch		\

gnu/packages/haskell-xyz.scm

@@ -2606,6 +2606,7 @@ and parsers with useful semantics.")
               (method url-fetch)
               (uri (string-append "https://hackage.haskell.org/package/"
                                   "Diff/Diff-" version ".tar.gz"))
+              (patches (search-patches "ghc-diff-swap-cover-args.patch"))
               (sha256
                (base32
                 "0bqcdvhxx8dmqc3793m6axg813wv9ldz2j37f1wygbbrbbndmdvp"))))

gnu/packages/patches/ghc-diff-swap-cover-args.patch

@@ -0,0 +1,20 @@
+The QuickCheck library swapped the order of the arguments of the 'cover'
+function in version 2.12.  Version 0.3.4 of the Diff library still uses
+the old argument order.  Swapping the argument order makes Diff work
+with newer versions of QuickCheck.
+
+See <https://github.com/commercialhaskell/stackage/issues/4289> for the
+upstream bug report.
+
+diff -ruN a/test/Test.hs b/test/Test.hs
+--- a/test/Test.hs	2016-04-23 01:21:45.000000000 -0400
++++ b/test/Test.hs	2019-11-01 19:13:04.590770903 -0400
+@@ -134,7 +134,7 @@
+ prop_ppDiffR (DiffInput le ri) =
+     let haskDiff=ppDiff $ getGroupedDiff le ri
+         utilDiff= unsafePerformIO (runDiff (unlines le) (unlines ri))
+-    in  cover (haskDiff == utilDiff) 90 "exact match" $
++    in  cover 90 (haskDiff == utilDiff) "exact match" $
+                 classify (haskDiff == utilDiff) "exact match"
+                         (div ((length haskDiff)*100) (length utilDiff) < 110) -- less than 10% bigger
+     where