Add a hack to support both Cabal-1.16 and Cabal-1.18 Setup.hs script …

…(fixed number of arguments for programFindLocation). Taken from hdbc-postgresql commit e9b2fbab07b8f55ae6a9e120ab0b98c433842a8b.

Setup.lhs

@@ -1,6 +1,7 @@
 #!/usr/bin/env runhaskell
 
 \begin{code}
+{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-}
 import Distribution.Simple
 import Distribution.PackageDescription
 import Distribution.Version
@@ -26,8 +27,24 @@ main = defaultMainWithHooks simpleUserHooks {
     }
 }
 
+
+-- 'ConstOrId' is a @Cabal-1.16@ vs @Cabal-1.18@ compatibility hack,
+-- 'programFindLocation' has a new (unused in this case)
+-- parameter. 'ConstOrId' adds this parameter when types say it is
+-- mandatory.
+
+class ConstOrId a b where
+    constOrId :: a -> b
+
+instance ConstOrId a a where
+    constOrId = id
+
+instance ConstOrId a (b -> a) where
+    constOrId = const
+
+
 mysqlConfigProgram = (simpleProgram "mysql_config") {
-    programFindLocation = \verbosity -> do
+    programFindLocation = \verbosity -> constOrId $ do
       mysql_config  <- findProgramOnPath "mysql_config"  verbosity
       mysql_config5 <- findProgramOnPath "mysql_config5" verbosity
       return (mysql_config `mplus` mysql_config5)
@@ -55,5 +72,4 @@ split xs cs = split' $ dropWhile (`elem` xs) cs
               let (run, cs1) = span (`notElem` xs) cs0
                   cs2        = dropWhile (`elem` xs) cs1
               in run:(split' cs2)
-
 \end{code}