Setup.hs: rewrite so as not to use process, directory, filepath.

Using anything outside base is dangerous, since older
versions of ghc will link against two different versions.

See e.g.
- https://groups.google.com/forum/#!topic/pandoc-discuss/0r9Hhl730LY
- https://www.reddit.com/r/haskell/comments/3634x2/cabal_is_giving_a_weird_error_when_attempting_to/
- jaspervdj/hakyll#356

Setup.hs

@@ -20,17 +20,14 @@ import Distribution.Simple
 import Distribution.Simple.PreProcess
 import Distribution.Simple.Setup (ConfigFlags(..))
 import Distribution.PackageDescription (PackageDescription(..), FlagName(..))
-import System.Process ( rawSystem )
-import System.FilePath ( (</>) )
-import System.Directory ( findExecutable )
+import Distribution.Simple.Utils ( rawSystemExitCode, findProgramVersion )
+import System.Exit
 import Distribution.Verbosity ( Verbosity )
 import Distribution.Simple.Utils (info, notice, installOrdinaryFiles)
 import Distribution.Simple.Setup
 import Distribution.Simple.Program (simpleProgram, Program(..))
 import Distribution.Simple.LocalBuildInfo
 import Data.Version
-import System.Process (readProcess)
-import Text.ParserCombinators.ReadP (readP_to_S, skipSpaces, eof)
 import Control.Monad (when)
 import qualified Control.Exception as E
 
@@ -39,23 +36,11 @@ main = defaultMainWithHooks $ simpleUserHooks {
       -- enable hsb2hs preprocessor for .hsb files
       hookedPreProcessors = [ppBlobSuffixHandler]
     , hookedPrograms = [(simpleProgram "hsb2hs"){
-                           programFindVersion = findHsb2hsVersion }]
+                           programFindVersion = \verbosity fp ->
+                             findProgramVersion "--version" id verbosity fp }]
     , postCopy = installManPage
     }
 
-findHsb2hsVersion :: Verbosity -> FilePath -> IO (Maybe Version)
-findHsb2hsVersion verb fp = do
-  let handleExitFailure :: IOError -> IO (Maybe Version)
-      handleExitFailure _ = return Nothing
-  E.handle handleExitFailure $ do
-    outp <- readProcess fp ["--version"] ""
-    case readP_to_S (do v <- parseVersion
-                        skipSpaces
-                        eof
-                        return v) outp of
-         ((v,""):_) -> return (Just v)
-         _          -> return Nothing
-
 ppBlobSuffixHandler :: PPSuffixHandler
 ppBlobSuffixHandler = ("hsb", \_ lbi ->
   PreProcessor {
@@ -67,11 +52,11 @@ ppBlobSuffixHandler = ("hsb", \_ lbi ->
                               _         -> False
          when embedData $
             do info verbosity $ "Preprocessing " ++ infile ++ " to " ++ outfile
-               hsb2hsPath <- findExecutable "hsb2hs"
-               case hsb2hsPath of
-                    Just p  -> rawSystem p [infile, infile, outfile]
-                    Nothing -> error "hsb2hs is needed to build this program: cabal install hsb2hs"
-               return ()
+               ec <- rawSystemExitCode verbosity "hsb2hs"
+                          [infile, infile, outfile]
+               case ec of
+                    ExitSuccess   -> return ()
+                    ExitFailure _ -> error "hsb2hs is needed to build this program"
   })
 
 installManPage :: Args -> CopyFlags
@@ -80,6 +65,6 @@ installManPage _ flags pkg lbi = do
   let verbosity = fromFlag (copyVerbosity flags)
   let copydest  = fromFlag (copyDest flags)
   let mandest   = mandir (absoluteInstallDirs pkg lbi copydest)
-                     </> "man1"
+                     ++ "/man1"
   notice verbosity $ "Copying man page to " ++ mandest
   installOrdinaryFiles verbosity mandest [("man", "pandoc.1")]