diff --git a/shake-ats.cabal b/shake-ats.cabal
--- a/shake-ats.cabal
+++ b/shake-ats.cabal
@@ -1,6 +1,6 @@
 cabal-version: 1.18
 name: shake-ats
-version: 1.9.0.0
+version: 1.9.0.1
 license: BSD3
 license-file: LICENSE
 copyright: Copyright: (c) 2018 Vanessa McHale
diff --git a/src/Development/Shake/ATS.hs b/src/Development/Shake/ATS.hs
--- a/src/Development/Shake/ATS.hs
+++ b/src/Development/Shake/ATS.hs
@@ -33,7 +33,7 @@
 import           Control.Monad.IO.Class
 import           Data.Bool                         (bool)
 import           Data.Either                       (fromRight)
-import           Data.Foldable                     (fold)
+import           Data.Foldable
 import           Data.Maybe                        (fromMaybe)
 import           Data.Semigroup                    (Semigroup (..))
 import qualified Data.Text.Lazy                    as TL
@@ -45,7 +45,6 @@
 import           Development.Shake.FilePath
 import           Development.Shake.Version
 import           Language.ATS
--- import           Lens.Micro
 import           System.Directory                  (copyFile, createDirectoryIfMissing, doesFileExist)
 import           System.Environment                (getEnv)
 import           System.Exit                       (ExitCode (ExitSuccess))
@@ -125,7 +124,7 @@
 patsEnv :: ATSToolConfig -> FilePath -> [CmdOption]
 patsEnv cfg path = EchoStderr False :
     zipWith AddEnv
-        ["PATSHOME", "PATH", "PATSHOMELOCS", "LIBGMP"]
+        ["PATSHOME", "PATH", "PATSHOMELOCS"]
         [_patsHome cfg, _patsHome cfg </> "bin:" ++ path, _patsHomeLocs cfg ]
 
 atsToC :: FilePath -> FilePath
@@ -156,11 +155,11 @@
 atsBin :: ATSTarget -> Rules ()
 atsBin ATSTarget{..} = do
 
-    mapM_ satsGen _linkTargets
+    traverse_ satsGen _linkTargets
 
-    mapM_ hsAts _genTargets
+    traverse_ hsAts _genTargets
 
-    mapM_ (cabalForeign (_cc _toolConfig)) _hsLibs
+    traverse_ (cabalForeign (_cc _toolConfig)) _hsLibs
 
     let cTargets = atsToC <$> _src
 
@@ -175,7 +174,10 @@
     cconfig' <- cconfig _toolConfig _libs _gc (makeCFlags _cFlags mempty (pure undefined) _gc)
 
     let atsGen = (hatsFile <$> _linkTargets) <> (_atsTarget <$> _genTargets)
+        -- FIXME the generated C should not depend on the C compiler but the
+        -- build artifacts should
         atsExtras = _otherDeps <> (TL.unpack . objectFile <$> _hsLibs)
+
     zipWithM_ (cgen _toolConfig atsExtras atsGen) _src cTargets
 
     doLib _tgtType (zipWithM_ (objectFileR (_cc _toolConfig) cconfig') cTargets (h' cTargets))
diff --git a/src/Development/Shake/ATS/Rules.hs b/src/Development/Shake/ATS/Rules.hs
--- a/src/Development/Shake/ATS/Rules.hs
+++ b/src/Development/Shake/ATS/Rules.hs
@@ -7,6 +7,7 @@
                                    ) where
 
 import           Control.Monad
+import           Data.Foldable
 import           Data.Semigroup                 (Semigroup (..))
 import qualified Data.Text.Lazy                 as TL
 import           Development.Shake              hiding (doesDirectoryExist)
@@ -35,7 +36,7 @@
     link %> \out -> liftIO $ do
         contents <- readFile dats
         let proc = generateLinks contents
-        writeFile out (either undefined id proc)
+        writeFile out (either undefined id proc) -- FIXME better error
 
 -- | Get subdirectories recursively.
 getSubdirs :: FilePath -> IO [FilePath]
@@ -45,12 +46,12 @@
         [] -> pure []
         xs -> do
             ds' <- filterM doesDirectoryExist ((p </>) <$> xs)
-            ss <- mapM getSubdirs ds'
-            pure $ ds' <> join ss
+            ss <- traverse getSubdirs ds'
+            pure $ ds' <> fold ss
 
 -- | These rules take a @.cabal@ file and the @.o@ file to be produced from
 -- them, building the @.o@ file.
-cabalForeign :: CCompiler -> ForeignCabal -> Rules ()
+cabalForeign :: CCompiler -> ForeignCabal -> Rules () -- TODO HsCompiler?
 cabalForeign (GHC _ suff) (ForeignCabal cbp' cf' obf') = do
 
     let cf = TL.unpack cf'
@@ -65,10 +66,9 @@
         ghcV' <- quietly ghcVersion
         let ghcV = maybe ghcV' (drop 1) suff
 
-        need (cf : fmap ((obfDir <> "/") <>) trDeps)
+        need (cf : fmap ((obfDir <> [pathSeparator]) <>) trDeps)
         command_ [Cwd obfDir] "cabal" ["new-build", "all", "-w", "ghc-" ++ ghcV]
 
-        -- TODO move this to the @shake-ext@ package?
         let subdir = takeDirectory cbp
             correctDir = (== "build")
             endsBuild = correctDir . last . splitPath
@@ -80,7 +80,7 @@
 
         let hdr = dropExtension obj ++ "_stub.h"
         liftIO $ copyFile hdr (takeDirectory out </> takeFileName hdr)
-cabalForeign _ _ = mempty
+cabalForeign _ _ = mempty -- FXIME error here?
 
 -- | Build a @.lats@ file using @atslex@.
 atsLex :: FilePath -- ^ Filepath of @.lats@ file