use CPP to compile on older version of GHCs

Author: damhiya

src/DFAMin.hs

@@ -1,5 +1,6 @@
 {-# OPTIONS_GHC -fno-warn-name-shadowing #-}
 
+{-# LANGUAGE CPP                 #-}
 {-# LANGUAGE PatternGuards       #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TupleSections       #-}
@@ -17,6 +18,12 @@ import qualified Data.IntMap as IM
 import qualified Data.List as List
 import Data.Foldable (foldl')
 
+#if __GLASGOW_HASKELL__ >= 802
+restrictKeys = IM.restrictKeys
+#else
+restrictKeys m s = IM.filterWithKey (\k _ -> IS.member k s) m
+#endif
+
 -- Hopcroft's Algorithm for DFA minimization (cut/pasted from Wikipedia):
 
 -- P := {{all accepting states}, {all nonaccepting states}};
@@ -155,7 +162,7 @@ groupEquivStates DFA { dfa_states = statemap }
       where
         xs :: [EquivalenceClass]
         xs = filter (not . IS.null)
-           . map (foldl' IS.union IS.empty . flip IM.restrictKeys a)
+           . map (foldl' IS.union IS.empty . flip restrictKeys a)
            $ bigmap
 
         go0 (r,q) x = go1 r [] []