keep first occurence when removing duplicates

src/Language/Fortran/Analysis/ModGraph.hs

@@ -99,8 +99,13 @@ genModGraph mversion includeDirs cppOpts paths = do
  pure ()
  execStateT (mapM_ iter (removeDuplicates paths)) modGraph0
 
+-- Remove duplicates from a list preserving the left most occurrence.
 removeDuplicates :: Eq a => [a] -> [a]
-removeDuplicates = foldl (\ acc x -> if x `elem` acc then acc else x : acc) []
+removeDuplicates [] = []
+removeDuplicates (x:xs) =
+ if x `elem` xs
+ then x : removeDuplicates (filter (/= x) xs)
+ else x : removeDuplicates xs
 
 modGraphToDOT :: ModGraph -> String
 modGraphToDOT ModGraph { mgGraph = gr } = unlines dot