scaffolding ready for s9pks

src/Util/Function.hs

@@ -6,4 +6,18 @@ import           Startlude
 (.*) = (.) . (.)
 
 (.**) :: (b -> c) -> (a0 -> a1 -> a2 -> b) -> a0 -> a1 -> a2 -> c
-(.**) = (.) . (.*)
+(.**) = (.) . (.*)
+
+preimage :: Eq b => (a -> b) -> b -> [a] -> [a]
+preimage f target = filter ((== target) . f)
+
+mapFind :: ([a] -> Maybe a) -> (b -> a) -> [b] -> Maybe b
+mapFind _ _ [] = Nothing
+mapFind finder mapping (b:bs) = let
+        mB = mapFind finder mapping bs
+        mA = finder [mapping b]
+    in case (mB, mA) of
+        (Just b',_)       -> Just b'
+        (Nothing, Just _) -> Just b
+        _                 -> Nothing
+