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+-- | This module provides the HsValueMap type, which can structurally map a
+--   Haskell value to something else.
+module HsValueMap where
+
+import qualified Type
+import qualified TyCon
+import Control.Applicative
+import Data.Traversable
+import Data.Foldable
+
+-- | A datatype that maps each of the single values in a haskell structure to
+-- a mapto. The map has the same structure as the haskell type mapped, ie
+-- nested tuples etc.
+data HsValueMap mapto =
+  Tuple [HsValueMap mapto]
+  | Single mapto
+  deriving (Show, Eq, Ord)
+
+instance Functor HsValueMap where
+  fmap f (Single s) = Single (f s)
+  fmap f (Tuple maps) = Tuple (map (fmap f) maps)
+
+instance Foldable HsValueMap where
+  foldMap f (Single s) = f s
+  -- The first foldMap folds a list of HsValueMaps, the second foldMap folds
+  -- each of the HsValueMaps in that list
+  foldMap f (Tuple maps) = foldMap (foldMap f) maps
+
+instance Traversable HsValueMap where
+  traverse f (Single s) = Single <$> f s
+  traverse f (Tuple maps) = Tuple <$> (traverse (traverse f) maps)
+
+data PassState s x = PassState (s -> (s, x))
+
+instance Functor (PassState s) where
+  fmap f (PassState a) = PassState (\s -> let (s', a') = a s in (s', f a'))
+
+instance Applicative (PassState s) where
+  pure x = PassState (\s -> (s, x))
+  PassState f <*> PassState x = PassState (\s -> let (s', f') = f s; (s'', x') = x s' in (s'', f' x'))
+
+-- | Creates a HsValueMap with the same structure as the given type, using the
+--   given function for mapping the single types.
+mkHsValueMap ::
+  Type.Type                         -- ^ The type to map to a HsValueMap
+  -> HsValueMap Type.Type           -- ^ The resulting map and state
+
+mkHsValueMap ty =
+  case Type.splitTyConApp_maybe ty of
+    Just (tycon, args) ->
+      if (TyCon.isTupleTyCon tycon) 
+        then
+          Tuple (map mkHsValueMap args)
+        else
+          Single ty
+    Nothing -> Single ty