Structure.hs

{-# LANGUAGE CPP, Rank2Types #-}

-- it seems that older GHCs do not like multiple LANGUAGE pragmas?
-- {- LANGUAGE CPP -}
-- ifdef __GLASGOW_HASKELL__
-- {- LANGUAGE Rank2Types -}
-- endif

-- | Changing the structure of a tree.
module Data.Generics.Fixplate.Structure 
  ( NatTrafo
  , restructure
  , liftAnn
  )
  where
  
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import Data.Generics.Fixplate.Base

--------------------------------------------------------------------------------

-- #ifdef __GLASGOW_HASKELL__

-- | The type of natural transformations.
type NatTrafo f g = forall a. (f a -> g a)

-- | Changing the structure of a tree.
restructure :: Functor f => NatTrafo f g -> Mu f -> Mu g
restructure trafo = go where
  go = Fix . trafo . fmap go . unFix

-- #else  

{-
data NatTrafo f g = NatTrafo (f Int) (g Int)   -- fake and opaque data type

-- | Unfortunately, this function requires Rank2Types,
-- thus we only provide it for GHC.
restructure :: Functor f => NatTrafo f g -> Mu f -> Mu g
restructure = error "restructure: this operation requires Rank2Types"
-}

-- #endif
  
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-- | Lifting natural transformations to annotations.
liftAnn :: (f e -> g e) -> Ann f a e -> Ann g a e
liftAnn trafo (Ann a x) = Ann a (trafo x)

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