-----------------------------------------------------------------------------
-- |
-- Module : Data.Generics.Aliases
--- Copyright : (c) The University of Glasgow, CWI 2001--2003
+-- Copyright : (c) The University of Glasgow, CWI 2001--2004
-- License : BSD-style (see the file libraries/base/LICENSE)
--
-- Maintainer : libraries@haskell.org
-- Stability : experimental
--- Portability : non-portable
+-- Portability : non-portable (local universal quantification)
--
-- \"Scrap your boilerplate\" --- Generic programming in Haskell
-- See <http://www.cs.vu.nl/boilerplate/>. The present module provides
module Data.Generics.Aliases (
-- * Combinators to \"make\" generic functions via cast
- mkT, mkQ, mkM, mkMp, mkB,
- extT, extQ, extM, extMp, extB,
+ mkT, mkQ, mkM, mkMp, mkR,
+ ext0, extT, extQ, extM, extMp, extB, extR,
-- * Type synonyms for generic function types
GenericT,
GenericQ,
GenericM,
GenericB,
+ GenericR,
Generic,
Generic'(..),
+ GenericT'(..),
+ GenericQ'(..),
+ GenericM'(..),
-- * Inredients of generic functions
orElse,
recoverMp,
recoverQ,
choiceMp,
- choiceQ
+ choiceQ,
- ) where
+ -- * Type extension for unary type constructors
+ ext1T,
+ ext1M,
+ ext1Q,
+ ext1R
+ ) where
+#ifdef __HADDOCK__
+import Prelude
+#endif
import Control.Monad
import Data.Generics.Basics
-
-
------------------------------------------------------------------------------
--
-- Combinators to "make" generic functions
-- start from a type-specific case;
-- preserve the term otherwise
--
-mkT :: (Typeable a, Typeable b) => (b -> b) -> a -> a
-mkT f = case cast f of
- Just g -> g
- Nothing -> id
+mkT :: ( Typeable a
+ , Typeable b
+ )
+ => (b -> b)
+ -> a
+ -> a
+mkT = extT id
-- | Make a generic query;
-- start from a type-specific case;
-- return a constant otherwise
--
-mkQ :: (Typeable a, Typeable b) => r -> (b -> r) -> a -> r
+mkQ :: ( Typeable a
+ , Typeable b
+ )
+ => r
+ -> (b -> r)
+ -> a
+ -> r
(r `mkQ` br) a = case cast a of
Just b -> br b
Nothing -> r
-- start from a type-specific case;
-- resort to return otherwise
--
-mkM :: ( Monad m,
- Typeable a,
- Typeable b
+mkM :: ( Monad m
+ , Typeable a
+ , Typeable b
)
- => (b -> m b) -> a -> m a
-mkM f = case castarr f of
- Just g -> g
- Nothing -> return
+ => (b -> m b)
+ -> a
+ -> m a
+mkM = extM return
{-
-- | Make a generic monadic transformation for MonadPlus;
-- use \"const mzero\" (i.e., failure) instead of return as default.
--
-mkMp :: ( MonadPlus m,
- Typeable a,
- Typeable b
+mkMp :: ( MonadPlus m
+ , Typeable a
+ , Typeable b
)
- => (b -> m b) -> a -> m a
-mkMp = maybe (const mzero) id . castarr
+ => (b -> m b)
+ -> a
+ -> m a
+mkMp = extM (const mzero)
-- | Make a generic builder;
-- start from a type-specific ase;
-- resort to no build (i.e., mzero) otherwise
--
-mkB :: ( MonadPlus m,
- Typeable a,
- Typeable b
+mkR :: ( MonadPlus m
+ , Typeable a
+ , Typeable b
)
=> m b -> m a
-mkB = maybe mzero id . castss
+mkR f = mzero `extR` f
+
+
+-- | Flexible type extension
+ext0 :: (Typeable a, Typeable b) => c a -> c b -> c a
+ext0 def ext = maybe def id (gcast ext)
-- | Extend a generic transformation by a type-specific case
-extT :: (Typeable a, Typeable b) => (a -> a) -> (b -> b) -> a -> a
-extT f = maybe f id . cast
+extT :: ( Typeable a
+ , Typeable b
+ )
+ => (a -> a)
+ -> (b -> b)
+ -> a
+ -> a
+extT def ext = unT ((T def) `ext0` (T ext))
-- | Extend a generic query by a type-specific case
-extQ :: (Typeable a, Typeable b) => (a -> q) -> (b -> q) -> a -> q
+extQ :: ( Typeable a
+ , Typeable b
+ )
+ => (a -> q)
+ -> (b -> q)
+ -> a
+ -> q
extQ f g a = maybe (f a) g (cast a)
-- | Extend a generic monadic transformation by a type-specific case
-extM :: ( Monad m,
- Typeable a,
- Typeable b
+extM :: ( Monad m
+ , Typeable a
+ , Typeable b
)
=> (a -> m a) -> (b -> m b) -> a -> m a
-extM f = maybe f id . castarr
+extM def ext = unM ((M def) `ext0` (M ext))
-- | Extend a generic MonadPlus transformation by a type-specific case
-extMp :: ( MonadPlus m,
- Typeable a,
- Typeable b
+extMp :: ( MonadPlus m
+ , Typeable a
+ , Typeable b
)
=> (a -> m a) -> (b -> m b) -> a -> m a
extMp = extM
+-- | Extend a generic builder
+extB :: ( Typeable a
+ , Typeable b
+ )
+ => a -> b -> a
+extB a = maybe a id . cast
+
--- | Extend a generic builder by a type-specific case
-extB :: (Monad m,
- Typeable a,
- Typeable b
+-- | Extend a generic reader
+extR :: ( Monad m
+ , Typeable a
+ , Typeable b
)
=> m a -> m b -> m a
-extB f = maybe f id . castss
+extR def ext = unR ((R def) `ext0` (R ext))
+
------------------------------------------------------------------------------
type GenericM m = forall a. Data a => a -> m a
--- | Generic monadic builders with input i,
+-- | Generic builders
+-- i.e., produce an \"a\".
+--
+type GenericB = forall a. Data a => a
+
+
+-- | Generic readers, say monadic builders,
-- i.e., produce an \"a\" with the help of a monad \"m\".
--
-type GenericB m = forall a. Data a => m a
+type GenericR m = forall a. Data a => m a
-- | The general scheme underlying generic functions
-- assumed by gfoldl; there are isomorphisms such as
--- GenericT = Generic ID.
+-- GenericT = Generic T.
--
type Generic c = forall a. Data a => a -> c a
data Generic' c = Generic' { unGeneric' :: Generic c }
+-- | Other first-class polymorphic wrappers
+newtype GenericT' = GT { unGT :: Data a => a -> a }
+newtype GenericQ' r = GQ { unGQ :: GenericQ r }
+newtype GenericM' m = GM { unGM :: Data a => a -> m a }
+
-- | Left-biased choice on maybies
orElse :: Maybe a -> Maybe a -> Maybe a
-- | Recover from the failure of monadic query by a constant
recoverQ :: MonadPlus m => r -> GenericQ (m r) -> GenericQ (m r)
recoverQ r f = f `choiceQ` const (return r)
+
+
+
+------------------------------------------------------------------------------
+--
+-- Type extension for unary type constructors
+--
+------------------------------------------------------------------------------
+
+
+
+-- | Flexible type extension
+ext1 :: (Data a, Typeable1 t)
+ => c a
+ -> (forall a. Data a => c (t a))
+ -> c a
+ext1 def ext = maybe def id (dataCast1 ext)
+
+
+-- | Type extension of transformations for unary type constructors
+ext1T :: (Data d, Typeable1 t)
+ => (forall d. Data d => d -> d)
+ -> (forall d. Data d => t d -> t d)
+ -> d -> d
+ext1T def ext = unT ((T def) `ext1` (T ext))
+
+
+-- | Type extension of monadic transformations for type constructors
+ext1M :: (Monad m, Data d, Typeable1 t)
+ => (forall d. Data d => d -> m d)
+ -> (forall d. Data d => t d -> m (t d))
+ -> d -> m d
+ext1M def ext = unM ((M def) `ext1` (M ext))
+
+
+-- | Type extension of queries for type constructors
+ext1Q :: (Data d, Typeable1 t)
+ => (d -> q)
+ -> (forall d. Data d => t d -> q)
+ -> d -> q
+ext1Q def ext = unQ ((Q def) `ext1` (Q ext))
+
+
+-- | Type extension of readers for type constructors
+ext1R :: (Monad m, Data d, Typeable1 t)
+ => m d
+ -> (forall d. Data d => m (t d))
+ -> m d
+ext1R def ext = unR ((R def) `ext1` (R ext))
+
+
+
+------------------------------------------------------------------------------
+--
+-- Type constructors for type-level lambdas
+--
+------------------------------------------------------------------------------
+
+
+-- | The type constructor for transformations
+newtype T x = T { unT :: x -> x }
+
+-- | The type constructor for transformations
+newtype M m x = M { unM :: x -> m x }
+
+-- | The type constructor for queries
+newtype Q q x = Q { unQ :: x -> q }
+
+-- | The type constructor for readers
+newtype R m x = R { unR :: m x }
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