Haskell Hierarchical Libraries (base package)Source codeContentsIndex
Data.Typeable
Portabilityportable
Stabilityexperimental
Maintainerlibraries@haskell.org
Contents
The Typeable class
Type-safe cast
Type representations
Construction of type representations
Observation of type representations
The other Typeable classes
Default instances
Description
The Typeable class reifies types to some extent by associating type representations to types. These type representations can be compared, and one can in turn define a type-safe cast operation. To this end, an unsafe cast is guarded by a test for type (representation) equivalence. The module Data.Dynamic uses Typeable for an implementation of dynamics. The module Data.Generics uses Typeable and type-safe cast (but not dynamics) to support the "Scrap your boilerplate" style of generic programming.
Synopsis
class Typeable a where
typeOf :: a -> TypeRep
cast :: (Typeable a, Typeable b) => a -> Maybe b
gcast :: (Typeable a, Typeable b) => c a -> Maybe (c b)
data TypeRep
data TyCon
mkTyCon :: String -> TyCon
mkTyConApp :: TyCon -> [TypeRep] -> TypeRep
mkAppTy :: TypeRep -> TypeRep -> TypeRep
mkFunTy :: TypeRep -> TypeRep -> TypeRep
splitTyConApp :: TypeRep -> (TyCon, [TypeRep])
funResultTy :: TypeRep -> TypeRep -> Maybe TypeRep
typeRepTyCon :: TypeRep -> TyCon
typeRepArgs :: TypeRep -> [TypeRep]
tyConString :: TyCon -> String
typeRepKey :: TypeRep -> IO Int
class Typeable1 t where
typeOf1 :: t a -> TypeRep
class Typeable2 t where
typeOf2 :: t a b -> TypeRep
class Typeable3 t where
typeOf3 :: t a b c -> TypeRep
class Typeable4 t where
typeOf4 :: t a b c d -> TypeRep
class Typeable5 t where
typeOf5 :: t a b c d e -> TypeRep
class Typeable6 t where
typeOf6 :: t a b c d e f -> TypeRep
class Typeable7 t where
typeOf7 :: t a b c d e f g -> TypeRep
gcast1 :: (Typeable1 t, Typeable1 t') => c (t a) -> Maybe (c (t' a))
gcast2 :: (Typeable2 t, Typeable2 t') => c (t a b) -> Maybe (c (t' a b))
typeOfDefault :: (Typeable1 t, Typeable a) => t a -> TypeRep
typeOf1Default :: (Typeable2 t, Typeable a) => t a b -> TypeRep
typeOf2Default :: (Typeable3 t, Typeable a) => t a b c -> TypeRep
typeOf3Default :: (Typeable4 t, Typeable a) => t a b c d -> TypeRep
typeOf4Default :: (Typeable5 t, Typeable a) => t a b c d e -> TypeRep
typeOf5Default :: (Typeable6 t, Typeable a) => t a b c d e f -> TypeRep
typeOf6Default :: (Typeable7 t, Typeable a) => t a b c d e f g -> TypeRep
The Typeable class
class Typeable a where
The class Typeable allows a concrete representation of a type to be calculated.
Methods
typeOf :: a -> TypeRep
Takes a value of type a and returns a concrete representation of that type. The value of the argument should be ignored by any instance of Typeable, so that it is safe to pass undefined as the argument.
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Type-safe cast
cast :: (Typeable a, Typeable b) => a -> Maybe b
The type-safe cast operation
gcast :: (Typeable a, Typeable b) => c a -> Maybe (c b)
A flexible variation parameterised in a type constructor
Type representations
data TypeRep
A concrete representation of a (monomorphic) type. TypeRep supports reasonably efficient equality.
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data TyCon
An abstract representation of a type constructor. TyCon objects can be built using mkTyCon.
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Construction of type representations
mkTyCon
:: Stringthe name of the type constructor (should be unique in the program, so it might be wise to use the fully qualified name).
-> TyConA unique TyCon object

Builds a TyCon object representing a type constructor. An implementation of Data.Typeable should ensure that the following holds: 

  mkTyCon "a" == mkTyCon "a"
mkTyConApp :: TyCon -> [TypeRep] -> TypeRep
Applies a type constructor to a sequence of types
mkAppTy :: TypeRep -> TypeRep -> TypeRep
Adds a TypeRep argument to a TypeRep.
mkFunTy :: TypeRep -> TypeRep -> TypeRep
A special case of mkTyConApp, which applies the function type constructor to a pair of types.
Observation of type representations
splitTyConApp :: TypeRep -> (TyCon, [TypeRep])
Splits a type constructor application
funResultTy :: TypeRep -> TypeRep -> Maybe TypeRep
Applies a type to a function type. Returns: Just u if the first argument represents a function of type t -> u and the second argument represents a function of type t. Otherwise, returns Nothing.
typeRepTyCon :: TypeRep -> TyCon
Observe the type constructor of a type representation
typeRepArgs :: TypeRep -> [TypeRep]
Observe the argument types of a type representation
tyConString :: TyCon -> String
Observe string encoding of a type representation
typeRepKey :: TypeRep -> IO Int

Returns a unique integer associated with a TypeRep. This can be used for making a mapping (IntMap) with TypeReps as the keys, for example. It is guaranteed that t1 == t2 if and only if typeRepKey t1 == typeRepKey t2.

It is in the IO monad because the actual value of the key may vary from run to run of the program. You should only rely on the equality property, not any actual key value. The relative ordering of keys has no meaning either.

The other Typeable classes
Note: The general instances are provided for GHC only.
class Typeable1 t where
Variant for unary type constructors
Methods
typeOf1 :: t a -> TypeRep
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class Typeable2 t where
Variant for binary type constructors
Methods
typeOf2 :: t a b -> TypeRep
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class Typeable3 t where
Variant for 3-ary type constructors
Methods
typeOf3 :: t a b c -> TypeRep
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class Typeable4 t where
Variant for 4-ary type constructors
Methods
typeOf4 :: t a b c d -> TypeRep
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Typeable4 (,,,)
(Typeable5 s, Typeable a) => Typeable4 (s a)
class Typeable5 t where
Variant for 5-ary type constructors
Methods
typeOf5 :: t a b c d e -> TypeRep
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Typeable5 (,,,,)
(Typeable6 s, Typeable a) => Typeable5 (s a)
class Typeable6 t where
Variant for 6-ary type constructors
Methods
typeOf6 :: t a b c d e f -> TypeRep
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Typeable6 (,,,,,)
(Typeable7 s, Typeable a) => Typeable6 (s a)
class Typeable7 t where
Variant for 7-ary type constructors
Methods
typeOf7 :: t a b c d e f g -> TypeRep
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Typeable7 (,,,,,,)
gcast1 :: (Typeable1 t, Typeable1 t') => c (t a) -> Maybe (c (t' a))
Cast for * -> *
gcast2 :: (Typeable2 t, Typeable2 t') => c (t a b) -> Maybe (c (t' a b))
Cast for * -> * -> *
Default instances
Note: These are not needed by GHC, for which these instances are generated by general instance declarations.
typeOfDefault :: (Typeable1 t, Typeable a) => t a -> TypeRep
For defining a Typeable instance from any Typeable1 instance.
typeOf1Default :: (Typeable2 t, Typeable a) => t a b -> TypeRep
For defining a Typeable1 instance from any Typeable2 instance.
typeOf2Default :: (Typeable3 t, Typeable a) => t a b c -> TypeRep
For defining a Typeable2 instance from any Typeable3 instance.
typeOf3Default :: (Typeable4 t, Typeable a) => t a b c d -> TypeRep
For defining a Typeable3 instance from any Typeable4 instance.
typeOf4Default :: (Typeable5 t, Typeable a) => t a b c d e -> TypeRep
For defining a Typeable4 instance from any Typeable5 instance.
typeOf5Default :: (Typeable6 t, Typeable a) => t a b c d e f -> TypeRep
For defining a Typeable5 instance from any Typeable6 instance.
typeOf6Default :: (Typeable7 t, Typeable a) => t a b c d e f g -> TypeRep
For defining a Typeable6 instance from any Typeable7 instance.
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