base-4.2.0.1: Basic librariesSource 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.Data 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
showsTypeRep :: TypeRep -> ShowS
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 :: forall t a. (Typeable1 t, Typeable a) => t a -> TypeRep
typeOf1Default :: forall t a b. (Typeable2 t, Typeable a) => t a b -> TypeRep
typeOf2Default :: forall t a b c. (Typeable3 t, Typeable a) => t a b c -> TypeRep
typeOf3Default :: forall t a b c d. (Typeable4 t, Typeable a) => t a b c d -> TypeRep
typeOf4Default :: forall t a b c d e. (Typeable5 t, Typeable a) => t a b c d e -> TypeRep
typeOf5Default :: forall t a b c d e f. (Typeable6 t, Typeable a) => t a b c d e f -> TypeRep
typeOf6Default :: forall t a b c d e f g. (Typeable7 t, Typeable a) => t a b c d e f g -> TypeRep
The Typeable class
class Typeable a whereSource
The class Typeable allows a concrete representation of a type to be calculated.
Methods
typeOf :: a -> TypeRepSource
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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Typeable Bool
Typeable Char
Typeable Double
Typeable Float
Typeable Int
Typeable Int8
Typeable Int16
Typeable Int32
Typeable Int64
Typeable Integer
Typeable Ordering
Typeable RealWorld
Typeable Word
Typeable Word8
Typeable Word16
Typeable Word32
Typeable Word64
Typeable ()
Typeable TyCon
Typeable TypeRep
Typeable ArithException
Typeable ErrorCall
Typeable SomeException
Typeable IOException
Typeable CUIntMax
Typeable CIntMax
Typeable CUIntPtr
Typeable CIntPtr
Typeable CTime
Typeable CClock
Typeable CSigAtomic
Typeable CWchar
Typeable CSize
Typeable CPtrdiff
Typeable CDouble
Typeable CFloat
Typeable CULLong
Typeable CLLong
Typeable CULong
Typeable CLong
Typeable CUInt
Typeable CInt
Typeable CUShort
Typeable CShort
Typeable CUChar
Typeable CSChar
Typeable CChar
Typeable Dynamic
Typeable IntPtr
Typeable WordPtr
Typeable Handle__
Typeable Handle
Typeable ExitCode
Typeable ArrayException
Typeable AsyncException
Typeable AssertionFailed
Typeable Deadlock
Typeable BlockedIndefinitelyOnSTM
Typeable BlockedIndefinitelyOnMVar
Typeable BlockedIndefinitely
Typeable BlockedOnDeadMVar
Typeable Fd
Typeable CRLim
Typeable CTcflag
Typeable CSpeed
Typeable CCc
Typeable CUid
Typeable CNlink
Typeable CGid
Typeable CSsize
Typeable CPid
Typeable COff
Typeable CMode
Typeable CIno
Typeable CDev
Typeable ThreadId
Typeable NestedAtomically
Typeable NonTermination
Typeable NoMethodError
Typeable RecUpdError
Typeable RecConError
Typeable RecSelError
Typeable PatternMatchFail
Typeable FD
Typeable QSem
Typeable QSemN
Typeable Timeout
Typeable E12
Typeable E9
Typeable E6
Typeable E3
Typeable E2
Typeable E1
Typeable E0
Typeable Version
Typeable Exception
(Typeable1 s, Typeable a) => Typeable (s a)
Type-safe cast
cast :: (Typeable a, Typeable b) => a -> Maybe bSource
The type-safe cast operation
gcast :: (Typeable a, Typeable b) => c a -> Maybe (c b)Source
A flexible variation parameterised in a type constructor
Type representations
data TypeRep Source
A concrete representation of a (monomorphic) type. TypeRep supports reasonably efficient equality.
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data TyCon Source
An abstract representation of a type constructor. TyCon objects can be built using mkTyCon.
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showsTypeRep :: TypeRep -> ShowSSource
Construction of type representations
mkTyConSource
:: 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] -> TypeRepSource
Applies a type constructor to a sequence of types
mkAppTy :: TypeRep -> TypeRep -> TypeRepSource
Adds a TypeRep argument to a TypeRep.
mkFunTy :: TypeRep -> TypeRep -> TypeRepSource
A special case of mkTyConApp, which applies the function type constructor to a pair of types.
Observation of type representations
splitTyConApp :: TypeRep -> (TyCon, [TypeRep])Source
Splits a type constructor application
funResultTy :: TypeRep -> TypeRep -> Maybe TypeRepSource
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 -> TyConSource
Observe the type constructor of a type representation
typeRepArgs :: TypeRep -> [TypeRep]Source
Observe the argument types of a type representation
tyConString :: TyCon -> StringSource
Observe string encoding of a type representation
typeRepKey :: TypeRep -> IO IntSource

Returns a unique integer associated with a TypeRep. This can be used for making a mapping 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 whereSource
Variant for unary type constructors
Methods
typeOf1 :: t a -> TypeRepSource
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class Typeable2 t whereSource
Variant for binary type constructors
Methods
typeOf2 :: t a b -> TypeRepSource
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class Typeable3 t whereSource
Variant for 3-ary type constructors
Methods
typeOf3 :: t a b c -> TypeRepSource
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class Typeable4 t whereSource
Variant for 4-ary type constructors
Methods
typeOf4 :: t a b c d -> TypeRepSource
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class Typeable5 t whereSource
Variant for 5-ary type constructors
Methods
typeOf5 :: t a b c d e -> TypeRepSource
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class Typeable6 t whereSource
Variant for 6-ary type constructors
Methods
typeOf6 :: t a b c d e f -> TypeRepSource
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class Typeable7 t whereSource
Variant for 7-ary type constructors
Methods
typeOf7 :: t a b c d e f g -> TypeRepSource
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gcast1 :: (Typeable1 t, Typeable1 t') => c (t a) -> Maybe (c (t' a))Source
Cast for * -> *
gcast2 :: (Typeable2 t, Typeable2 t') => c (t a b) -> Maybe (c (t' a b))Source
Cast for * -> * -> *
Default instances
Note: These are not needed by GHC, for which these instances are generated by general instance declarations.
typeOfDefault :: forall t a. (Typeable1 t, Typeable a) => t a -> TypeRepSource
For defining a Typeable instance from any Typeable1 instance.
typeOf1Default :: forall t a b. (Typeable2 t, Typeable a) => t a b -> TypeRepSource
For defining a Typeable1 instance from any Typeable2 instance.
typeOf2Default :: forall t a b c. (Typeable3 t, Typeable a) => t a b c -> TypeRepSource
For defining a Typeable2 instance from any Typeable3 instance.
typeOf3Default :: forall t a b c d. (Typeable4 t, Typeable a) => t a b c d -> TypeRepSource
For defining a Typeable3 instance from any Typeable4 instance.
typeOf4Default :: forall t a b c d e. (Typeable5 t, Typeable a) => t a b c d e -> TypeRepSource
For defining a Typeable4 instance from any Typeable5 instance.
typeOf5Default :: forall t a b c d e f. (Typeable6 t, Typeable a) => t a b c d e f -> TypeRepSource
For defining a Typeable5 instance from any Typeable6 instance.
typeOf6Default :: forall t a b c d e f g. (Typeable7 t, Typeable a) => t a b c d e f g -> TypeRepSource
For defining a Typeable6 instance from any Typeable7 instance.
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