| base-3.0.0.0: Basic libraries | Contents | Index

Data.Typeable |

Portability | portable
Stability | experimental
Maintainer | libraries@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
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 :: (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.

Instances

Typeable ArithException
Typeable ArrayException
Typeable AsyncException
Typeable Bool
Typeable CDev
Typeable CIno
Typeable CMode
Typeable COff
Typeable CPid
Typeable CSsize
Typeable Char
Typeable ConsoleEvent
Typeable DataType
Typeable Double
Typeable Dynamic
Typeable Exception
Typeable Fd
Typeable Float
Typeable Handle
Typeable IOException
Typeable Int
Typeable Int16
Typeable Int32
Typeable Int64
Typeable Int8
Typeable IntPtr
Typeable Integer
Typeable Ordering
Typeable QSem
Typeable QSemN
Typeable RealWorld
Typeable ThreadId
Typeable Timeout
Typeable TyCon
Typeable TypeRep
Typeable Version
Typeable Word
Typeable Word16
Typeable Word32
Typeable Word64
Typeable Word8
Typeable WordPtr
Typeable ()
(Typeable1 s, Typeable a) => Typeable (s a)

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.
Instances

Data TypeRep
Eq TypeRep
Show TypeRep
Typeable TypeRep

data TyCon

An abstract representation of a type constructor. `TyCon` objects can be built using `mkTyCon`.
Instances

Data TyCon
Eq TyCon
Show TyCon
Typeable TyCon

showsTypeRep :: TypeRep -> ShowS
Construction of type representations
mkTyCon

:: String | the name of the type constructor (should be unique in the program, so it might be wise to use the fully qualified name).
-> TyCon | A 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

Instances

Typeable1 Chan
Typeable1 Complex
Typeable1 ForeignPtr
Typeable1 FunPtr
Typeable1 IO
Typeable1 IORef
Typeable1 MVar
Typeable1 Maybe
Typeable1 Ptr
Typeable1 Ratio
Typeable1 STM
Typeable1 StableName
Typeable1 StablePtr
Typeable1 TVar
Typeable1 Weak
Typeable1 []
(Typeable2 s, Typeable a) => Typeable1 (s a)

class Typeable2 t where

Variant for binary type constructors
Methods

typeOf2 :: t a b -> TypeRep

Instances

Typeable2 Array
Typeable2 Either
Typeable2 ST
Typeable2 STRef
Typeable2 (,)
Typeable2 (->)
(Typeable3 s, Typeable a) => Typeable2 (s a)

class Typeable3 t where

Variant for 3-ary type constructors
Methods

typeOf3 :: t a b c -> TypeRep

Instances

Typeable3 STArray
Typeable3 (,,)
(Typeable4 s, Typeable a) => Typeable3 (s a)

class Typeable4 t where

Variant for 4-ary type constructors
Methods

typeOf4 :: t a b c d -> TypeRep

Instances

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

Instances

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

Instances

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

Instances

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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