Portability	portable
Stability	experimental
Maintainer	libraries@haskell.org

Data.Typeable

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

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.

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.

data TyCon

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

showsTypeRep :: TypeRep -> ShowS

Construction of type representations

mkTyCon

Arguments

:: 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 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

class Typeable2 t where

Variant for binary type constructors

Methods

typeOf2 :: t a b -> TypeRep

class Typeable3 t where

Variant for 3-ary type constructors

Methods

typeOf3 :: t a b c -> TypeRep

class Typeable4 t where

Variant for 4-ary type constructors

Methods

typeOf4 :: t a b c d -> TypeRep

class Typeable5 t where

Variant for 5-ary type constructors

Methods

typeOf5 :: t a b c d e -> TypeRep

class Typeable6 t where

Variant for 6-ary type constructors

Methods

typeOf6 :: t a b c d e f -> TypeRep

class Typeable7 t where

Variant for 7-ary type constructors

Methods

typeOf7 :: t a b c d e f g -> TypeRep

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 :: forall t a. (Typeable1 t, Typeable a) => t a -> TypeRep

For defining a Typeable instance from any Typeable1 instance.

typeOf1Default :: forall t a b. (Typeable2 t, Typeable a) => t a b -> TypeRep

For defining a Typeable1 instance from any Typeable2 instance.

typeOf2Default :: forall t a b c. (Typeable3 t, Typeable a) => t a b c -> TypeRep

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 -> TypeRep

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 -> TypeRep

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 -> TypeRep

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 -> TypeRep

For defining a Typeable6 instance from any Typeable7 instance.