base-4.9.1.0: Basic libraries

Data.Typeable

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.

## Compatibility Notes

Since GHC 7.8, Typeable is poly-kinded. The changes required for this might break some old programs involving Typeable. More details on this, including how to fix your code, can be found on the PolyTypeable wiki page

Synopsis

# The Typeable class

class Typeable a #

The class Typeable allows a concrete representation of a type to be calculated.

Minimal complete definition

typeRep#

typeRep :: forall proxy a. Typeable a => proxy a -> TypeRep #

Takes a value of type a and returns a concrete representation of that type.

Since: 4.7.0.0

# Propositional equality

data a :~: b where infix 4 #

Propositional equality. If a :~: b is inhabited by some terminating value, then the type a is the same as the type b. To use this equality in practice, pattern-match on the a :~: b to get out the Refl constructor; in the body of the pattern-match, the compiler knows that a ~ b.

Since: 4.7.0.0

Constructors

 Refl :: a :~: a

Instances

# For backwards compatibility

typeOf :: forall a. Typeable a => a -> TypeRep #

typeOf1 :: forall t a. Typeable t => t a -> TypeRep #

typeOf2 :: forall t a b. Typeable t => t a b -> TypeRep #

typeOf3 :: forall t a b c. Typeable t => t a b c -> TypeRep #

typeOf4 :: forall t a b c d. Typeable t => t a b c d -> TypeRep #

typeOf5 :: forall t a b c d e. Typeable t => t a b c d e -> TypeRep #

typeOf6 :: forall t a b c d e f. Typeable t => t a b c d e f -> TypeRep #

typeOf7 :: forall t a b c d e f g. Typeable t => t a b c d e f g -> TypeRep #

type Typeable1 a = Typeable a #

Deprecated: renamed to Typeable

type Typeable2 a = Typeable a #

Deprecated: renamed to Typeable

type Typeable3 a = Typeable a #

Deprecated: renamed to Typeable

type Typeable4 a = Typeable a #

Deprecated: renamed to Typeable

type Typeable5 a = Typeable a #

Deprecated: renamed to Typeable

type Typeable6 a = Typeable a #

Deprecated: renamed to Typeable

type Typeable7 a = Typeable a #

Deprecated: renamed to Typeable

# Type-safe cast

cast :: forall a b. (Typeable a, Typeable b) => a -> Maybe b #

The type-safe cast operation

eqT :: forall a b. (Typeable a, Typeable b) => Maybe (a :~: b) #

Extract a witness of equality of two types

Since: 4.7.0.0

gcast :: forall a b c. (Typeable a, Typeable b) => c a -> Maybe (c b) #

A flexible variation parameterised in a type constructor

# Generalized casts for higher-order kinds

gcast1 :: forall c t t' a. (Typeable t, Typeable t') => c (t a) -> Maybe (c (t' a)) #

Cast over k1 -> k2

gcast2 :: forall c t t' a b. (Typeable t, Typeable t') => c (t a b) -> Maybe (c (t' a b)) #

Cast over k1 -> k2 -> k3

# A canonical proxy type

data Proxy t #

A concrete, poly-kinded proxy type

Constructors

 Proxy

Instances

# Type representations

data TypeRep #

A concrete representation of a (monomorphic) type. TypeRep supports reasonably efficient equality.

Instances

 # Methods # Methods # MethodsshowList :: [TypeRep] -> ShowS #

Observe the Fingerprint of a type representation

Since: 4.8.0.0

rnfTypeRep :: TypeRep -> () #

Helper to fully evaluate TypeRep for use as NFData(rnf) implementation

Since: 4.8.0.0

data TyCon :: * Source #

Instances

 Methods Methods(<) :: TyCon -> TyCon -> Bool Source #(>) :: TyCon -> TyCon -> Bool Source # # MethodsshowsPrec :: Int -> TyCon -> ShowS #show :: TyCon -> String #showList :: [TyCon] -> ShowS #

Deprecated: renamed to tyConName; tyConModule and tyConPackage are also available.

Observe string encoding of a type representation

rnfTyCon :: TyCon -> () #

# Construction of type representations

Arguments

 :: String package name -> String module name -> String the name of the type constructor -> TyCon A unique TyCon object Used when the strings are dynamically allocated, eg from binary deserialisation

mkTyConApp :: TyCon -> [TypeRep] -> TypeRep #

Applies a kind-monomorphic type constructor to a sequence of types

Adds a TypeRep argument to a 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. Note that if the type constructor is polymorphic, this will not return the kinds that were used. See splitPolyTyConApp if you need all parts.

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.

Observe the type constructor of a type representation

typeRepArgs :: TypeRep -> [TypeRep] #

Observe the argument types of a type representation