base-4.10.1.0: Basic libraries

Copyright (c) The University of Glasgow CWI 2001--2004 BSD-style (see the file libraries/base/LICENSE) [email protected] experimental portable Trustworthy Haskell2010

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 8.2, GHC has supported type-indexed type representations. Data.Typeable provides type representations which are qualified over this index, providing an interface very similar to the Typeable notion seen in previous releases. For the type-indexed interface, see Type.Reflection.

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 :: k) Source #

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

Minimal complete definition

typeRep#

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

Observe a type representation for the type of a value.

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

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 Source #

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

data (a :: k1) :~~: (b :: k2) where infix 4 Source #

Kind heterogeneous propositional equality. Like '(:~:)', a :~~: b is inhabited by a terminating value if and only if a is the same type as b.

Since: 4.10.0.0

Constructors

 HRefl :: a :~~: a

Instances

# Type-safe cast

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

The type-safe cast operation

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

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) Source #

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)) Source #

Cast over k1 -> k2

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

Cast over k1 -> k2 -> k3

# A canonical proxy type

data Proxy t Source #

A concrete, poly-kinded proxy type

Constructors

 Proxy

Instances

# Type representations

A quantified type representation.

rnfTypeRep :: TypeRep -> () Source #

Force a TypeRep to normal form.

Show a type representation

Build a function type.

# Observing type representations

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.

splitTyConApp :: TypeRep -> (TyCon, [TypeRep]) Source #

Splits a type constructor application. Note that if the type constructor is polymorphic, this will not return the kinds that were used.

Observe the argument types of a type representation

Observe the type constructor of a quantified type representation.

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

Since: 4.7.0.0

# Type constructors

data TyCon :: * Source #

Instances

 Methods Methods(<) :: TyCon -> TyCon -> Bool Source #(>) :: TyCon -> TyCon -> Bool Source # # Since: 2.1 MethodsshowList :: [TyCon] -> ShowS Source #

# For backwards compatibility

typeOf1 :: forall t (a :: *). Typeable t => t a -> TypeRep Source #

typeOf2 :: forall t (a :: *) (b :: *). Typeable t => t a b -> TypeRep Source #

typeOf3 :: forall t (a :: *) (b :: *) (c :: *). Typeable t => t a b c -> TypeRep Source #

typeOf4 :: forall t (a :: *) (b :: *) (c :: *) (d :: *). Typeable t => t a b c d -> TypeRep Source #

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

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

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

type Typeable1 (a :: * -> *) = Typeable a Source #

Deprecated: renamed to Typeable

type Typeable2 (a :: * -> * -> *) = Typeable a Source #

Deprecated: renamed to Typeable

type Typeable3 (a :: * -> * -> * -> *) = Typeable a Source #

Deprecated: renamed to Typeable

type Typeable4 (a :: * -> * -> * -> * -> *) = Typeable a Source #

Deprecated: renamed to Typeable

type Typeable5 (a :: * -> * -> * -> * -> * -> *) = Typeable a Source #

Deprecated: renamed to Typeable

type Typeable6 (a :: * -> * -> * -> * -> * -> * -> *) = Typeable a Source #

Deprecated: renamed to Typeable

type Typeable7 (a :: * -> * -> * -> * -> * -> * -> * -> *) = Typeable a Source #

Deprecated: renamed to Typeable