Copyright | (c) The University of Glasgow CWI 2001--2004 |
---|---|

License | BSD-style (see the file libraries/base/LICENSE) |

Maintainer | libraries@haskell.org |

Stability | stable |

Portability | portable |

Safe Haskell | Trustworthy |

Language | Haskell2010 |

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 7.10, all types automatically have `Typeable`

instances derived.
This is in contrast to previous releases where `Typeable`

had to be
explicitly derived using the `DeriveDataTypeable`

language extension.

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

- class Typeable (a :: k)
- typeOf :: Typeable a => a -> TypeRep
- typeRep :: forall {k} proxy (a :: k). Typeable a => proxy a -> TypeRep
- data (a :: k) :~: (b :: k) where
- data (a :: k1) :~~: (b :: k2) where
- cast :: (Typeable a, Typeable b) => a -> Maybe b
- eqT :: forall {k} (a :: k) (b :: k). (Typeable a, Typeable b) => Maybe (a :~: b)
- heqT :: forall {k1} {k2} (a :: k1) (b :: k2). (Typeable a, Typeable b) => Maybe (a :~~: b)
- decT :: forall {k} (a :: k) (b :: k). (Typeable a, Typeable b) => Either ((a :~: b) -> Void) (a :~: b)
- hdecT :: forall {k1} {k2} (a :: k1) (b :: k2). (Typeable a, Typeable b) => Either ((a :~~: b) -> Void) (a :~~: b)
- gcast :: forall {k} (a :: k) (b :: k) c. (Typeable a, Typeable b) => c a -> Maybe (c b)
- gcast1 :: forall {k1} {k2} c (t :: k2 -> k1) (t' :: k2 -> k1) (a :: k2). (Typeable t, Typeable t') => c (t a) -> Maybe (c (t' a))
- gcast2 :: forall {k1} {k2} {k3} c (t :: k2 -> k3 -> k1) (t' :: k2 -> k3 -> k1) (a :: k2) (b :: k3). (Typeable t, Typeable t') => c (t a b) -> Maybe (c (t' a b))
- data Proxy (t :: k) = Proxy
- type TypeRep = SomeTypeRep
- rnfTypeRep :: TypeRep -> ()
- showsTypeRep :: TypeRep -> ShowS
- mkFunTy :: TypeRep -> TypeRep -> TypeRep
- funResultTy :: TypeRep -> TypeRep -> Maybe TypeRep
- splitTyConApp :: TypeRep -> (TyCon, [TypeRep])
- typeRepArgs :: TypeRep -> [TypeRep]
- typeRepTyCon :: TypeRep -> TyCon
- typeRepFingerprint :: TypeRep -> Fingerprint
- data TyCon
- tyConPackage :: TyCon -> String
- tyConModule :: TyCon -> String
- tyConName :: TyCon -> String
- rnfTyCon :: TyCon -> ()
- tyConFingerprint :: TyCon -> Fingerprint
- typeOf1 :: Typeable t => t a -> TypeRep
- typeOf2 :: Typeable t => t a b -> TypeRep
- typeOf3 :: Typeable t => t a b c -> TypeRep
- typeOf4 :: Typeable t => t a b c d -> TypeRep
- typeOf5 :: Typeable t => t a b c d e -> TypeRep
- typeOf6 :: Typeable t => t a b c d e f -> TypeRep
- typeOf7 :: Typeable t => t a b c d e f g -> TypeRep
- trLiftedRep :: TypeRep LiftedRep

# The Typeable class

class Typeable (a :: k) Source #

The class `Typeable`

allows a concrete representation of a type to
be calculated.

typeRep#

typeRep :: forall {k} proxy (a :: k). Typeable a => proxy a -> TypeRep Source #

Takes a value of type `a`

and returns a concrete representation
of that type.

@since base-4.7.0.0

# Propositional equality

data (a :: k) :~: (b :: k) 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 base-4.7.0.0

#### Instances

Category ((:~:) :: k -> k -> Type) Source # | @since base-4.7.0.0 |

TestCoercion ((:~:) a :: k -> Type) Source # | @since base-4.7.0.0 |

Defined in GHC.Internal.Data.Type.Coercion | |

TestEquality ((:~:) a :: k -> Type) Source # | @since base-4.7.0.0 |

Defined in GHC.Internal.Data.Type.Equality | |

(a ~ b, Data a) => Data (a :~: b) Source # | @since base-4.7.0.0 |

Defined in GHC.Internal.Data.Data gfoldl :: (forall d b0. Data d => c (d -> b0) -> d -> c b0) -> (forall g. g -> c g) -> (a :~: b) -> c (a :~: b) Source # gunfold :: (forall b0 r. Data b0 => c (b0 -> r) -> c r) -> (forall r. r -> c r) -> Constr -> c (a :~: b) Source # toConstr :: (a :~: b) -> Constr Source # dataTypeOf :: (a :~: b) -> DataType Source # dataCast1 :: Typeable t => (forall d. Data d => c (t d)) -> Maybe (c (a :~: b)) Source # dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c (t d e)) -> Maybe (c (a :~: b)) Source # gmapT :: (forall b0. Data b0 => b0 -> b0) -> (a :~: b) -> a :~: b Source # gmapQl :: (r -> r' -> r) -> r -> (forall d. Data d => d -> r') -> (a :~: b) -> r Source # gmapQr :: forall r r'. (r' -> r -> r) -> r -> (forall d. Data d => d -> r') -> (a :~: b) -> r Source # gmapQ :: (forall d. Data d => d -> u) -> (a :~: b) -> [u] Source # gmapQi :: Int -> (forall d. Data d => d -> u) -> (a :~: b) -> u Source # gmapM :: Monad m => (forall d. Data d => d -> m d) -> (a :~: b) -> m (a :~: b) Source # gmapMp :: MonadPlus m => (forall d. Data d => d -> m d) -> (a :~: b) -> m (a :~: b) Source # gmapMo :: MonadPlus m => (forall d. Data d => d -> m d) -> (a :~: b) -> m (a :~: b) Source # | |

a ~ b => Bounded (a :~: b) Source # | @since base-4.7.0.0 |

a ~ b => Enum (a :~: b) Source # | @since base-4.7.0.0 |

Defined in GHC.Internal.Data.Type.Equality succ :: (a :~: b) -> a :~: b Source # pred :: (a :~: b) -> a :~: b Source # toEnum :: Int -> a :~: b Source # fromEnum :: (a :~: b) -> Int Source # enumFrom :: (a :~: b) -> [a :~: b] Source # enumFromThen :: (a :~: b) -> (a :~: b) -> [a :~: b] Source # enumFromTo :: (a :~: b) -> (a :~: b) -> [a :~: b] Source # enumFromThenTo :: (a :~: b) -> (a :~: b) -> (a :~: b) -> [a :~: b] Source # | |

a ~ b => Read (a :~: b) Source # | @since base-4.7.0.0 |

Show (a :~: b) Source # | @since base-4.7.0.0 |

Eq (a :~: b) Source # | @since base-4.7.0.0 |

Ord (a :~: b) Source # | @since base-4.7.0.0 |

Defined in GHC.Internal.Data.Type.Equality |

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 base-4.10.0.0

#### Instances

Category ((:~~:) :: k -> k -> Type) Source # | @since base-4.10.0.0 |

TestCoercion ((:~~:) a :: k -> Type) Source # | @since base-4.10.0.0 |

Defined in GHC.Internal.Data.Type.Coercion | |

TestEquality ((:~~:) a :: k -> Type) Source # | @since base-4.10.0.0 |

Defined in GHC.Internal.Data.Type.Equality | |

(Typeable i, Typeable j, Typeable a, Typeable b, a ~~ b) => Data (a :~~: b) Source # | @since base-4.10.0.0 |

Defined in GHC.Internal.Data.Data gfoldl :: (forall d b0. Data d => c (d -> b0) -> d -> c b0) -> (forall g. g -> c g) -> (a :~~: b) -> c (a :~~: b) Source # gunfold :: (forall b0 r. Data b0 => c (b0 -> r) -> c r) -> (forall r. r -> c r) -> Constr -> c (a :~~: b) Source # toConstr :: (a :~~: b) -> Constr Source # dataTypeOf :: (a :~~: b) -> DataType Source # dataCast1 :: Typeable t => (forall d. Data d => c (t d)) -> Maybe (c (a :~~: b)) Source # dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c (t d e)) -> Maybe (c (a :~~: b)) Source # gmapT :: (forall b0. Data b0 => b0 -> b0) -> (a :~~: b) -> a :~~: b Source # gmapQl :: (r -> r' -> r) -> r -> (forall d. Data d => d -> r') -> (a :~~: b) -> r Source # gmapQr :: forall r r'. (r' -> r -> r) -> r -> (forall d. Data d => d -> r') -> (a :~~: b) -> r Source # gmapQ :: (forall d. Data d => d -> u) -> (a :~~: b) -> [u] Source # gmapQi :: Int -> (forall d. Data d => d -> u) -> (a :~~: b) -> u Source # gmapM :: Monad m => (forall d. Data d => d -> m d) -> (a :~~: b) -> m (a :~~: b) Source # gmapMp :: MonadPlus m => (forall d. Data d => d -> m d) -> (a :~~: b) -> m (a :~~: b) Source # gmapMo :: MonadPlus m => (forall d. Data d => d -> m d) -> (a :~~: b) -> m (a :~~: b) Source # | |

a ~~ b => Bounded (a :~~: b) Source # | @since base-4.10.0.0 |

a ~~ b => Enum (a :~~: b) Source # | @since base-4.10.0.0 |

Defined in GHC.Internal.Data.Type.Equality succ :: (a :~~: b) -> a :~~: b Source # pred :: (a :~~: b) -> a :~~: b Source # toEnum :: Int -> a :~~: b Source # fromEnum :: (a :~~: b) -> Int Source # enumFrom :: (a :~~: b) -> [a :~~: b] Source # enumFromThen :: (a :~~: b) -> (a :~~: b) -> [a :~~: b] Source # enumFromTo :: (a :~~: b) -> (a :~~: b) -> [a :~~: b] Source # enumFromThenTo :: (a :~~: b) -> (a :~~: b) -> (a :~~: b) -> [a :~~: b] Source # | |

a ~~ b => Read (a :~~: b) Source # | @since base-4.10.0.0 |

Show (a :~~: b) Source # | @since base-4.10.0.0 |

Eq (a :~~: b) Source # | @since base-4.10.0.0 |

Ord (a :~~: b) Source # | @since base-4.10.0.0 |

Defined in GHC.Internal.Data.Type.Equality compare :: (a :~~: b) -> (a :~~: b) -> Ordering Source # (<) :: (a :~~: b) -> (a :~~: b) -> Bool Source # (<=) :: (a :~~: b) -> (a :~~: b) -> Bool Source # (>) :: (a :~~: b) -> (a :~~: b) -> Bool Source # (>=) :: (a :~~: b) -> (a :~~: b) -> Bool Source # |

# Type-safe cast

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

Extract a witness of equality of two types

@since base-4.7.0.0

heqT :: forall {k1} {k2} (a :: k1) (b :: k2). (Typeable a, Typeable b) => Maybe (a :~~: b) Source #

Extract a witness of heterogeneous equality of two types

@since base-4.18.0.0

decT :: forall {k} (a :: k) (b :: k). (Typeable a, Typeable b) => Either ((a :~: b) -> Void) (a :~: b) Source #

Decide an equality of two types

@since base-4.19.0.0

hdecT :: forall {k1} {k2} (a :: k1) (b :: k2). (Typeable a, Typeable b) => Either ((a :~~: b) -> Void) (a :~~: b) Source #

Decide heterogeneous equality of two types.

@since base-4.19.0.0

gcast :: forall {k} (a :: k) (b :: k) 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 {k1} {k2} c (t :: k2 -> k1) (t' :: k2 -> k1) (a :: k2). (Typeable t, Typeable t') => c (t a) -> Maybe (c (t' a)) Source #

Cast over `k1 -> k2`

gcast2 :: forall {k1} {k2} {k3} c (t :: k2 -> k3 -> k1) (t' :: k2 -> k3 -> k1) (a :: k2) (b :: k3). (Typeable t, Typeable t') => c (t a b) -> Maybe (c (t' a b)) Source #

Cast over `k1 -> k2 -> k3`

# A canonical proxy type

`Proxy`

is a type that holds no data, but has a phantom parameter of
arbitrary type (or even kind). Its use is to provide type information, even
though there is no value available of that type (or it may be too costly to
create one).

Historically,

is a safer alternative to the
`Proxy`

:: `Proxy`

a

idiom.`undefined`

:: a

`>>>`

Proxy`Proxy :: Proxy (Void, Int -> Int)`

Proxy can even hold types of higher kinds,

`>>>`

Proxy`Proxy :: Proxy Either`

`>>>`

Proxy`Proxy :: Proxy Functor`

`>>>`

Proxy`Proxy :: Proxy complicatedStructure`

#### Instances

Generic1 (Proxy :: k -> Type) Source # | |

Defined in GHC.Internal.Generics | |

Alternative (Proxy :: Type -> Type) Source # | @since base-4.9.0.0 |

Applicative (Proxy :: Type -> Type) Source # | @since base-4.7.0.0 |

Functor (Proxy :: Type -> Type) Source # | @since base-4.7.0.0 |

Monad (Proxy :: Type -> Type) Source # | @since base-4.7.0.0 |

MonadPlus (Proxy :: Type -> Type) Source # | @since base-4.9.0.0 |

Foldable (Proxy :: Type -> Type) Source # | @since base-4.7.0.0 |

Defined in GHC.Internal.Data.Foldable fold :: Monoid m => Proxy m -> m Source # foldMap :: Monoid m => (a -> m) -> Proxy a -> m Source # foldMap' :: Monoid m => (a -> m) -> Proxy a -> m Source # foldr :: (a -> b -> b) -> b -> Proxy a -> b Source # foldr' :: (a -> b -> b) -> b -> Proxy a -> b Source # foldl :: (b -> a -> b) -> b -> Proxy a -> b Source # foldl' :: (b -> a -> b) -> b -> Proxy a -> b Source # foldr1 :: (a -> a -> a) -> Proxy a -> a Source # foldl1 :: (a -> a -> a) -> Proxy a -> a Source # toList :: Proxy a -> [a] Source # null :: Proxy a -> Bool Source # length :: Proxy a -> Int Source # elem :: Eq a => a -> Proxy a -> Bool Source # maximum :: Ord a => Proxy a -> a Source # minimum :: Ord a => Proxy a -> a Source # | |

Traversable (Proxy :: Type -> Type) Source # | @since base-4.7.0.0 |

Defined in GHC.Internal.Data.Traversable | |

Monoid (Proxy s) Source # | @since base-4.7.0.0 |

Semigroup (Proxy s) Source # | @since base-4.9.0.0 |

Data t => Data (Proxy t) Source # | @since base-4.7.0.0 |

Defined in GHC.Internal.Data.Data gfoldl :: (forall d b. Data d => c (d -> b) -> d -> c b) -> (forall g. g -> c g) -> Proxy t -> c (Proxy t) Source # gunfold :: (forall b r. Data b => c (b -> r) -> c r) -> (forall r. r -> c r) -> Constr -> c (Proxy t) Source # toConstr :: Proxy t -> Constr Source # dataTypeOf :: Proxy t -> DataType Source # dataCast1 :: Typeable t0 => (forall d. Data d => c (t0 d)) -> Maybe (c (Proxy t)) Source # dataCast2 :: Typeable t0 => (forall d e. (Data d, Data e) => c (t0 d e)) -> Maybe (c (Proxy t)) Source # gmapT :: (forall b. Data b => b -> b) -> Proxy t -> Proxy t Source # gmapQl :: (r -> r' -> r) -> r -> (forall d. Data d => d -> r') -> Proxy t -> r Source # gmapQr :: forall r r'. (r' -> r -> r) -> r -> (forall d. Data d => d -> r') -> Proxy t -> r Source # gmapQ :: (forall d. Data d => d -> u) -> Proxy t -> [u] Source # gmapQi :: Int -> (forall d. Data d => d -> u) -> Proxy t -> u Source # gmapM :: Monad m => (forall d. Data d => d -> m d) -> Proxy t -> m (Proxy t) Source # gmapMp :: MonadPlus m => (forall d. Data d => d -> m d) -> Proxy t -> m (Proxy t) Source # gmapMo :: MonadPlus m => (forall d. Data d => d -> m d) -> Proxy t -> m (Proxy t) Source # | |

Bounded (Proxy t) Source # | @since base-4.7.0.0 |

Enum (Proxy s) Source # | @since base-4.7.0.0 |

Defined in GHC.Internal.Data.Proxy succ :: Proxy s -> Proxy s Source # pred :: Proxy s -> Proxy s Source # toEnum :: Int -> Proxy s Source # fromEnum :: Proxy s -> Int Source # enumFrom :: Proxy s -> [Proxy s] Source # enumFromThen :: Proxy s -> Proxy s -> [Proxy s] Source # enumFromTo :: Proxy s -> Proxy s -> [Proxy s] Source # enumFromThenTo :: Proxy s -> Proxy s -> Proxy s -> [Proxy s] Source # | |

Generic (Proxy t) Source # | |

Defined in GHC.Internal.Generics | |

Ix (Proxy s) Source # | @since base-4.7.0.0 |

Defined in GHC.Internal.Data.Proxy | |

Read (Proxy t) Source # | @since base-4.7.0.0 |

Show (Proxy s) Source # | @since base-4.7.0.0 |

Eq (Proxy s) Source # | @since base-4.7.0.0 |

Ord (Proxy s) Source # | @since base-4.7.0.0 |

Defined in GHC.Internal.Data.Proxy | |

type Rep1 (Proxy :: k -> Type) Source # | @since base-4.6.0.0 |

type Rep (Proxy t) Source # | @since base-4.6.0.0 |

# Type representations

type TypeRep = SomeTypeRep Source #

A quantified type representation.

rnfTypeRep :: TypeRep -> () Source #

Force a `TypeRep`

to normal form.

showsTypeRep :: TypeRep -> ShowS Source #

Show a type representation

# Observing type representations

funResultTy :: TypeRep -> TypeRep -> Maybe TypeRep Source #

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.

typeRepArgs :: TypeRep -> [TypeRep] Source #

Observe the argument types of a type representation

typeRepTyCon :: TypeRep -> TyCon Source #

Observe the type constructor of a quantified type representation.

typeRepFingerprint :: TypeRep -> Fingerprint Source #

Takes a value of type `a`

and returns a concrete representation
of that type.

@since base-4.7.0.0

# Type constructors

tyConPackage :: TyCon -> String Source #

tyConModule :: TyCon -> String Source #

tyConFingerprint :: TyCon -> Fingerprint Source #