| Copyright | (c) The University of Glasgow CWI 2001--2004 |
|---|---|
| License | BSD-style (see the file libraries/base/LICENSE) |
| Maintainer | libraries@haskell.org |
| Stability | experimental |
| Portability | portable |
| Safe Haskell | Trustworthy |
| Language | Haskell2010 |
Data.Typeable
Contents
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
- class Typeable (a :: k)
- typeOf :: forall a. Typeable a => a -> TypeRep
- typeRep :: forall proxy a. Typeable a => proxy a -> TypeRep
- data a :~: b where
- data (a :: k1) :~~: (b :: k2) where
- cast :: forall a b. (Typeable a, Typeable b) => a -> Maybe b
- eqT :: forall a b. (Typeable a, Typeable b) => Maybe (a :~: b)
- gcast :: forall a b c. (Typeable a, Typeable b) => c a -> Maybe (c b)
- gcast1 :: forall c t t' a. (Typeable t, Typeable t') => c (t a) -> Maybe (c (t' a))
- gcast2 :: forall c t t' a b. (Typeable t, Typeable t') => c (t a b) -> Maybe (c (t' a b))
- data Proxy t = 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 :: 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
- type Typeable2 (a :: * -> * -> *) = Typeable a
- type Typeable3 (a :: * -> * -> * -> *) = Typeable a
- type Typeable4 (a :: * -> * -> * -> * -> *) = Typeable a
- type Typeable5 (a :: * -> * -> * -> * -> * -> *) = Typeable a
- type Typeable6 (a :: * -> * -> * -> * -> * -> * -> *) = Typeable a
- type Typeable7 (a :: * -> * -> * -> * -> * -> * -> * -> *) = Typeable a
The Typeable class
The class Typeable allows a concrete representation of a type to
be calculated.
Minimal complete definition
typeRep#
typeOf :: forall a. Typeable a => a -> TypeRep #
Observe a type representation for the type of a value.
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
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
Instances
| Category k ((:~:) k) # | Since: 4.7.0.0 |
| TestEquality k ((:~:) k a) # | Since: 4.7.0.0 |
| TestCoercion k ((:~:) k a) # | Since: 4.7.0.0 |
| (~) k a b => Bounded ((:~:) k a b) # | Since: 4.7.0.0 |
| (~) k a b => Enum ((:~:) k a b) # | Since: 4.7.0.0 |
| Eq ((:~:) k a b) # | |
| ((~) * a b, Data a) => Data ((:~:) * a b) # | Since: 4.7.0.0 |
| Ord ((:~:) k a b) # | |
| (~) k a b => Read ((:~:) k a b) # | Since: 4.7.0.0 |
| Show ((:~:) k a b) # | |
data (a :: k1) :~~: (b :: k2) where infix 4 #
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
Instances
| Category k ((:~~:) k k) # | Since: 4.10.0.0 |
| TestEquality k ((:~~:) k1 k a) # | Since: 4.10.0.0 |
| TestCoercion k ((:~~:) k1 k a) # | Since: 4.10.0.0 |
| (~~) k1 k2 a b => Bounded ((:~~:) k1 k2 a b) # | Since: 4.10.0.0 |
| (~~) k1 k2 a b => Enum ((:~~:) k1 k2 a b) # | Since: 4.10.0.0 |
| Eq ((:~~:) k1 k2 a b) # | Since: 4.10.0.0 |
| (Typeable * i2, Typeable * j2, Typeable i2 a, Typeable j2 b, (~~) i2 j2 a b) => Data ((:~~:) i2 j2 a b) # | Since: 4.10.0.0 |
| Ord ((:~~:) k1 k2 a b) # | Since: 4.10.0.0 |
| (~~) k1 k2 a b => Read ((:~~:) k1 k2 a b) # | Since: 4.10.0.0 |
| Show ((:~~:) k1 k2 a b) # | Since: 4.10.0.0 |
Type-safe cast
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
A concrete, poly-kinded proxy type
Constructors
| Proxy |
Instances
| Generic1 k (Proxy k) # | |
| Monad (Proxy *) # | Since: 4.7.0.0 |
| Functor (Proxy *) # | Since: 4.7.0.0 |
| Applicative (Proxy *) # | Since: 4.7.0.0 |
| Foldable (Proxy *) # | Since: 4.7.0.0 |
| Traversable (Proxy *) # | Since: 4.7.0.0 |
| MonadPlus (Proxy *) # | Since: 4.9.0.0 |
| Alternative (Proxy *) # | Since: 4.9.0.0 |
| MonadZip (Proxy *) # | Since: 4.9.0.0 |
| Show1 (Proxy *) # | Since: 4.9.0.0 |
| Read1 (Proxy *) # | Since: 4.9.0.0 |
| Ord1 (Proxy *) # | Since: 4.9.0.0 |
| Eq1 (Proxy *) # | Since: 4.9.0.0 |
| Bounded (Proxy k t) # | |
| Enum (Proxy k s) # | Since: 4.7.0.0 |
| Eq (Proxy k s) # | Since: 4.7.0.0 |
| Data t => Data (Proxy * t) # | Since: 4.7.0.0 |
| Ord (Proxy k s) # | Since: 4.7.0.0 |
| Read (Proxy k s) # | Since: 4.7.0.0 |
| Show (Proxy k s) # | Since: 4.7.0.0 |
| Ix (Proxy k s) # | Since: 4.7.0.0 |
| Generic (Proxy k t) # | |
| Semigroup (Proxy k s) # | Since: 4.9.0.0 |
| Monoid (Proxy k s) # | Since: 4.7.0.0 |
| type Rep1 k (Proxy k) # | |
| type Rep (Proxy k t) # | |
Type representations
type TypeRep = SomeTypeRep #
A quantified type representation.
rnfTypeRep :: TypeRep -> () #
Force a TypeRep to normal form.
showsTypeRep :: TypeRep -> ShowS #
Show a type representation
Observing type representations
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.
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.
typeRepArgs :: TypeRep -> [TypeRep] #
Observe the argument types of a type representation
typeRepTyCon :: TypeRep -> TyCon #
Observe the type constructor of a quantified type representation.
typeRepFingerprint :: TypeRep -> Fingerprint #
Takes a value of type a and returns a concrete representation
of that type.
Since: 4.7.0.0
Type constructors
tyConPackage :: TyCon -> String #
tyConModule :: TyCon -> String #
tyConFingerprint :: TyCon -> Fingerprint #
For backwards compatibility
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 #