Data.Type.Equality

Definition of propositional equality (:~:). Pattern-matching on a variable of type (a :~: b) produces a proof that a ~ b.

Since: base-4.7.0.0

The equality types

class a ~# b => (a :: k) ~ (b :: k) infix 4

Lifted, homogeneous equality. By lifted, we mean that it can be bogus (deferred type error). By homogeneous, the two types a and b must have the same kinds.

class a ~# b => (a :: k0) ~~ (b :: k1) infix 4

Lifted, heterogeneous equality. By lifted, we mean that it can be bogus (deferred type error). By heterogeneous, the two types a and b might have different kinds. Because ~~ can appear unexpectedly in error messages to users who do not care about the difference between heterogeneous equality ~~ and homogeneous equality ~, this is printed as ~ unless -fprint-equality-relations is set.

In 0.7.0, the fixity was set to infix 4 to match the fixity of :~~:.

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: base-4.7.0.0

Constructors

Refl :: a :~: a

Instances

Category ((:~:) :: k -> k -> Type)

Since: base-4.7.0.0

Defined in Control.Category

Methods

id :: forall (a :: k0). a :~: a

(.) :: forall (b :: k0) (c :: k0) (a :: k0). (b :~: c) -> (a :~: b) -> a :~: c

TestCoercion ((:~:) a :: k -> Type)

Since: base-4.7.0.0

Defined in Data.Type.Coercion

Methods

testCoercion :: forall (a0 :: k0) (b :: k0). (a :~: a0) -> (a :~: b) -> Maybe (Coercion a0 b)

TestEquality ((:~:) a :: k -> Type)

Since: base-4.7.0.0

Defined in Data.Type.Equality

Methods

testEquality :: forall (a0 :: k0) (b :: k0). (a :~: a0) -> (a :~: b) -> Maybe (a0 :~: b)

(a ~ b, Data a) => Data (a :~: b)

Since: base-4.7.0.0

Defined in Data.Data

Methods

gfoldl :: (forall d b0. Data d => c (d -> b0) -> d -> c b0) -> (forall g. g -> c g) -> (a :~: b) -> c (a :~: b)

gunfold :: (forall b0 r. Data b0 => c (b0 -> r) -> c r) -> (forall r. r -> c r) -> Constr -> c (a :~: b)

toConstr :: (a :~: b) -> Constr

dataTypeOf :: (a :~: b) -> DataType

dataCast1 :: Typeable t => (forall d. Data d => c (t d)) -> Maybe (c (a :~: b))

dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c (t d e)) -> Maybe (c (a :~: b))

gmapT :: (forall b0. Data b0 => b0 -> b0) -> (a :~: b) -> a :~: b

gmapQl :: (r -> r' -> r) -> r -> (forall d. Data d => d -> r') -> (a :~: b) -> r

gmapQr :: forall r r'. (r' -> r -> r) -> r -> (forall d. Data d => d -> r') -> (a :~: b) -> r

gmapQ :: (forall d. Data d => d -> u) -> (a :~: b) -> [u]

gmapQi :: Int -> (forall d. Data d => d -> u) -> (a :~: b) -> u

gmapM :: Monad m => (forall d. Data d => d -> m d) -> (a :~: b) -> m (a :~: b)

gmapMp :: MonadPlus m => (forall d. Data d => d -> m d) -> (a :~: b) -> m (a :~: b)

gmapMo :: MonadPlus m => (forall d. Data d => d -> m d) -> (a :~: b) -> m (a :~: b)

a ~ b => Bounded (a :~: b)

Since: base-4.7.0.0

Defined in Data.Type.Equality

Methods

minBound :: a :~: b

maxBound :: a :~: b

a ~ b => Enum (a :~: b)

Since: base-4.7.0.0

Defined in Data.Type.Equality

Methods

succ :: (a :~: b) -> a :~: b

pred :: (a :~: b) -> a :~: b

toEnum :: Int -> a :~: b

fromEnum :: (a :~: b) -> Int

enumFrom :: (a :~: b) -> [a :~: b]

enumFromThen :: (a :~: b) -> (a :~: b) -> [a :~: b]

enumFromTo :: (a :~: b) -> (a :~: b) -> [a :~: b]

enumFromThenTo :: (a :~: b) -> (a :~: b) -> (a :~: b) -> [a :~: b]

a ~ b => Read (a :~: b)

Since: base-4.7.0.0

Defined in Data.Type.Equality

Methods

readsPrec :: Int -> ReadS (a :~: b)

readList :: ReadS [a :~: b]

readPrec :: ReadPrec (a :~: b)

readListPrec :: ReadPrec [a :~: b]

Show (a :~: b)

Since: base-4.7.0.0

Defined in Data.Type.Equality

Methods

showsPrec :: Int -> (a :~: b) -> ShowS

show :: (a :~: b) -> String

showList :: [a :~: b] -> ShowS

Eq (a :~: b)

Since: base-4.7.0.0

Defined in Data.Type.Equality

Methods

(==) :: (a :~: b) -> (a :~: b) -> Bool

(/=) :: (a :~: b) -> (a :~: b) -> Bool

Ord (a :~: b)

Since: base-4.7.0.0

Defined in Data.Type.Equality

Methods

compare :: (a :~: b) -> (a :~: b) -> Ordering

(<) :: (a :~: b) -> (a :~: b) -> Bool

(<=) :: (a :~: b) -> (a :~: b) -> Bool

(>) :: (a :~: b) -> (a :~: b) -> Bool

(>=) :: (a :~: b) -> (a :~: b) -> Bool

max :: (a :~: b) -> (a :~: b) -> a :~: b

min :: (a :~: b) -> (a :~: b) -> a :~: b

data a :~~: b 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: base-4.10.0.0

Constructors

HRefl :: a :~~: a

Instances

Category ((:~~:) :: k -> k -> Type)

Since: base-4.10.0.0

Defined in Control.Category

Methods

id :: forall (a :: k0). a :~~: a

(.) :: forall (b :: k0) (c :: k0) (a :: k0). (b :~~: c) -> (a :~~: b) -> a :~~: c

TestCoercion ((:~~:) a :: k -> Type)

Since: base-4.10.0.0

Defined in Data.Type.Coercion

Methods

testCoercion :: forall (a0 :: k0) (b :: k0). (a :~~: a0) -> (a :~~: b) -> Maybe (Coercion a0 b)

TestEquality ((:~~:) a :: k -> Type)

Since: base-4.10.0.0

Defined in Data.Type.Equality

Methods

testEquality :: forall (a0 :: k0) (b :: k0). (a :~~: a0) -> (a :~~: b) -> Maybe (a0 :~: b)

(Typeable i, Typeable j, Typeable a, Typeable b, a ~~ b) => Data (a :~~: b)

Since: base-4.10.0.0

Defined in Data.Data

Methods

gfoldl :: (forall d b0. Data d => c (d -> b0) -> d -> c b0) -> (forall g. g -> c g) -> (a :~~: b) -> c (a :~~: b)

gunfold :: (forall b0 r. Data b0 => c (b0 -> r) -> c r) -> (forall r. r -> c r) -> Constr -> c (a :~~: b)

toConstr :: (a :~~: b) -> Constr

dataTypeOf :: (a :~~: b) -> DataType

dataCast1 :: Typeable t => (forall d. Data d => c (t d)) -> Maybe (c (a :~~: b))

dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c (t d e)) -> Maybe (c (a :~~: b))

gmapT :: (forall b0. Data b0 => b0 -> b0) -> (a :~~: b) -> a :~~: b

gmapQl :: (r -> r' -> r) -> r -> (forall d. Data d => d -> r') -> (a :~~: b) -> r

gmapQr :: forall r r'. (r' -> r -> r) -> r -> (forall d. Data d => d -> r') -> (a :~~: b) -> r

gmapQ :: (forall d. Data d => d -> u) -> (a :~~: b) -> [u]

gmapQi :: Int -> (forall d. Data d => d -> u) -> (a :~~: b) -> u

gmapM :: Monad m => (forall d. Data d => d -> m d) -> (a :~~: b) -> m (a :~~: b)

gmapMp :: MonadPlus m => (forall d. Data d => d -> m d) -> (a :~~: b) -> m (a :~~: b)

gmapMo :: MonadPlus m => (forall d. Data d => d -> m d) -> (a :~~: b) -> m (a :~~: b)

a ~~ b => Bounded (a :~~: b)

Since: base-4.10.0.0

Defined in Data.Type.Equality

Methods

minBound :: a :~~: b

maxBound :: a :~~: b

a ~~ b => Enum (a :~~: b)

Since: base-4.10.0.0

Defined in Data.Type.Equality

Methods

succ :: (a :~~: b) -> a :~~: b

pred :: (a :~~: b) -> a :~~: b

toEnum :: Int -> a :~~: b

fromEnum :: (a :~~: b) -> Int

enumFrom :: (a :~~: b) -> [a :~~: b]

enumFromThen :: (a :~~: b) -> (a :~~: b) -> [a :~~: b]

enumFromTo :: (a :~~: b) -> (a :~~: b) -> [a :~~: b]

enumFromThenTo :: (a :~~: b) -> (a :~~: b) -> (a :~~: b) -> [a :~~: b]

a ~~ b => Read (a :~~: b)

Since: base-4.10.0.0

Defined in Data.Type.Equality

Methods

readsPrec :: Int -> ReadS (a :~~: b)

readList :: ReadS [a :~~: b]

readPrec :: ReadPrec (a :~~: b)

readListPrec :: ReadPrec [a :~~: b]

Show (a :~~: b)

Since: base-4.10.0.0

Defined in Data.Type.Equality

Methods

showsPrec :: Int -> (a :~~: b) -> ShowS

show :: (a :~~: b) -> String

showList :: [a :~~: b] -> ShowS

Eq (a :~~: b)

Since: base-4.10.0.0

Defined in Data.Type.Equality

Methods

(==) :: (a :~~: b) -> (a :~~: b) -> Bool

(/=) :: (a :~~: b) -> (a :~~: b) -> Bool

Ord (a :~~: b)

Since: base-4.10.0.0

Defined in Data.Type.Equality

Methods

compare :: (a :~~: b) -> (a :~~: b) -> Ordering

(<) :: (a :~~: b) -> (a :~~: b) -> Bool

(<=) :: (a :~~: b) -> (a :~~: b) -> Bool

(>) :: (a :~~: b) -> (a :~~: b) -> Bool

(>=) :: (a :~~: b) -> (a :~~: b) -> Bool

max :: (a :~~: b) -> (a :~~: b) -> a :~~: b

min :: (a :~~: b) -> (a :~~: b) -> a :~~: b

Working with equality

sym :: (a :~: b) -> b :~: a

Symmetry of equality

trans :: (a :~: b) -> (b :~: c) -> a :~: c

Transitivity of equality

castWith :: (a :~: b) -> a -> b

Type-safe cast, using propositional equality

gcastWith :: (a :~: b) -> (a ~ b => r) -> r

Generalized form of type-safe cast using propositional equality

apply :: (f :~: g) -> (a :~: b) -> f a :~: g b

Apply one equality to another, respectively

inner :: (f a :~: g b) -> a :~: b

Extract equality of the arguments from an equality of applied types

outer :: (f a :~: g b) -> f :~: g

Extract equality of type constructors from an equality of applied types

Inferring equality from other types

class TestEquality f where

This class contains types where you can learn the equality of two types from information contained in terms.

The result should be Just Refl if and only if the types applied to f are equal:

TestEquality (x :: f a) (y :: f b) = Just Refl ⟺ a = b

Typically, only singleton types should inhabit this class. In that case type argument equality coincides with term equality:

TestEquality (x :: f a) (y :: f b) = Just Refl ⟺ a = b ⟺ x = y

isJust (TestEquality x y) = x == y

Singleton types are not required, however, and so the latter two would-be laws are not in fact valid in general.

Methods

testEquality :: f a -> f b -> Maybe (a :~: b)

Conditionally prove the equality of a and b.

Instances

TestEquality (TypeRep :: k -> Type)

Defined in Data.Typeable.Internal

Methods

testEquality :: forall (a :: k0) (b :: k0). TypeRep a -> TypeRep b -> Maybe (a :~: b)

TestEquality ((:~:) a :: k -> Type)

Since: base-4.7.0.0

Defined in Data.Type.Equality

Methods

testEquality :: forall (a0 :: k0) (b :: k0). (a :~: a0) -> (a :~: b) -> Maybe (a0 :~: b)

TestEquality ((:~~:) a :: k -> Type)

Since: base-4.10.0.0

Defined in Data.Type.Equality

Methods

testEquality :: forall (a0 :: k0) (b :: k0). (a :~~: a0) -> (a :~~: b) -> Maybe (a0 :~: b)

TestEquality f => TestEquality (Compose f g :: k2 -> Type)

The deduction (via generativity) that if g x :~: g y then x :~: y.

Since: base-4.14.0.0

Defined in Data.Functor.Compose

Methods

testEquality :: forall (a :: k) (b :: k). Compose f g a -> Compose f g b -> Maybe (a :~: b)

Boolean type-level equality

type family a == b where ... infix 4

A type family to compute Boolean equality.

Equations

(f a) == (g b) = (f == g) && (a == b)

a == a = 'True

_ == _ = 'False