Data.Constraint.Deferrable

The idea for this trick comes from Dimitrios Vytiniotis.

Documentation

data UnsatisfiedConstraint

Constructors

UnsatisfiedConstraint String

Instances

Show UnsatisfiedConstraint

Methods

showsPrec :: Int -> UnsatisfiedConstraint -> ShowS

show :: UnsatisfiedConstraint -> String

showList :: [UnsatisfiedConstraint] -> ShowS

Exception UnsatisfiedConstraint

Methods

toException :: UnsatisfiedConstraint -> SomeException

fromException :: SomeException -> Maybe UnsatisfiedConstraint

displayException :: UnsatisfiedConstraint -> String

class Deferrable p where

Allow an attempt at resolution of a constraint at a later time

Minimal complete definition

deferEither

Methods

deferEither :: proxy p -> (p => r) -> Either String r

Resolve a Deferrable constraint with observable failure.

Instances

Deferrable ()

Methods

deferEither :: proxy () -> (() -> r) -> Either String r

(Deferrable a, Deferrable b) => Deferrable (a, b)

Methods

deferEither :: proxy (a, b) -> ((a, b) -> r) -> Either String r

(Typeable * k2, Typeable k2 a, Typeable k2 b) => Deferrable ((~) k2 a b)

Deferrable homogeneous equality constraints.

Note that due to a GHC bug (https:/ghc.haskell.orgtracghcticket/10343), using this instance on GHC 7.10 will only work with *-kinded types.

Methods

deferEither :: proxy ((k2 ~ a) b) -> ((k2 ~ a) b -> r) -> Either String r

(Deferrable a, Deferrable b, Deferrable c) => Deferrable (a, b, c)

Methods

deferEither :: proxy (a, b, c) -> ((a, b, c) -> r) -> Either String r

(Typeable * i1, Typeable * j1, Typeable i1 a, Typeable j1 b) => Deferrable ((~~) i1 j1 a b)

Deferrable heterogenous equality constraints.

Only available on GHC 8.0 or later.

Methods

deferEither :: proxy ((i1 ~~ j1) a b) -> ((i1 ~~ j1) a b -> r) -> Either String r

defer :: forall p r proxy. Deferrable p => proxy p -> (p => r) -> r

Defer a constraint for later resolution in a context where we want to upgrade failure into an error

deferred :: forall p. Deferrable p :- p

defer_ :: forall p r. Deferrable p => (p => r) -> r

A version of defer that uses visible type application in place of a Proxy.

Only available on GHC 8.0 or later.

deferEither_ :: forall p r. Deferrable p => (p => r) -> Either String r

A version of deferEither that uses visible type application in place of a Proxy.

Only available on GHC 8.0 or later.

data a :~~: b where

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

Only available on GHC 8.0 or later.

Constructors

HRefl :: a :~~: a

data (k :~: a) b :: forall k. k -> k -> * 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 :: (:~:) k a a

Instances

Category k ((:~:) k)

Methods

id :: cat a a

(.) :: cat b c -> cat a b -> cat a c

TestEquality k ((:~:) k a)

Methods

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

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

Methods

minBound :: (k :~: a) b

maxBound :: (k :~: a) b

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

Methods

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

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

toEnum :: Int -> (k :~: a) b

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

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

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

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

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

Eq ((:~:) k a b)

Methods

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

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

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

Methods

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

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

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

gmapQr :: (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)

Ord ((:~:) k a b)

Methods

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

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

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

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

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

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

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

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

Methods

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

readList :: ReadS [(k :~: a) b]

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

readListPrec :: ReadPrec [(k :~: a) b]

Show ((:~:) k a b)

Methods

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

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

showList :: [(k :~: a) b] -> ShowS