constraints-0.9: Constraint manipulation

Copyright | (C) 2015-2016 Edward Kmett
License | BSD-style (see the file LICENSE)
Maintainer | Edward Kmett <ekmett@gmail.com>
Stability | experimental
Portability | non-portable
Safe Haskell | None
Language | Haskell2010

Data.Constraint.Deferrable

Description

The idea for this trick comes from Dimitrios Vytiniotis.

Synopsis

- data UnsatisfiedConstraint = UnsatisfiedConstraint String
- class Deferrable p where
- defer :: forall p r proxy. Deferrable p => proxy p -> (p => r) -> r
- deferred :: forall p. Deferrable p :- p
- defer_ :: forall p r. Deferrable p => (p => r) -> r
- deferEither_ :: forall p r. Deferrable p => (p => r) -> Either String r
- data a :~~: b where
  - HRefl :: a :~~: a
- data (k :~: a) b :: forall k. k -> k -> * where
  - Refl :: (:~:) k a a

# 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 #
TestCoercion k ((:~:) k a) |
Methods testCoercion :: f a -> f b -> Maybe (Coercion (k :~: a) a b) #
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 #