| 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.
- 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 (k1 :~~: k2) (a :: k1) (b :: k2) :: forall k1 k2. k1 -> k2 -> * where
- data (k :~: (a :: k)) (b :: k) :: forall k. k -> k -> * where
Documentation
class Deferrable p where #
Allow an attempt at resolution of a constraint at a later time
Minimal complete definition
Methods
deferEither :: proxy p -> (p => r) -> Either String r #
Resolve a Deferrable constraint with observable failure.
Instances
| Deferrable () # | |
| (Deferrable a, Deferrable b) => Deferrable (a, b) # | |
| (Typeable * k3, Typeable k3 a, Typeable k3 b) => Deferrable ((~) k3 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 |
| (Deferrable a, Deferrable b, Deferrable c) => Deferrable (a, b, c) # | |
| (Typeable * i2, Typeable * j2, Typeable i2 a, Typeable j2 b) => Deferrable ((~~) i2 j2 a b) # | Deferrable heterogenous equality constraints. Only available on GHC 8.0 or later. |
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 #
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 (k1 :~~: k2) (a :: k1) (b :: k2) :: forall k1 k2. k1 -> 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 |
| NFData2 ((:~~:) * *) | Since: 1.4.3.0 |
| NFData1 ((:~~:) k1 * a) | Since: 1.4.3.0 |
| (~~) 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 |
| NFData ((:~~:) k1 k2 a b) | Since: 1.4.3.0 |
data (k :~: (a :: k)) (b :: k) :: 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
Instances
| Category k ((:~:) k) | Since: 4.7.0.0 |
| TestEquality k ((:~:) k a) | Since: 4.7.0.0 |
| NFData2 ((:~:) *) | Since: 1.4.3.0 |
| NFData1 ((:~:) * a) | Since: 1.4.3.0 |
| (~) 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) | |
| NFData ((:~:) k a b) | Since: 1.4.3.0 |