constraints-0.13.3: Constraint manipulation

Copyright | (C) 2011-2015 Edward Kmett
License | BSD-style (see the file LICENSE)
Maintainer | Edward Kmett <ekmett@gmail.com>
Stability | experimental
Portability | non-portable
Safe Haskell | Unsafe
Language | Haskell2010

Data.Constraint.Unsafe

Contents

- Sugar

Description

Synopsis

- class a ~R# b => Coercible (a :: k) (b :: k)
- unsafeCoerceConstraint :: a :- b
- unsafeDerive :: Coercible n o => (o -> n) -> t o :- t n
- unsafeUnderive :: Coercible n o => (o -> n) -> t n :- t o
- unsafeApplicative :: forall m a. Monad m => (Applicative m => m a) -> m a
- unsafeAlternative :: forall m a. MonadPlus m => (Alternative m => m a) -> m a

# Documentation

class a ~R# b => Coercible (a :: k) (b :: k) #

`Coercible` is a two-parameter class that has instances for types `a` and `b` if the compiler can infer that they have the same representation. This class does not have regular instances; instead they are created on-the-fly during type-checking. Trying to manually declare an instance of `Coercible` is an error.

Nevertheless one can pretend that the following three kinds of instances exist. First, as a trivial base-case:

    instance Coercible a a

Furthermore, for every type constructor there is an instance that allows to coerce under the type constructor. For example, let `D` be a prototypical type constructor (`data` or `newtype`) with three type arguments, which have roles `nominal`, `representational` resp. `phantom`. Then there is an instance of the form

    instance Coercible b b' => Coercible (D a b c) (D a b' c')

Note that the `nominal` type arguments are equal, the `representational` type arguments can differ, but need to have a `Coercible` instance themself, and the `phantom` type arguments can be changed arbitrarily.

The third kind of instance exists for every `newtype NT = MkNT T` and comes in two variants, namely

    instance Coercible a T => Coercible a NT

    instance Coercible T b => Coercible NT b

This instance is only usable if the constructor `MkNT` is in scope.

If, as a library author of a type constructor like `Set a`, you want to prevent a user of your module to write `coerce :: Set T -> Set NT`, you need to set the role of `Set`'s type parameter to `nominal`, by writing

    type role Set nominal

For more details about this feature, please refer to Safe Coercions by Joachim Breitner, Richard A. Eisenberg, Simon Peyton Jones and Stephanie Weirich.

Since: ghc-prim-4.7.0.0

#### Instances

Instances details

HasDict (Coercible a b) (Coercion a b) # |
Instance details Defined in Data.Constraint Methods evidence :: Coercion a b -> Dict (Coercible a b) #

unsafeCoerceConstraint :: a :- b #

Coerce a dictionary unsafely from one type to another

unsafeDerive :: Coercible n o => (o -> n) -> t o :- t n #

Coerce a dictionary unsafely from one type to a newtype of that type

unsafeUnderive :: Coercible n o => (o -> n) -> t n :- t o #

Coerce a dictionary unsafely from a newtype of a type to the base type

# Sugar

unsafeApplicative :: forall m a. Monad m => (Applicative m => m a) -> m a #

Construct an Applicative instance from a Monad

unsafeAlternative :: forall m a. MonadPlus m => (Alternative m => m a) -> m a #

Construct an Alternative instance from a MonadPlus