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 |

## Synopsis

- class a ~R# b => Coercible (a :: k0) (b :: k0)
- 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 :: k0) (b :: k0) #

`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*

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