Copyright | (c) The University of Glasgow 2001 |
---|---|
License | BSD-style (see the file libraries/base/LICENSE) |
Maintainer | [email protected] |
Stability | provisional |
Portability | portable |
Safe Haskell | Unsafe |
Language | Haskell2010 |
Safe coercions between data types.
More in-depth information can be found on the Roles wiki page
Since: 4.7.0.0
coerce :: Coercible * a b => a -> b Source
The function coerce
allows you to safely convert between values of types that have the same representation with no run-time overhead. In the simplest case you can use it instead of a newtype constructor, to go from the newtype's concrete type to the abstract type. But it also works in more complicated settings, e.g. converting a list of newtypes to a list of concrete types.
class (~R#) k0 k0 a b => Coercible k0 (a :: k0) (b :: k0) Source
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: 4.7.0.0
© The University of Glasgow and others
Licensed under a BSD-style license (see top of the page).
https://downloads.haskell.org/~ghc/8.2.1/docs/html/libraries/base-4.10.0.0/Data-Coerce.html