|Copyright||(c) The University of Glasgow 2001|
|License||BSD-style (see the file libraries/base/LICENSE)|
Safe coercions between data types.
More in-depth information can be found on the Roles wiki page
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.
This two-parameter class has instances for types
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 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 (
newtype) with three type arguments, which have roles
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.
© The University of Glasgow and others
Licensed under a BSD-style license (see top of the page).