Data.Constraint.Bare

Extract a Constraint from a Dict to manipulate it as a plain value. It is supposed to be used in compiler plugins -- to move around instances of type classes.

Documentation

data BareConstraint :: Constraint -> Type

An unsafeCoerced pointer to a Constraint, such as a class function dictionary.

pattern DictValue :: BareConstraint c -> Dict c

Extract the constraint inside the Dict GADT as if it was an ordinary value of kind Type.

It exploits the feature of the GHC core language -- representing constraints as ordinary type arguments of a function. Thus, I unsafeCoerce between a function with one argument and a function with no arguments and one constraint.

This pattern has never been tested with multiple constraints.

dictToBare :: forall c. Dict c -> BareConstraint c

Extract a Constraint from a Dict

bareToDict :: forall c. BareConstraint c -> Dict c

Wrap a Constraint into a Dict

withBareConstraint :: forall c r. BareConstraint c -> (c => r) -> r

Provide a constraint to a function using BareConstraint. This allows to provide constraints on-demand (lazily), rather than eagerly pattern-matching against Dict before executing the function.