Control.Bidirectional.Class

Documentation

class (c a, Constr c a) => Bidirectional (c :: k -> Constraint) (a :: k)

Class for non-recursive bidirectional instances, i. e. for instances, such that their components constraints (Constr) is ordinary instance.

Arguments:

• c class for which we declare bidirectional instance
• a data type for which instance is provided

For example:

instance Show a => Bidirectional Show [a] where
  type ConstrRec Show [a] = Show a

is correct Bidirectional instance. If you want to have bidirectional Show instance in backward constraint, use BidirectionalRec

Instances for this class are supposed to be generated by makeBidirectionalInstances or by decBidirectionalInstances.

Associated Types

type Constr c a :: Constraint

Constraint for backwards inference. Should not be recursively bidirectional (it means that all constraints should not be wrapped in Bidirectional, e. g. Show a but not Bidirectional Show a)

class (c a, ConstrRec c a, Bidirectional c a) => BidirectionalRec (c :: k -> Constraint) (a :: k)

Class for recursive bidirectional instances, i. e. for instances, such that components also have bidirectional instance. Use Bidirectional non-recursive variant, if you need only one step in backward direction.

Arguments:

• c class for which we declare bidirectional instance
• a data type for which instance is provided

For example, this is a nice recursive instance:

instance BidirectionalRec Show a => BidirectionalRec Show [a] where
  type ConstrRec Show [a] = BidirectionalRec Show a

but this one isn't (actually it should be a Bidirectional instance):

instance Show a => BidirectionalRec Show [a] where
  type ConstrRec Show [a] = Show a

Instances for this class are supposed to be generated by makeBidirectionalInstances or by decBidirectionalInstances.

Associated Types

type ConstrRec c a :: Constraint

Constraint for backwards inference. Should be recursively bidirectional (it means that all constraints should be wrapped in BidirectionalRec, e. g. BidirectionalRec Show a but not simply Show a)