Safe Haskell | None |
---|
- type StackPrism a b = forall p f. (Choice p, Applicative f) => p a (f a) -> p b (f b)
- stackPrism :: (a -> b) -> (b -> Maybe a) -> StackPrism a b
- forward :: StackPrism a b -> a -> b
- backward :: StackPrism a b -> b -> Maybe a
- data h :- t = h :- t
Stack prisms
type StackPrism a b = forall p f. (Choice p, Applicative f) => p a (f a) -> p b (f b)Source
A stack prism is a bidirectional isomorphism that is partial in the backward direction.
These prisms are compatible with the lens
library.
This can be used to express constructor-deconstructor pairs. For example:
nil :: StackPrism t ([a] :- t) nil = stackPrism f g where f t = [] :- t g ([] :- t) = Just t g _ = Nothing cons :: StackPrism (a :- [a] :- t) ([a] :- t) cons = stackPrism f g where f (x :- xs :- t) = (x : xs) :- t g ((x : xs) :- t) = Just (x :- xs :- t) g _ = Nothing
Here :-
can be read as 'cons', forming a stack of values. For example,
nil
pushes []
onto the stack; or, in the backward direction, tries to
remove []
from the stack. Representing constructor-destructor pairs as
stack manipulators allows them to be composed more easily.
Modules Data.StackPrism.Generic and Data.StackPrism.TH offer generic ways of deriving StackPrism
s for custom datatypes.
stackPrism :: (a -> b) -> (b -> Maybe a) -> StackPrism a bSource
Construct a prism.
forward :: StackPrism a b -> a -> bSource
Apply a prism in forward direction.
backward :: StackPrism a b -> b -> Maybe aSource
Apply a prism in backward direction.