Safe Haskell | None |
---|---|

Language | Haskell98 |

- deriveStackPrisms :: Name -> Q [Dec]
- deriveStackPrismsWith :: (String -> String) -> Name -> Q [Dec]
- deriveStackPrismsFor :: [String] -> Name -> Q [Dec]
- type StackPrism a b = forall p f. (Choice p, Applicative f) => p a (f a) -> p b (f b)
- data h :- t = h :- t

# Deriving stack prisms

deriveStackPrisms :: Name -> Q [Dec] Source #

Derive stack prisms for a given datatype.

For example:

deriveStackPrisms ''Maybe

will create

_Just :: StackPrism (a :- t) (Maybe a :- t) _Nothing :: StackPrism t (Nothing :- t)

together with their implementations.

deriveStackPrismsWith :: (String -> String) -> Name -> Q [Dec] Source #

Derive stack prisms given a function that derives variable names from constructor names.

deriveStackPrismsFor :: [String] -> Name -> Q [Dec] Source #

Derive stack prisms given a list of variable names, one for each constructor.

# Re-exported types from Data.StackPrism

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.

Stack prisms can 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. `cons`

takes a head `x`

and tail `xs`

from the
stack and pushes `x : xs`

onto the stack, or, in the backward direction,
tries to take `x : xs`

from the stack and replaces it with its two
individual components.

Every constructor has its own stack prism version. You don't have to write them by hand; you can automatically generate them, either using Template Haskell (see module Data.StackPrism.TH) or using GHC generic programming (see module Data.StackPrism.Generic).