Safe Haskell | Safe |
---|---|

Language | Haskell98 |

- mkPrismList :: (Generic a, MkPrismList (Rep a)) => StackPrisms a
- type StackPrisms a = PrismList (Rep a) a
- data family PrismList (f :: * -> *) (a :: *)
- 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

mkPrismList :: (Generic a, MkPrismList (Rep a)) => StackPrisms a Source #

Derive a list of stack prisms. For more information on the shape of a
`PrismList`

, please see the documentation below.

type StackPrisms a = PrismList (Rep a) a Source #

Convenient shorthand for a `PrismList`

indexed by a type and its generic
representation.

data family PrismList (f :: * -> *) (a :: *) Source #

A data family that is indexed on the building blocks from representation
types from `GHC.Generics`

. It builds up to a list of prisms, one for each
constructor in the generic representation. The list is wrapped in the unary
constructor `PrismList`

. Within that constructor, the prisms are separated by
the right-associative binary infix constructor `:&`

. Finally, the individual
prisms are wrapped in the unary constructor `P`

.

As an example, here is how to define the prisms `nil`

and `cons`

for `[a]`

,
which is an instance of `Generic`

:

nil :: StackPrism t ([a] :- t) cons :: StackPrism (a :- [a] :- t) ([a] :- t) PrismList (P nil :& P cons) = mkPrismList :: StackPrisms [a]

# 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).