stack-prism

Haskell lens prisms that use stack types

Monthly Downloads: 17

Programming language: Haskell

License: BSD 3-clause "New" or "Revised" License

Tags: Data    

Latest version: v0.1.4

README

A Haskell library for expressing and deriving stack prisms.

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, takes 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).

Representing constructor-destructor pairs as stack manipulators allows them to be composed more easily.