Lens-like Access to Dictionary-like Object

gistfile1.hs

{-# LANGUAGE RankNTypes #-}

module AList where

import Control.Lens

type AList t = [(String, t)]

alistGet :: String -> AList t -> Maybe t
alistGet key ((k,v):xs)
	| k == key = Just v
	| otherwise = alistGet key xs
alistGet _ [] = Nothing

alistSet :: String -> AList t -> t -> AList t
alistSet key ((k,v):xs) value
	| k == key = (key, value):xs
	| otherwise = (k,v):(alistSet key xs value)
alistSet key [] value = [(key, value)]

-- Bad Lens. Violates Lens rules 1 and 2.
alistLens :: String -> Lens (AList t) (AList t) (Maybe t) t
alistLens key = lens (alistGet key) (alistSet key)