Container.hs

{- |
This module allows to access elements of arrays and finite maps
like elements of records.
This is especially useful for working with nested structures
consisting of arrays, maps and records.
-}
module Data.Accessor.Container
   (array, mapDefault, mapMaybe, ) where

import qualified Data.Accessor.Basic as Accessor

import qualified Data.Array as Array
import qualified Data.Map   as Map

import Prelude hiding (map)


array :: Array.Ix i => i -> Accessor.T (Array.Array i e) e
array i = Accessor.fromSetGet (\e a -> a Array.// [(i,e)]) (Array.! i)

{- |
Treats a finite map like an infinite map,
where all undefined elements are replaced by a default value.
-}
mapDefault :: Ord key => elem -> key -> Accessor.T (Map.Map key elem) elem
mapDefault deflt key =
   Accessor.fromSetGet (Map.insert key) (Map.findWithDefault deflt key)

{- |
Treats a finite map like an infinite map,
where all undefined elements are 'Nothing'
and defined elements are 'Just'.
-}
mapMaybe :: Ord key => key -> Accessor.T (Map.Map key elem) (Maybe elem)
mapMaybe key =
   Accessor.fromSetGet
      (\e m -> maybe (Map.delete key m) (flip (Map.insert key) m) e)
      (Map.lookup key)