{-
Copyright (c) Erik Hesselink & Sebastiaan Visser 2008

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   may be used to endorse or promote products derived from this software
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THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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-}
{-# LANGUAGE TypeOperators #-}
module Data.Label.Pure
( (:->)
, lens
, get
, set
, modify
-- , glue
, fmapL
)
where

import qualified Data.Label.Abstract as A
import Control.Applicative

type PureLens f a = A.Lens (->) f a

-- | Pure lens type specialized for pure accessor functions.

type (f :-> a) = PureLens f a

-- | Create a pure lens from a getter and a setter.
--
-- We expect the following law to hold:
--
-- > get l (set l a f) == a
--
-- Or, equivalently:
--
-- > set l (get l f) f == f

lens :: (f -> a) -> (a -> f -> f) -> f :-> a
lens g s = A.lens g (uncurry s)

-- | Getter for a pure lens.

get :: (f :-> a) -> f -> a
get = A.get

-- | Setter for a pure lens.

set :: (f :-> a) -> a -> f -> f
set = curry . A.set

-- | Modifier for a pure lens.

modify :: (f :-> a) -> (a -> a) -> f -> f
modify = curry . A.modify


fmapL :: (Alternative f) => (a :-> b) -> (f a :-> f b)
fmapL (A.Lens (A.Point g' s')) = lens g s where
    g a = fmap g' a
    s fb fa = ((\b a->s' (b,a)) <$> fb <*> fa) <|> fa

-- glue :: (Alternative f) => (o :-> f a) -> (a :-> b) -> (o :-> f b)
-- glue l1 l@(A.Lens (A.Point g2 s2)) = lens g s where
--     g o = fmap g2 (get l1 o)
--     s fb o = set l1 (set ll fb fa) o where
--         fa = get l1 o
--         ll = fmapL l

-- glue2 :: (Alternative f) => (o :-> f a) -> (a :-> f b) -> (o :-> f b)
-- glue2 l1 l2 = lens g s where