Lens.hs

{-# LANGUAGE RankNTypes #-}

-- | Compose polymorphic record updates with /van Laarhoven/ lenses
module Mini.Optics.Lens (
  -- * Type
  Lens,

  -- * Construction
  lens,

  -- * Operations
  view,
  over,
  set,
) where

import Control.Applicative (
  Const (
    Const,
    getConst
  ),
 )
import Data.Functor.Identity (
  Identity (
    Identity,
    runIdentity
  ),
 )

{-
 - Type
 -}

-- | A reference updating structures from /s/ to /t/ and fields from /a/ to /b/
type Lens s t a b = forall f. (Functor f) => (a -> f b) -> (s -> f t)

{-
 - Construction
 -}

-- | Make a lens from a getter and a setter
lens :: (s -> a) -> (s -> b -> t) -> Lens s t a b
lens sa sbt ab s = sbt s <$> ab (sa s)

{-
 - Operations
 -}

-- | Fetch the field referenced by a lens from a structure
view :: Lens s t a b -> s -> a
view o = getConst . o Const

-- | Update the field referenced by a lens with an operation on a structure
over :: Lens s t a b -> (a -> b) -> s -> t
over o ab = runIdentity . o (Identity . ab)

-- | Overwrite the field referenced by a lens with a value on a structure
set :: Lens s t a b -> b -> s -> t
set o b = runIdentity . o (const $ Identity b)