Getter.hs

{-# LANGUAGE CPP #-}
{-# LANGUAGE Rank2Types #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE Trustworthy #-}

-- Disable the warnings generated by 'Control.Lens.Getter.to', 'ito', 'like', 'ilike'.
-- These functions are intended to produce 'Getters'. Without this constraint
-- users would get warnings when annotating types at uses of these functions.
{-# OPTIONS_GHC -Wno-redundant-constraints #-}

-------------------------------------------------------------------------------
-- |
-- Module      :  Control.Lens.Getter
-- Copyright   :  (C) 2012-16 Edward Kmett
-- License     :  BSD-style (see the file LICENSE)
-- Maintainer  :  Edward Kmett <ekmett@gmail.com>
-- Stability   :  provisional
-- Portability :  Rank2Types
--
--
-- A @'Getter' s a@ is just any function @(s -> a)@, which we've flipped
-- into continuation passing style, @(a -> r) -> s -> r@ and decorated
-- with 'Const' to obtain:
--
-- @type 'Getting' r s a = (a -> 'Const' r a) -> s -> 'Const' r s@
--
-- If we restrict access to knowledge about the type 'r', we could get:
--
-- @type 'Getter' s a = forall r. 'Getting' r s a@
--
-- However, for 'Getter' (but not for 'Getting') we actually permit any
-- functor @f@ which is an instance of both 'Functor' and 'Contravariant':
--
-- @type 'Getter' s a = forall f. ('Contravariant' f, 'Functor' f) => (a -> f a) -> s -> f s@
--
-- Everything you can do with a function, you can do with a 'Getter', but
-- note that because of the continuation passing style ('.') composes them
-- in the opposite order.
--
-- Since it is only a function, every 'Getter' obviously only retrieves a
-- single value for a given input.
--
-- A common question is whether you can combine multiple 'Getter's to
-- retrieve multiple values. Recall that all 'Getter's are 'Fold's and that
-- we have a @'Monoid' m => 'Applicative' ('Const' m)@ instance to play
-- with. Knowing this, we can use @'Data.Semigroup.<>'@ to glue 'Fold's
-- together:
--
-- >>> (1, 2, 3, 4, 5) ^.. (_2 <> _3 <> _5)
-- [2,3,5]
--
-------------------------------------------------------------------------------
module Control.Lens.Getter
  (
  -- * Getters
    Getter, IndexedGetter
  , Getting, IndexedGetting
  , Accessing
  -- * Building Getters
  , to
  , ito
  , like
  , ilike
  -- * Combinators for Getters and Folds
  , (^.)
  , view, views
  , use, uses
  , listening, listenings
  -- * Indexed Getters
  -- ** Indexed Getter Combinators
  , (^@.)
  , iview, iviews
  , iuse, iuses
  , ilistening, ilistenings
  -- * Implementation Details
  , Contravariant(..)
  , getting
  , Const(..)
  ) where

import Prelude ()

import Control.Lens.Internal.Indexed
import Control.Lens.Internal.Prelude
import Control.Lens.Type
import Control.Monad.Reader.Class as Reader
import Control.Monad.State        as State
import Control.Monad.Writer (MonadWriter (..))

-- $setup
-- >>> :set -XNoOverloadedStrings
-- >>> import Control.Lens
-- >>> import Control.Monad.State
-- >>> import Data.List.Lens
-- >>> import Data.Semigroup (Semigroup (..))
-- >>> import Debug.SimpleReflect.Expr
-- >>> import Debug.SimpleReflect.Vars as Vars hiding (f,g)
-- >>> let f :: Expr -> Expr; f = Debug.SimpleReflect.Vars.f
-- >>> let g :: Expr -> Expr; g = Debug.SimpleReflect.Vars.g

infixl 8 ^., ^@.

-------------------------------------------------------------------------------
-- Getters
-------------------------------------------------------------------------------

-- | Build an (index-preserving) 'Getter' from an arbitrary Haskell function.
--
-- @
-- 'Control.Lens.Getter.to' f '.' 'Control.Lens.Getter.to' g ≡ 'Control.Lens.Getter.to' (g '.' f)
-- @
--
-- @
-- a '^.' 'Control.Lens.Getter.to' f ≡ f a
-- @
--
-- >>> a ^.to f
-- f a
--
-- >>> ("hello","world")^.to snd
-- "world"
--
-- >>> 5^.to succ
-- 6
--
-- >>> (0, -5)^._2.to abs
-- 5
--
-- @
-- 'Control.Lens.Getter.to' :: (s -> a) -> 'IndexPreservingGetter' s a
-- @
to :: (Profunctor p, Contravariant f) => (s -> a) -> Optic' p f s a
to k = dimap k (contramap k)
{-# INLINE to #-}

-- |
-- @
-- 'ito' :: (s -> (i, a)) -> 'IndexedGetter' i s a
-- @
ito :: (Indexable i p, Contravariant f) => (s -> (i, a)) -> Over' p f s a
ito k = dimap k (contramap (snd . k)) . uncurry . indexed
{-# INLINE ito #-}


-- | Build an constant-valued (index-preserving) 'Getter' from an arbitrary Haskell value.
--
-- @
-- 'like' a '.' 'like' b ≡ 'like' b
-- a '^.' 'like' b ≡ b
-- a '^.' 'like' b ≡ a '^.' 'Control.Lens.Getter.to' ('const' b)
-- @
--
-- This can be useful as a second case 'failing' a 'Fold'
-- e.g. @foo `failing` 'like' 0@
--
-- @
-- 'like' :: a -> 'IndexPreservingGetter' s a
-- @
like :: (Profunctor p, Contravariant f, Functor f) => a -> Optic' p f s a
like a = to (const a)
{-# INLINE like #-}

-- |
-- @
-- 'ilike' :: i -> a -> 'IndexedGetter' i s a
-- @
ilike :: (Indexable i p, Contravariant f, Functor f) => i -> a -> Over' p f s a
ilike i a = ito (const (i, a))
{-# INLINE ilike #-}

-- | When you see this in a type signature it indicates that you can
-- pass the function a 'Lens', 'Getter',
-- 'Control.Lens.Traversal.Traversal', 'Control.Lens.Fold.Fold',
-- 'Control.Lens.Prism.Prism', 'Control.Lens.Iso.Iso', or one of
-- the indexed variants, and it will just \"do the right thing\".
--
-- Most 'Getter' combinators are able to be used with both a 'Getter' or a
-- 'Control.Lens.Fold.Fold' in limited situations, to do so, they need to be
-- monomorphic in what we are going to extract with 'Control.Applicative.Const'. To be compatible
-- with 'Lens', 'Control.Lens.Traversal.Traversal' and
-- 'Control.Lens.Iso.Iso' we also restricted choices of the irrelevant @t@ and
-- @b@ parameters.
--
-- If a function accepts a @'Getting' r s a@, then when @r@ is a 'Data.Monoid.Monoid', then
-- you can pass a 'Control.Lens.Fold.Fold' (or
-- 'Control.Lens.Traversal.Traversal'), otherwise you can only pass this a
-- 'Getter' or 'Lens'.
type Getting r s a = (a -> Const r a) -> s -> Const r s

-- | Used to consume an 'Control.Lens.Fold.IndexedFold'.
type IndexedGetting i m s a = Indexed i a (Const m a) -> s -> Const m s

-- | This is a convenient alias used when consuming (indexed) getters and (indexed) folds
-- in a highly general fashion.
type Accessing p m s a = p a (Const m a) -> s -> Const m s

-------------------------------------------------------------------------------
-- Getting Values
-------------------------------------------------------------------------------

-- | View the value pointed to by a 'Getter', 'Control.Lens.Iso.Iso' or
-- 'Lens' or the result of folding over all the results of a
-- 'Control.Lens.Fold.Fold' or 'Control.Lens.Traversal.Traversal' that points
-- at a monoidal value.
--
-- @
-- 'view' '.' 'Control.Lens.Getter.to' ≡ 'id'
-- @
--
-- >>> view (to f) a
-- f a
--
-- >>> view _2 (1,"hello")
-- "hello"
--
-- >>> view (to succ) 5
-- 6
--
-- >>> view (_2._1) ("hello",("world","!!!"))
-- "world"
--
--
-- As 'view' is commonly used to access the target of a 'Getter' or obtain a monoidal summary of the targets of a 'Fold',
-- It may be useful to think of it as having one of these more restricted signatures:
--
-- @
-- 'view' ::             'Getter' s a     -> s -> a
-- 'view' :: 'Data.Monoid.Monoid' m => 'Control.Lens.Fold.Fold' s m       -> s -> m
-- 'view' ::             'Control.Lens.Iso.Iso'' s a       -> s -> a
-- 'view' ::             'Lens'' s a      -> s -> a
-- 'view' :: 'Data.Monoid.Monoid' m => 'Control.Lens.Traversal.Traversal'' s m -> s -> m
-- @
--
-- In a more general setting, such as when working with a 'Monad' transformer stack you can use:
--
-- @
-- 'view' :: 'MonadReader' s m             => 'Getter' s a     -> m a
-- 'view' :: ('MonadReader' s m, 'Data.Monoid.Monoid' a) => 'Control.Lens.Fold.Fold' s a       -> m a
-- 'view' :: 'MonadReader' s m             => 'Control.Lens.Iso.Iso'' s a       -> m a
-- 'view' :: 'MonadReader' s m             => 'Lens'' s a      -> m a
-- 'view' :: ('MonadReader' s m, 'Data.Monoid.Monoid' a) => 'Control.Lens.Traversal.Traversal'' s a -> m a
-- @
view :: MonadReader s m => Getting a s a -> m a
view l = Reader.asks (getConst #. l Const)
{-# INLINE view #-}

-- | View a function of the value pointed to by a 'Getter' or 'Lens' or the result of
-- folding over the result of mapping the targets of a 'Control.Lens.Fold.Fold' or
-- 'Control.Lens.Traversal.Traversal'.
--
-- @
-- 'views' l f ≡ 'view' (l '.' 'Control.Lens.Getter.to' f)
-- @
--
-- >>> views (to f) g a
-- g (f a)
--
-- >>> views _2 length (1,"hello")
-- 5
--
-- As 'views' is commonly used to access the target of a 'Getter' or obtain a monoidal summary of the targets of a 'Fold',
-- It may be useful to think of it as having one of these more restricted signatures:
--
-- @
-- 'views' ::             'Getter' s a     -> (a -> r) -> s -> r
-- 'views' :: 'Data.Monoid.Monoid' m => 'Control.Lens.Fold.Fold' s a       -> (a -> m) -> s -> m
-- 'views' ::             'Control.Lens.Iso.Iso'' s a       -> (a -> r) -> s -> r
-- 'views' ::             'Lens'' s a      -> (a -> r) -> s -> r
-- 'views' :: 'Data.Monoid.Monoid' m => 'Control.Lens.Traversal.Traversal'' s a -> (a -> m) -> s -> m
-- @
--
-- In a more general setting, such as when working with a 'Monad' transformer stack you can use:
--
-- @
-- 'views' :: 'MonadReader' s m             => 'Getter' s a     -> (a -> r) -> m r
-- 'views' :: ('MonadReader' s m, 'Data.Monoid.Monoid' r) => 'Control.Lens.Fold.Fold' s a       -> (a -> r) -> m r
-- 'views' :: 'MonadReader' s m             => 'Control.Lens.Iso.Iso'' s a       -> (a -> r) -> m r
-- 'views' :: 'MonadReader' s m             => 'Lens'' s a      -> (a -> r) -> m r
-- 'views' :: ('MonadReader' s m, 'Data.Monoid.Monoid' r) => 'Control.Lens.Traversal.Traversal'' s a -> (a -> r) -> m r
-- @
--
-- @
-- 'views' :: 'MonadReader' s m => 'Getting' r s a -> (a -> r) -> m r
-- @
views :: MonadReader s m => LensLike' (Const r) s a -> (a -> r) -> m r
views l f = Reader.asks (coerce l f)
{-# INLINE views #-}

-- | View the value pointed to by a 'Getter' or 'Lens' or the
-- result of folding over all the results of a 'Control.Lens.Fold.Fold' or
-- 'Control.Lens.Traversal.Traversal' that points at a monoidal values.
--
-- This is the same operation as 'view' with the arguments flipped.
--
-- The fixity and semantics are such that subsequent field accesses can be
-- performed with ('Prelude..').
--
-- >>> (a,b)^._2
-- b
--
-- >>> ("hello","world")^._2
-- "world"
--
-- >>> import Data.Complex
-- >>> ((0, 1 :+ 2), 3)^._1._2.to magnitude
-- 2.23606797749979
--
-- @
-- ('^.') ::             s -> 'Getter' s a     -> a
-- ('^.') :: 'Data.Monoid.Monoid' m => s -> 'Control.Lens.Fold.Fold' s m       -> m
-- ('^.') ::             s -> 'Control.Lens.Iso.Iso'' s a       -> a
-- ('^.') ::             s -> 'Lens'' s a      -> a
-- ('^.') :: 'Data.Monoid.Monoid' m => s -> 'Control.Lens.Traversal.Traversal'' s m -> m
-- @
(^.) :: s -> Getting a s a -> a
s ^. l = getConst (l Const s)
{-# INLINE (^.) #-}

-------------------------------------------------------------------------------
-- MonadState
-------------------------------------------------------------------------------

-- | Use the target of a 'Lens', 'Control.Lens.Iso.Iso', or
-- 'Getter' in the current state, or use a summary of a
-- 'Control.Lens.Fold.Fold' or 'Control.Lens.Traversal.Traversal' that points
-- to a monoidal value.
--
-- >>> evalState (use _1) (a,b)
-- a
--
-- >>> evalState (use _1) ("hello","world")
-- "hello"
--
-- @
-- 'use' :: 'MonadState' s m             => 'Getter' s a     -> m a
-- 'use' :: ('MonadState' s m, 'Data.Monoid.Monoid' r) => 'Control.Lens.Fold.Fold' s r       -> m r
-- 'use' :: 'MonadState' s m             => 'Control.Lens.Iso.Iso'' s a       -> m a
-- 'use' :: 'MonadState' s m             => 'Lens'' s a      -> m a
-- 'use' :: ('MonadState' s m, 'Data.Monoid.Monoid' r) => 'Control.Lens.Traversal.Traversal'' s r -> m r
-- @
use :: MonadState s m => Getting a s a -> m a
use l = State.gets (view l)
{-# INLINE use #-}

-- | Use the target of a 'Lens', 'Control.Lens.Iso.Iso' or
-- 'Getter' in the current state, or use a summary of a
-- 'Control.Lens.Fold.Fold' or 'Control.Lens.Traversal.Traversal' that
-- points to a monoidal value.
--
-- >>> evalState (uses _1 length) ("hello","world")
-- 5
--
-- @
-- 'uses' :: 'MonadState' s m             => 'Getter' s a     -> (a -> r) -> m r
-- 'uses' :: ('MonadState' s m, 'Data.Monoid.Monoid' r) => 'Control.Lens.Fold.Fold' s a       -> (a -> r) -> m r
-- 'uses' :: 'MonadState' s m             => 'Lens'' s a      -> (a -> r) -> m r
-- 'uses' :: 'MonadState' s m             => 'Control.Lens.Iso.Iso'' s a       -> (a -> r) -> m r
-- 'uses' :: ('MonadState' s m, 'Data.Monoid.Monoid' r) => 'Control.Lens.Traversal.Traversal'' s a -> (a -> r) -> m r
-- @
--
-- @
-- 'uses' :: 'MonadState' s m => 'Getting' r s t a b -> (a -> r) -> m r
-- @
uses :: MonadState s m => LensLike' (Const r) s a -> (a -> r) -> m r
uses l f = State.gets (views l f)
{-# INLINE uses #-}

-- | This is a generalized form of 'listen' that only extracts the portion of
-- the log that is focused on by a 'Getter'. If given a 'Fold' or a 'Traversal'
-- then a monoidal summary of the parts of the log that are visited will be
-- returned.
--
-- @
-- 'listening' :: 'MonadWriter' w m             => 'Getter' w u     -> m a -> m (a, u)
-- 'listening' :: 'MonadWriter' w m             => 'Lens'' w u      -> m a -> m (a, u)
-- 'listening' :: 'MonadWriter' w m             => 'Iso'' w u       -> m a -> m (a, u)
-- 'listening' :: ('MonadWriter' w m, 'Monoid' u) => 'Fold' w u       -> m a -> m (a, u)
-- 'listening' :: ('MonadWriter' w m, 'Monoid' u) => 'Traversal'' w u -> m a -> m (a, u)
-- 'listening' :: ('MonadWriter' w m, 'Monoid' u) => 'Prism'' w u     -> m a -> m (a, u)
-- @
listening :: MonadWriter w m => Getting u w u -> m a -> m (a, u)
listening l m = do
  (a, w) <- listen m
  return (a, view l w)
{-# INLINE listening #-}

-- | This is a generalized form of 'listen' that only extracts the portion of
-- the log that is focused on by a 'Getter'. If given a 'Fold' or a 'Traversal'
-- then a monoidal summary of the parts of the log that are visited will be
-- returned.
--
-- @
-- 'ilistening' :: 'MonadWriter' w m             => 'IndexedGetter' i w u     -> m a -> m (a, (i, u))
-- 'ilistening' :: 'MonadWriter' w m             => 'IndexedLens'' i w u      -> m a -> m (a, (i, u))
-- 'ilistening' :: ('MonadWriter' w m, 'Monoid' u) => 'IndexedFold' i w u       -> m a -> m (a, (i, u))
-- 'ilistening' :: ('MonadWriter' w m, 'Monoid' u) => 'IndexedTraversal'' i w u -> m a -> m (a, (i, u))
-- @
ilistening :: MonadWriter w m => IndexedGetting i (i, u) w u -> m a -> m (a, (i, u))
ilistening l m = do
  (a, w) <- listen m
  return (a, iview l w)
{-# INLINE ilistening #-}

-- | This is a generalized form of 'listen' that only extracts the portion of
-- the log that is focused on by a 'Getter'. If given a 'Fold' or a 'Traversal'
-- then a monoidal summary of the parts of the log that are visited will be
-- returned.
--
-- @
-- 'listenings' :: 'MonadWriter' w m             => 'Getter' w u     -> (u -> v) -> m a -> m (a, v)
-- 'listenings' :: 'MonadWriter' w m             => 'Lens'' w u      -> (u -> v) -> m a -> m (a, v)
-- 'listenings' :: 'MonadWriter' w m             => 'Iso'' w u       -> (u -> v) -> m a -> m (a, v)
-- 'listenings' :: ('MonadWriter' w m, 'Monoid' v) => 'Fold' w u       -> (u -> v) -> m a -> m (a, v)
-- 'listenings' :: ('MonadWriter' w m, 'Monoid' v) => 'Traversal'' w u -> (u -> v) -> m a -> m (a, v)
-- 'listenings' :: ('MonadWriter' w m, 'Monoid' v) => 'Prism'' w u     -> (u -> v) -> m a -> m (a, v)
-- @
listenings :: MonadWriter w m => Getting v w u -> (u -> v) -> m a -> m (a, v)
listenings l uv m = do
  (a, w) <- listen m
  return (a, views l uv w)
{-# INLINE listenings #-}

-- | This is a generalized form of 'listen' that only extracts the portion of
-- the log that is focused on by a 'Getter'. If given a 'Fold' or a 'Traversal'
-- then a monoidal summary of the parts of the log that are visited will be
-- returned.
--
-- @
-- 'ilistenings' :: 'MonadWriter' w m             => 'IndexedGetter' w u     -> (i -> u -> v) -> m a -> m (a, v)
-- 'ilistenings' :: 'MonadWriter' w m             => 'IndexedLens'' w u      -> (i -> u -> v) -> m a -> m (a, v)
-- 'ilistenings' :: ('MonadWriter' w m, 'Monoid' v) => 'IndexedFold' w u       -> (i -> u -> v) -> m a -> m (a, v)
-- 'ilistenings' :: ('MonadWriter' w m, 'Monoid' v) => 'IndexedTraversal'' w u -> (i -> u -> v) -> m a -> m (a, v)
-- @
ilistenings :: MonadWriter w m => IndexedGetting i v w u -> (i -> u -> v) -> m a -> m (a, v)
ilistenings l iuv m = do
  (a, w) <- listen m
  return (a, iviews l iuv w)
{-# INLINE ilistenings #-}

------------------------------------------------------------------------------
-- Indexed Getters
------------------------------------------------------------------------------

-- | View the index and value of an 'IndexedGetter' into the current environment as a pair.
--
-- When applied to an 'IndexedFold' the result will most likely be a nonsensical monoidal summary of
-- the indices tupled with a monoidal summary of the values and probably not whatever it is you wanted.
iview :: MonadReader s m => IndexedGetting i (i,a) s a -> m (i,a)
iview l = asks (getConst #. l (Indexed $ \i -> Const #. (,) i))
{-# INLINE iview #-}

-- | View a function of the index and value of an 'IndexedGetter' into the current environment.
--
-- When applied to an 'IndexedFold' the result will be a monoidal summary instead of a single answer.
--
-- @
-- 'iviews' ≡ 'Control.Lens.Fold.ifoldMapOf'
-- @
iviews :: MonadReader s m => IndexedGetting i r s a -> (i -> a -> r) -> m r
iviews l f = asks (coerce l f)
{-# INLINE iviews #-}

-- | Use the index and value of an 'IndexedGetter' into the current state as a pair.
--
-- When applied to an 'IndexedFold' the result will most likely be a nonsensical monoidal summary of
-- the indices tupled with a monoidal summary of the values and probably not whatever it is you wanted.
iuse :: MonadState s m => IndexedGetting i (i,a) s a -> m (i,a)
iuse l = gets (getConst #. l (Indexed $ \i -> Const #. (,) i))
{-# INLINE iuse #-}

-- | Use a function of the index and value of an 'IndexedGetter' into the current state.
--
-- When applied to an 'IndexedFold' the result will be a monoidal summary instead of a single answer.
iuses :: MonadState s m => IndexedGetting i r s a -> (i -> a -> r) -> m r
iuses l f = gets (coerce l f)
{-# INLINE iuses #-}

-- | View the index and value of an 'IndexedGetter' or 'IndexedLens'.
--
-- This is the same operation as 'iview' with the arguments flipped.
--
-- The fixity and semantics are such that subsequent field accesses can be
-- performed with ('Prelude..').
--
-- @
-- ('^@.') :: s -> 'IndexedGetter' i s a -> (i, a)
-- ('^@.') :: s -> 'IndexedLens'' i s a  -> (i, a)
-- @
--
-- The result probably doesn't have much meaning when applied to an 'IndexedFold'.
(^@.) :: s -> IndexedGetting i (i, a) s a -> (i, a)
s ^@. l = getConst $ l (Indexed $ \i -> Const #. (,) i) s
{-# INLINE (^@.) #-}

-- | Coerce a 'Getter'-compatible 'Optical' to an 'Optical''. This
-- is useful when using a 'Traversal' that is not simple as a 'Getter' or a
-- 'Fold'.
--
-- @
-- 'getting' :: 'Traversal' s t a b          -> 'Fold' s a
-- 'getting' :: 'Lens' s t a b               -> 'Getter' s a
-- 'getting' :: 'IndexedTraversal' i s t a b -> 'IndexedFold' i s a
-- 'getting' :: 'IndexedLens' i s t a b      -> 'IndexedGetter' i s a
-- @
getting :: (Profunctor p, Profunctor q, Functor f, Contravariant f)
        => Optical p q f s t a b -> Optical' p q f s a
getting l f = rmap phantom . l $ rmap phantom f