{-# LANGUAGE Trustworthy #-}
{-# LANGUAGE NoImplicitPrelude #-}

-----------------------------------------------------------------------------
-- This is a non-exposed internal module.
--
-- This code contains utility function and data structures that are used
-- to improve the efficiency of several instances in the Data.* namespace.
-----------------------------------------------------------------------------
module Data.Functor.Utils where

import Data.Coerce (Coercible, coerce)
import GHC.Base ( Applicative(..), Functor(..), Maybe(..), Monoid(..), Ord(..)
                , ($), otherwise )

-- We don't expose Max and Min because, as Edward Kmett pointed out to me,
-- there are two reasonable ways to define them. One way is to use Maybe, as we
-- do here; the other way is to impose a Bounded constraint on the Monoid
-- instance. We may eventually want to add both versions, but we don't want to
-- trample on anyone's toes by imposing Max = MaxMaybe.

newtype Max a = Max {getMax :: Maybe a}
newtype Min a = Min {getMin :: Maybe a}

-- | @since 4.8.0.0
instance Ord a => Monoid (Max a) where
  mempty = Max Nothing

  {-# INLINE mappend #-}
  m `mappend` Max Nothing = m
  Max Nothing `mappend` n = n
  (Max m@(Just x)) `mappend` (Max n@(Just y))
    | x >= y    = Max m
    | otherwise = Max n

-- | @since 4.8.0.0
instance Ord a => Monoid (Min a) where
  mempty = Min Nothing

  {-# INLINE mappend #-}
  m `mappend` Min Nothing = m
  Min Nothing `mappend` n = n
  (Min m@(Just x)) `mappend` (Min n@(Just y))
    | x <= y    = Min m
    | otherwise = Min n

-- left-to-right state transformer
newtype StateL s a = StateL { runStateL :: s -> (s, a) }

-- | @since 4.0
instance Functor (StateL s) where
    fmap f (StateL k) = StateL $ \ s -> let (s', v) = k s in (s', f v)

-- | @since 4.0
instance Applicative (StateL s) where
    pure x = StateL (\ s -> (s, x))
    StateL kf <*> StateL kv = StateL $ \ s ->
        let (s', f) = kf s
            (s'', v) = kv s'
        in (s'', f v)
    liftA2 f (StateL kx) (StateL ky) = StateL $ \s ->
        let (s', x) = kx s
            (s'', y) = ky s'
        in (s'', f x y)

-- right-to-left state transformer
newtype StateR s a = StateR { runStateR :: s -> (s, a) }

-- | @since 4.0
instance Functor (StateR s) where
    fmap f (StateR k) = StateR $ \ s -> let (s', v) = k s in (s', f v)

-- | @since 4.0
instance Applicative (StateR s) where
    pure x = StateR (\ s -> (s, x))
    StateR kf <*> StateR kv = StateR $ \ s ->
        let (s', v) = kv s
            (s'', f) = kf s'
        in (s'', f v)
    liftA2 f (StateR kx) (StateR ky) = StateR $ \ s ->
        let (s', y) = ky s
            (s'', x) = kx s'
        in (s'', f x y)

-- See Note [Function coercion]
(#.) :: Coercible b c => (b -> c) -> (a -> b) -> (a -> c)
(#.) _f = coerce
{-# INLINE (#.) #-}

{-
Note [Function coercion]
~~~~~~~~~~~~~~~~~~~~~~~

Several functions here use (#.) instead of (.) to avoid potential efficiency
problems relating to #7542. The problem, in a nutshell:

If N is a newtype constructor, then N x will always have the same
representation as x (something similar applies for a newtype deconstructor).
However, if f is a function,

N . f = \x -> N (f x)

This looks almost the same as f, but the eta expansion lifts it--the lhs could
be _|_, but the rhs never is. This can lead to very inefficient code.  Thus we
steal a technique from Shachaf and Edward Kmett and adapt it to the current
(rather clean) setting. Instead of using  N . f,  we use  N #. f, which is
just

coerce f `asTypeOf` (N . f)

That is, we just *pretend* that f has the right type, and thanks to the safety
of coerce, the type checker guarantees that nothing really goes wrong. We still
have to be a bit careful, though: remember that #. completely ignores the
*value* of its left operand.
-}
