{-# LANGUAGE DataKinds #-}
{-# LANGUAGE DeriveDataTypeable #-}
{-# LANGUAGE EmptyCase #-}
{-# LANGUAGE ExplicitForAll #-}
{-# LANGUAGE ExplicitNamespaces #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeOperators #-}
{-# OPTIONS_GHC -fno-warn-orphans #-}

-- | Provides type synonyms for logical connectives.
module Proof.Propositional
  ( type (/\),
    type (\/),
    Not,
    exfalso,
    orIntroL,
    orIntroR,
    orElim,
    andIntro,
    andElimL,
    andElimR,
    orAssocL,
    orAssocR,
    andAssocL,
    andAssocR,
    IsTrue (..),
    withWitness,
    Empty (..),
    withEmpty,
    withEmpty',
    refute,
    Inhabited (..),
    withInhabited,
    prove,
  )
where

import Data.Data (Data)
import Data.Type.Equality (gcastWith, (:~:) (..))
import Data.Typeable (Typeable)
import Data.Void
import Proof.Propositional.Empty
import Proof.Propositional.Inhabited
import Proof.Propositional.TH
import Unsafe.Coerce

type a /\ b = (a, b)

type a \/ b = Either a b

type Not a = a -> Void

infixr 2 \/

infixr 3 /\

exfalso :: a -> Not a -> b
exfalso a neg = absurd (neg a)

orIntroL :: a -> a \/ b
orIntroL = Left

orIntroR :: b -> a \/ b
orIntroR = Right

orElim :: a \/ b -> (a -> c) -> (b -> c) -> c
orElim aORb aTOc bTOc = either aTOc bTOc aORb

andIntro :: a -> b -> a /\ b
andIntro = (,)

andElimL :: a /\ b -> a
andElimL = fst

andElimR :: a /\ b -> b
andElimR = snd

andAssocL :: a /\ (b /\ c) -> (a /\ b) /\ c
andAssocL (a, (b, c)) = ((a, b), c)

andAssocR :: (a /\ b) /\ c -> a /\ (b /\ c)
andAssocR ((a, b), c) = (a, (b, c))

orAssocL :: a \/ (b \/ c) -> (a \/ b) \/ c
orAssocL (Left a) = Left (Left a)
orAssocL (Right (Left b)) = Left (Right b)
orAssocL (Right (Right c)) = Right c

orAssocR :: (a \/ b) \/ c -> a \/ (b \/ c)
orAssocR (Left (Left a)) = Left a
orAssocR (Left (Right b)) = Right (Left b)
orAssocR (Right c) = Right (Right c)

{- | Utility type to convert type-level (@'Bool'@-valued) predicate function
   into concrete witness data-type.
-}
data IsTrue (b :: Bool) where
  Witness :: IsTrue 'True

withWitness :: forall b r. IsTrue b -> (b ~ 'True => r) -> r
withWitness _ = gcastWith (unsafeCoerce (Refl :: () :~: ()) :: b :~: 'True)
{-# NOINLINE withWitness #-}

deriving instance Show (IsTrue b)

deriving instance Eq (IsTrue b)

deriving instance Ord (IsTrue b)

deriving instance Read (IsTrue 'True)

deriving instance Typeable IsTrue

deriving instance Data (IsTrue 'True)

instance {-# OVERLAPPABLE #-} (Inhabited a, Empty b) => Empty (a -> b) where
  eliminate f = eliminate (f trivial)

refute [t|0 :~: 1|]
refute [t|() :~: Int|]
refute [t| 'True :~: 'False|]
refute [t| 'False :~: 'True|]
refute [t| 'LT :~: 'GT|]
refute [t| 'LT :~: 'EQ|]
refute [t| 'EQ :~: 'LT|]
refute [t| 'EQ :~: 'GT|]
refute [t| 'GT :~: 'LT|]
refute [t| 'GT :~: 'EQ|]

prove [t|Bool|]
prove [t|Int|]
prove [t|Integer|]
prove [t|Word|]
prove [t|Double|]
prove [t|Float|]
prove [t|Char|]
prove [t|Ordering|]
prove [t|forall a. [a]|]
prove [t|Rational|]
prove [t|forall a. Maybe a|]
prove [t|forall n. n :~: n|]
prove [t|IsTrue 'True|]

instance Empty (IsTrue 'False) where
  eliminate = \case