Summer.hs

{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE UndecidableSuperClasses #-}
{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE InstanceSigs #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE ViewPatterns #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE PatternSynonyms #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE RoleAnnotations #-}
{-# LANGUAGE DataKinds #-}
{- |
Module: Data.Summer
Copyright: (c) Samuel Schlesinger 2020
License: MIT
Maintainer: sgschlesinger@gmail.com
Stability: experimental
Portability: non-portable
Description: Extensible sums
-}
module Data.Summer
  ( -- * The extensible sum type and its associated pattern for convenience
    Sum
  , pattern Inj
  , tag
  -- * Construction and Deconstruction
  , inject
  , inspect
  , consider
  , considerFirst
  , variant
  , Match(match, override, unmatch)
  , Unmatch
  -- * Type families
  , TagIn
  , HasTagIn
  , Delete
  , HaveSameTagsIn
  , Matcher
  -- * Weakening extensible sums
  , Weaken(weaken)
  , noOpWeaken
  -- * Transforming extensible sums
  , inmap
  , smap
  -- * Applying Polymorphic Functions
  , ApplyFunction(apply)
  , type ForAll
  ) where

import Control.Exception (catch, SomeException)
import Unsafe.Coerce (unsafeCoerce)
import GHC.Exts (Any, Constraint)
import GHC.TypeLits (Nat, KnownNat, natVal, type (+))
import Data.Proxy (Proxy(Proxy))
import Data.Kind (Type)
import Control.Monad (unless)
import Generics.SOP (Generic(..))
import qualified Generics.SOP as SOP
import Data.Profunctor (Profunctor(dimap), Choice(..))
import Data.ForAll (type ForAll)

-- | The extensible sum type, allowing inhabitants to be of any of the
-- types in the given type list.
data Sum (xs :: [*]) = UnsafeInj {-# UNPACK #-} !Word Any

-- | A prism which operates on a chosen variant of a 'Sum'
variant :: forall a b xs p f. (a `HasTagIn` xs, Applicative f, Choice p) => p a (f b) -> p (Sum xs) (f (Sum (Replace a b xs)))
variant p = dimap try replace (left' p) where
  try :: Sum xs -> Either a (Sum (Replace a b xs))
  try (UnsafeInj t x) = if t == tag @a @xs then Left (unsafeCoerce x) else Right (UnsafeInj t x)
  replace :: Either (f b) (Sum (Replace a b xs)) -> f (Sum (Replace a b xs))
  replace = \case
    Left fb -> fmap (UnsafeInj (tag @a @xs) . unsafeCoerce) fb
    Right s -> pure s

-- | Type family for replacing one type in a type level list with another
type family Replace x y xs where
  Replace x y (x ': xs) = y ': xs
  Replace x y (z ': xs) = z ': Replace x y xs
  Replace x y '[] = '[]

instance Generic (Sum '[]) where
  type Code (Sum '[]) = '[]
  from a = error "Generics.SOP.from: Impossible to create an empty sum"
  to a = error "Generics.SOP.to: Impossible to create an empty sum"

instance Generic (Sum xs) => Generic (Sum (x ': xs)) where
  type Code (Sum (x ': xs)) = '[x] ': Code (Sum xs)
  from (UnsafeInj 0 x) = SOP.SOP (SOP.Z (SOP.I (unsafeCoerce x) SOP.:* SOP.Nil))
  from xs = SOP.SOP (SOP.S ns) where
    SOP.SOP ns = from (unsafeForgetFirst xs)
  to (SOP.SOP ns) = case ns of
    SOP.Z (SOP.I x SOP.:* SOP.Nil) -> UnsafeInj 0 (unsafeCoerce x)
    SOP.S s -> recall (to (SOP.SOP s))
    where
      recall :: Sum xs -> Sum (x ': xs)
      recall (UnsafeInj i a) = UnsafeInj (i + 1) a

type role Sum representational

-- | A pattern to match on for convenience. Without this, the user facing
-- interface is rather baroque.
pattern Inj :: forall x xs. (x `HasTagIn` xs) => x -> Sum xs
pattern Inj x <- (inspect -> Just x)
  where
    Inj x = inject x

-- | A type family for computing the tag of a given type in an extensible
-- sum. In practice, this means computing the first index of the given type in
-- the list.
type family TagIn (x :: k) (xs :: [k]) where
  TagIn x (x ': xs) = 0
  TagIn x (y ': xs) = 1 + TagIn x xs

-- | A class that is used for convenience in order to make certain
-- type signatures read more clearly.
class KnownNat (x `TagIn` xs) => x `HasTagIn` xs
instance KnownNat (x `TagIn` xs) => x `HasTagIn` xs

-- | Computes the tag of the given type in the given type level list.
tag :: forall x xs. x `HasTagIn` xs => Word
tag = fromInteger $ natVal (Proxy @(TagIn x xs))
{-# INLINE CONLIKE tag #-}

-- | Injects a type into the extensible sum.
inject :: forall x xs. (x `HasTagIn` xs) => x -> Sum xs
inject x = UnsafeInj (tag @x @xs) (unsafeCoerce x)
{-# INLINE CONLIKE inject #-}

-- | Inspects an extensible sum for a particular type.
inspect :: forall x xs. (x `HasTagIn` xs) => Sum xs -> Maybe x
inspect (UnsafeInj tag' x) = if tag @x @xs == tag' then Just (unsafeCoerce x) else Nothing
{-# INLINE CONLIKE inspect #-}

-- | A type family for deleting the given type from a list
type family Delete (x :: k) (xs :: [k]) :: [k] where
  Delete x (x ': xs) = xs
  Delete x (y ': xs) = y ': Delete x xs
  Delete x '[] = '[]

-- | Consider a certain type, discarding it as an option if it is not the
-- correct one.
consider :: forall x xs. (x `HasTagIn` xs) => Sum xs -> Either (Sum (Delete x xs)) x
consider (UnsafeInj tag' x) =
  if tag' == tag @x @xs
    then Right (unsafeCoerce x)
    else Left (UnsafeInj (if tag' >= tag @x @xs then tag' - 1 else tag') x)
{-# INLINE CONLIKE consider #-}

-- | Consider the first type in the list of possibilities, a useful
-- specialization for type inference.
considerFirst :: forall x xs. Sum (x ': xs) -> Either (Sum xs) x
considerFirst = consider
{-# INLINE CONLIKE considerFirst #-}

-- | Transforms one type in the sum into another.
inmap :: forall x y xs. (x `HasTagIn` xs, y `HasTagIn` xs) => (x -> y) -> Sum xs -> Sum xs
inmap f uv@(UnsafeInj tag' x) = if tag' == tag @x @xs then UnsafeInj (tag @y @xs) (unsafeCoerce (f (unsafeCoerce x))) else uv
{-# INLINE CONLIKE inmap #-}

-- | Transform one type in one sum into another type in another sum.
smap :: forall x y xs ys. (Weaken (Delete x xs) ys, x `HasTagIn` xs, y `HasTagIn` ys) => (x -> y) -> Sum xs -> Sum ys
smap f uv@(UnsafeInj tag' x) = if tag' == tag @x @xs then UnsafeInj (tag @y @ys) (unsafeCoerce (f (unsafeCoerce x))) else weaken (unsafeForget @x uv)
{-# INLINE CONLIKE smap #-}

-- | A class which checks that every type has the same tag in the first
-- list as the second. In other words, checks if the first list is a prefix
-- of the other.
class HaveSameTagsIn xs ys
instance {-# OVERLAPPABLE #-} HaveSameTagsIn '[] ys
instance {-# OVERLAPPABLE #-} HaveSameTagsIn xs ys => HaveSameTagsIn (x ': xs) (x ': ys)

-- | A free version of weakening, where all you're doing is adding
-- more possibilities at exclusively higher tags.
noOpWeaken :: forall xs ys. (xs `HaveSameTagsIn` ys) => Sum xs -> Sum ys
noOpWeaken = unsafeCoerce
{-# INLINE CONLIKE noOpWeaken #-}

unsafeForget :: forall x xs. x `HasTagIn` xs => Sum xs -> Sum (Delete x xs)
unsafeForget (UnsafeInj tag' x) = if tag' < tag @x @xs then UnsafeInj tag' x
                             else if tag' == tag @x @xs then error "unsafeForget: you can't forget the truth"
                             else UnsafeInj (tag' - 1) x


unsafeForgetFirst :: Sum (x ': xs) -> Sum xs
unsafeForgetFirst (UnsafeInj tag' x) = UnsafeInj (tag' - 1) x
{-# INLINE CONLIKE unsafeForget #-}

-- | Testing extensible sums for equality.
instance (Eq (Sum xs), Eq x) => Eq (Sum (x ': xs)) where
  uv@(UnsafeInj tag' x) == uv'@(UnsafeInj tag'' x')
    | tag' == tag'' =
        if tag' == 0
          then unsafeCoerce @_ @x x == unsafeCoerce @_ @x x'
          else unsafeForgetFirst uv == unsafeForgetFirst uv'
    | otherwise = False
  {-# INLINE CONLIKE (==) #-}
instance Eq (Sum '[]) where
  (==) = error "(==) base case: impossible by construction"

-- | Transforming one sum into a sum which contains all of the same types
class Weaken xs ys where
  weaken :: Sum xs -> Sum ys
instance (Weaken xs ys, x `HasTagIn` ys) => Weaken (x ': xs) ys where
  weaken uv@(UnsafeInj tag' x) =
    if tag' == 0
      then UnsafeInj (tag @x @ys) x
      else let UnsafeInj tag'' _ = weaken @xs @ys (unsafeForgetFirst uv) in UnsafeInj tag'' x
  {-# INLINE CONLIKE weaken #-}
instance Weaken '[] ys where
  weaken = error "weaken base case: impossible by construction"
  {-# INLINE CONLIKE weaken #-}

-- | The scott encoding of an extensible sum
type family Matcher xs r :: Type where
  Matcher '[] r = r
  Matcher (x ': xs) r = (x -> r) -> Matcher xs r

-- | A typeclass for scott encoding extensible sums
class Match xs where
  match :: forall r. Sum xs -> Matcher xs r
  unmatch :: (forall r. Matcher xs r) -> Sum xs
  override :: forall r. r -> Matcher xs r -> Matcher xs r
instance Match '[] where
  match = error "match base case: impossible by construction"
  {-# INLINE CONLIKE match #-}
  unmatch r = r
  {-# INLINE CONLIKE unmatch #-}
  override = const
  {-# INLINE CONLIKE override #-}
instance (Unmatch xs (x ': xs), Match xs) => Match (x ': xs) where
  match :: forall r. Sum (x ': xs) -> (x -> r) -> Matcher xs r
  match uv@(UnsafeInj tag' x) f =
    if tag' == 0
      then override @xs @r (f (unsafeCoerce x)) $ match @xs @r (unsafeForgetFirst uv)
      else match @xs @r (unsafeForgetFirst uv)
  {-# INLINE CONLIKE match #-}
  unmatch :: (forall r. (x -> r) -> Matcher xs r) -> Sum (x ': xs)
  unmatch g = unmatchGo @xs $ g @(Sum (x ': xs)) (UnsafeInj 0 . unsafeCoerce @x)
  {-# INLINE CONLIKE unmatch #-}
  override r m = fmap (override @xs r) m
  {-# INLINE CONLIKE override #-}

-- | A utility typeclass which makes the implementation of 'Match' cleaner.
class Unmatch xs ys where
  unmatchGo :: Matcher xs (Sum ys) -> Sum ys
instance Unmatch '[] ys where
  unmatchGo = id
  {-# INLINE CONLIKE unmatchGo #-}
instance (Unmatch xs ys, x `HasTagIn` ys) => Unmatch (x ': xs) ys where
  unmatchGo f = unmatchGo @xs (f (UnsafeInj (tag @x @ys) . unsafeCoerce @x))
  {-# INLINE CONLIKE unmatchGo #-}

-- | Using functions which only require constraints which are satisfied by
-- all members of the sum.
class ForAll c xs => ApplyFunction c xs where
  apply :: (forall a. c a => a -> y) -> Sum xs -> y

instance ApplyFunction c '[] where
  apply _f x = error "Impossible: empty sum"

instance (c x, ApplyFunction c xs) => ApplyFunction c (x ': xs) where
  apply f x = case considerFirst x of
    Right x' -> f x'
    Left xs -> apply @c f xs