Instances.hs

-- | Type class instances that use direct recursion in a potentially partial
--   way. This is separated from the rest of `Yaya.Unsafe.Fold` because you can
--   neither control nor qualify the import of instances. Therefore this module
--   is _extra_ dangerous, as having these instances available applies to the
--   entire module they’re imported into.
--
--   This contains instances that you might _expect_ to see, but which aren’t
--   actually total. For example, folding a lazy list `[a]` is _not_ guaranteed
--   to terminate.
module Yaya.Unsafe.Fold.Instances where

import           Control.Arrow
import           Control.Comonad
import           Control.Comonad.Cofree
import           Control.Comonad.Env
import           Control.Monad
import           Control.Monad.Trans.Free
import           Data.Functor.Classes
import           Data.List.NonEmpty

import           Yaya.Fold
import           Yaya.Fold.Native
import           Yaya.Pattern
import qualified Yaya.Unsafe.Fold as Unsafe

instance Functor f => Recursive (Fix f) f where
  cata = flip Unsafe.hylo project

instance (Functor f, Foldable f, Eq1 f) => Eq (Fix f) where
  (==) = recursiveEq

instance (Functor f, Show1 f) => Show (Fix f) where
  showsPrec = recursiveShowsPrec

instance Functor f => Corecursive (Mu f) f where
  ana = Unsafe.hylo embed

instance Functor f => Recursive (Nu f) f where
  cata = flip Unsafe.hylo project

instance (Functor f, Foldable f, Eq1 f) => Eq (Nu f) where
  (==) = recursiveEq

instance (Functor f, Show1 f) => Show (Nu f) where
  showsPrec = recursiveShowsPrec

instance Recursive [a] (XNor a) where
  cata = flip Unsafe.hylo project

instance Recursive (NonEmpty a) (AndMaybe a) where
  cata = flip Unsafe.hylo project

instance Functor f => Recursive (Cofree f a) (EnvT a f) where
  cata = flip Unsafe.hylo project

instance Functor f => Recursive (Free f a) (FreeF f a) where
  cata = flip Unsafe.hylo project

-- TODO: If we can generalize this to an arbitrary 'Recursive t (FreeF h a)'
--       then it would no longer be unsafe.
seqFreeT
  :: (Functor f, Functor h)
  => DistributiveLaw h f
  -> DistributiveLaw (Free h) f
seqFreeT k =
  cata
  (\case
      Pure a -> free . Pure <$> a
      Free ft -> free . Free <$> k ft)