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{-# OPTIONS_GHC -frefinement-level-hole-fits=2 #-}
{-# LANGUAGE MonoLocalBinds #-}
module TG where
import Prelude ( Monad, Applicative, Functor
, fmap, (<*>), pure, (>>=)
, (=<<), ($), (<$>) )
data Free f a = Pure a | Free (f (Free f a))
instance Functor f => Functor (Free f) where
fmap f = go where
go (Pure a) = Pure (f a)
-- Should suggest (fmap)
go (Free fa) = Free (_a go fa)
instance Functor f => Applicative (Free f) where
pure = Pure
Pure a <*> Pure b = Pure (a b)
Pure a <*> Free mb = Free (fmap a <$> mb)
Free ma <*> b = Free ((<*> b) <$> ma)
instance Applicative f => Monad (Free f) where
Pure a >>= f = f a
-- Should suggest ((=<< (_ :: a -> Free f b))
Free f >>= g = Free (fmap _a f)
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