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Corecursive.purs
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Corecursive.purs
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{-
Copyright 2016 SlamData, Inc.
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
-}
module Matryoshka.Class.Corecursive where
import Prelude
import Control.Comonad.Cofree (Cofree, mkCofree)
import Control.Comonad.Env (EnvT(..))
import Control.Monad.Free (Free, liftF)
import Data.Either (either)
import Data.Functor.Mu (Mu, roll)
import Data.Functor.Nu (Nu, unfold, observe)
import Data.Tuple (Tuple(..))
import Matryoshka.Pattern.CoEnvT (CoEnvT(..))
class Functor f ⇐ Corecursive t f | t → f where
embed ∷ f t → t
instance corecursiveMu ∷ Functor f ⇒ Corecursive (Mu f) f where
embed = roll
instance corecursiveNu ∷ Functor f ⇒ Corecursive (Nu f) f where
embed = flip unfold (map observe)
instance corecursiveFree ∷ Functor f ⇒ Corecursive (Free f a) (CoEnvT a f) where
embed (CoEnvT e) = either pure (join <<< liftF) e
instance corecursiveCofree ∷ Functor f ⇒ Corecursive (Cofree f a) (EnvT a f) where
embed et = mkCofree (ask et) (lower et)
where
ask (EnvT (Tuple e _)) = e
lower (EnvT (Tuple _ x)) = x