module Pandora.Paradigm.Primary.Functor.Fix where

import Pandora.Core.Functor (type (<-|), type (|->))
import Pandora.Pattern.Category ((.))
import Pandora.Pattern.Functor.Covariant (Covariant (comap))

newtype Fix t = Fix { Fix t -> t (Fix t)
unfix :: t (Fix t) }

cata :: Covariant t => (a <-| t) -> Fix t -> a
cata :: (a <-| t) -> Fix t -> a
cata a <-| t
f = a <-| t
f (a <-| t) -> (Fix t -> t a) -> Fix t -> a
forall (m :: * -> * -> *) b c a.
Category m =>
m b c -> m a b -> m a c
. (Fix t -> a) -> t (Fix t) -> t a
forall (t :: * -> *) a b. Covariant t => (a -> b) -> t a -> t b
comap ((a <-| t) -> Fix t -> a
forall (t :: * -> *) a. Covariant t => (a <-| t) -> Fix t -> a
cata a <-| t
f) (t (Fix t) -> t a) -> (Fix t -> t (Fix t)) -> Fix t -> t a
forall (m :: * -> * -> *) b c a.
Category m =>
m b c -> m a b -> m a c
. Fix t -> t (Fix t)
forall (t :: * -> *). Fix t -> t (Fix t)
unfix

ana :: Covariant t => (a |-> t) -> a -> Fix t
ana :: (a |-> t) -> a -> Fix t
ana a |-> t
f = t (Fix t) -> Fix t
forall (t :: * -> *). t (Fix t) -> Fix t
Fix (t (Fix t) -> Fix t) -> (a -> t (Fix t)) -> a -> Fix t
forall (m :: * -> * -> *) b c a.
Category m =>
m b c -> m a b -> m a c
. (a -> Fix t) -> t a -> t (Fix t)
forall (t :: * -> *) a b. Covariant t => (a -> b) -> t a -> t b
comap ((a |-> t) -> a -> Fix t
forall (t :: * -> *) a. Covariant t => (a |-> t) -> a -> Fix t
ana a |-> t
f) (t a -> t (Fix t)) -> (a |-> t) -> a -> t (Fix t)
forall (m :: * -> * -> *) b c a.
Category m =>
m b c -> m a b -> m a c
. a |-> t
f

hylo :: Covariant t => (b <-| t) -> (a |-> t) -> (a -> b)
hylo :: (b <-| t) -> (a |-> t) -> a -> b
hylo b <-| t
phi a |-> t
psi = (b <-| t) -> Fix t -> b
forall (t :: * -> *) a. Covariant t => (a <-| t) -> Fix t -> a
cata b <-| t
phi (Fix t -> b) -> (a -> Fix t) -> a -> b
forall (m :: * -> * -> *) b c a.
Category m =>
m b c -> m a b -> m a c
. (a |-> t) -> a -> Fix t
forall (t :: * -> *) a. Covariant t => (a |-> t) -> a -> Fix t
ana a |-> t
psi