Learn.hs

{-# LANGUAGE ExistentialQuantification #-}

import Prelude hiding (id,(.))
import Control.Category

data Learn a b
  = forall p. Learn
  { impl    :: p -> a -> b
  , update  :: p -> a -> b -> p
  , request :: p -> a -> b -> a
  }

instance Category Learn where
  id = Learn
    { impl = \ _ a -> a
    , update = \ _ _ _ -> ()
    , request = \ _ a _ -> a
    }
  Learn j v s . Learn i u r = Learn
    { impl = \ (p, q) a -> j q (i p a)
    , update = \ (p, q) a c ->
      ( u p a (s q (i p a) c)
      , v q (i p a) c
      )
    , request = \ (p, q) a c -> r p a (s q (i p a) c)
    }

data Para a b = forall p. Para (p -> a -> b)

instance Category Para where
  id = Para (\ _ a -> a)
  Para j . Para i = Para (\ (p, q) a -> j q (i p a))

learn :: Para a b -> Learn a b
learn (Para i) = Learn
  { impl = i
  , update = undefined
  , request = undefined
  }