Differences with Scala Implicits

Hylo.hs

{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE ConstrainedClassMethods #-}

module Hylo(CoAlgebra(..), Anamorphism(..), Algebra(..), Catamorphism(..)) where

newtype CoAlgebra x u = CoAlgebra {
  unfold :: u -> Either () (x, u)
}

class Anamorphism m where
  ana :: CoAlgebra x u -> u -> m x
  {-# MINIMAL ana #-}

data Algebra t u = Algebra {
  empty :: u,
  fold :: t -> u -> u
}

class Catamorphism m where
  cata :: Algebra x u -> m x -> u
  {-# MINIMAL cata #-}


class Hylomorphism where
  hylo :: (Anamorphism m, Catamorphism m) => CoAlgebra x u -> Algebra x v -> u -> v

{-|
Prelude> :l Hylo.hs
[1 of 1] Compiling Hylo             ( Hylo.hs, interpreted )

Hylo.hs:24:3: error:
    • Could not deduce (Anamorphism m0)
      from the context: (Hylomorphism, Anamorphism m, Catamorphism m)
        bound by the type signature for:
                   hylo :: Hylomorphism =>
                           forall (m :: * -> *) x u v.
                           (Anamorphism m, Catamorphism m) =>
                           CoAlgebra x u -> Algebra x v -> u -> v
        at Hylo.hs:24:3-83
      The type variable ‘m0’ is ambiguous
    • In the ambiguity check for ‘hylo’
      To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
      When checking the class method:
        hylo :: Hylomorphism =>
                forall (m :: * -> *) x u v.
                (Anamorphism m, Catamorphism m) =>
                CoAlgebra x u -> Algebra x v -> u -> v
      In the class declaration for ‘Hylomorphism’
   |
24 |   hylo :: (Anamorphism m, Catamorphism m) => CoAlgebra x u -> Algebra x v -> u -> v
   |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

|-}