I have the following code:
>class FixCls fix where
> fix :: f (fix f) -> fix f
> xif :: fix f -> f (fix f)
>
>class Functor f => Algebra f a where
> phi :: f a -> a
>
>class Functor f => CoAlgebra f a where
> psi :: a -> f a
>
>cata::(Algebra f a, FixCls fix) => fix f -> a
>cata = phi . map cata . xif
>
>ana::(CoAlgebra f a, FixCls fix) => a -> fix f
>ana = fix . map ana . psi
It works fine but now I want to define the following function:
>hylo::(Algebra f a, CoAlgebra f b) => b -> a
>hylo = cata . ana
The type system rejects it:
ERROR "hylomorphism.lhs" (line 22): Ambiguous type signature in type declaration
*** ambiguous type : (CoAlgebra a b, Algebra a c) => b -> c
*** assigned to : hylo
I think I understand the reason, but my question is:
How can I define that function?
