HigherKindedPolymorphisms

It's full featured but has performance issues now.

Also, without structural function types, some advanced polymorhisms might not be that useful, unless you feel okay to annotate functions here and there.

I made this for using tagless-final style in Julia.

Usage

using HigherKindedPolymorphisms

abstract type VectSig end
@def_hkt VectSig{T} where T = Vector{T}


using CanonicalTraits
import FunctionWrappers: FunctionWrapper

Fn{A, B} = FunctionWrapper{B, Tuple{A}}
@trait Functor{F} begin
    fmap :: [Fn{A, B}, App{F, A}] where {A, B} => App{F, B}
end

@implement Functor{VectSig} begin
    fmap(f :: Fn{A, B}, a::App{VectSig, A}) where {A, B} =
        B[f(e) for e in prj(a)] |> inj
end

f = Fn{Int, String}(string)
a = inj([1, 2, 3])
fmap(f, a) |> prj

#=> ["1", "2", "3"]