power@thautwarm

hkt.jl

struct Fn{A, B}
    f :: Function
end

function (f::Fn{A, B})(a :: A) :: B where {A, B}
    f.f(a)
end

abstract type Sig end
abstract type HKT{S <: Sig, A} end


abstract type Functor <: Sig end

# interface method:
# getsig(::Type{Sig}) :: Sig

function fmap(f :: Fn{A, B}, ma :: HKT{F, A}) ::  HKT{F, B} where {A, B, F <: Functor}
    fmap(F, f, ma)
end


struct Maybe <: Functor # Union{Functor, Applicative, Monad}
end

struct Some{A} <: HKT{Maybe, A}
    a :: A
end

struct None{A} <: HKT{Maybe, A} end

function fmap(::Type{Maybe}, f :: Fn{A, B}, ma :: Some{A}) :: HKT{Maybe, B} where {A, B}
    Some{B}(f(ma.a))
end

function fmap(::Type{Maybe}, f :: Fn{A, B}, ma :: None{A}) :: HKT{Maybe, B} where {A, B}
    None{B}()
end