How Can Haskell's `fmap` Be Effectively Emulated in Go Using Generics?

Emulating Haskell's fmap in Go

The inability to have method arguments use parameterized types presents a challenge when emulating Haskell typeclasses in Go. Consider the following example attempting to emulate fmap:

type S[A any] struct {
  contents A
}

type Functor [A any, B any] interface{
  fmap(f func(A)B) B
}

func (x S[A]) fmap (f func(A)B) S[B] {
  x.contents = f(x.contents)
  return x
}

This code fails due to the undefined type B in the interface implementation.

Workaround Using a Top-Level Function

Since Go methods cannot introduce new type parameters, one workaround is to implement fmap as a top-level function:

func Fmap[A, B any](sa S[A], f func(A) B) S[B] {
    return S[B]{contents: f(sa.contents)}
}

Alternative Approaches

While this workaround solves the immediate issue, it highlights that emulating Haskell typeclasses using generics and methods is not a straightforward endeavor in Go. Alternative approaches could involve:

• Using inheritance: Design an inheritance hierarchy where each type implements a method for a specific function.
• Using interfaces with explicit method implementations: Define interfaces with explicitly implemented methods that accept generic types as arguments.

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