How Can We Effectively Emulate Haskell's `fmap` in Go?

Emulating fmap in Go: A Practical Approach

In Go, emulating the expressive power of Haskell's typeclasses presents challenges. One such example is the emulation of fmap, a fundamental operation in functional programming.

The Challenge

Consider the following attempt to implement fmap in Go:

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 implementation fails because Go methods cannot introduce new type parameters. As a result, the fmap method cannot access the B type.

A Practical Solution

While using generics and methods to emulate typeclasses in Go has limitations, it is possible 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)}
}

This approach provides the desired functionality without the constraints of Go's type system. However, it is crucial to evaluate whether such emulation aligns with the idiomatic approach in Go.

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