contramap.go

/*

ContraMapEq is a combinator that build a new instance of type trait Eq[B] using
existing instance of Eq[A] and f: b ⟼ a
*/
type ContraMapEq[A, B any] struct{ Eq[A] }

// implementation of contra variant functor
func (c ContraMapEq[A, B]) FMap(f func(B) A) Eq[B] {
  return FromEq[B](func(a, b B) bool {
    return c.Eq.Equal(f(a), f(b))
  })
}


/*

Use the combinator to make an instance of `Eq[ExampleType]`
*/
ContraMapEq[int, ExampleType]{Int}.FMap(
  func(x ExampleType) int { /* ... */ },
)