fogfish

/*

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 {

fogfish

/*

UnApply2 is like contra-map function for data type T that unwrap product type
*/  
type UnApply2[T, A, B any] func(T) (A, B)

/*

ProductEq2 is a container, product of type trait instances.
Here, the implementation is a shortcut due to the absence of heterogeneous lists in Golang. 

fogfish

// ExampleType product type is product of primitive types int × string
type ExampleType struct {
  A int
  B string
}

// Instances of Eq type trait for primitive types
var (
  Int    Eq[int]    = FromEq[int](equal[int])
  String Eq[string] = FromEq[string](equal[string])

fogfish

/*

FromEq is a combinator that lifts T ⟼ T ⟼ bool function to
an instance of Eq type trait
*/
type FromEq[T any] func(T, T) bool

// implementation of Eq type class 
func (f FromEq[T]) Equal(a, b T) bool { return f(a, b)}

fogfish

package ord

/*

Ord : T ⟼ T ⟼ Ordering
Each type implements compare rules, mapping pair of value to enum{ LT, EQ, GT }
*/
type Ord [T any] interface {
  Eq[T]

fogfish

type Haystack[T any] struct{ Eq[T] }

func (h Haystack[T]) Lookup(a T, b []T) bool {
  for _, x := range b {
    if h.Eq.Equal(a, x) {
      return true
    }
  }
  return false
}

fogfish

package eq

/*

eqInt declares a new instance of Eq trait, which is a real type.
The real type "knows" everything about equality in its own domain.
The instance of Eq is created as type over string, it is an intentional
technique to create a namespace using Golang constants.
The instance of trait is referenced as eq.Int in the code.
*/

fogfish

/*

Eq : T ⟼ T ⟼ bool
Each trait implements mapping pair of value to bool category using own
equality rules 
*/
type Eq [T any] interface {
  Equal(T, T) bool
}

fogfish

import * as cdk from '@aws-cdk/core'

class MyResource extends cdk.Construct {
  constructor(scope: cdk.Construct, id: string, l: number) {
    super(scope, id)
    for (const x of Array(10).keys()) {
      (l === 0)
        ? new cdk.CfnJson(this, `${id}${x}`, {value: '0123456789abcdef'})
        : new MyResource(this, `${id}${x}`, l - 1)
    }

fogfish

/*

go test -bench=. -test.benchmem=true -test.benchtime=10s

goos: darwin
goarch: amd64

BenchmarkDirectCall-12    	1000000000	         1.31 ns/op	       0 B/op	       0 allocs/op
BenchmarkInterface-12     	1000000000	         1.69 ns/op	       0 B/op	       0 allocs/op
BenchmarkHoF-12           	1000000000	         1.61 ns/op	       0 B/op	       0 allocs/op