Adding Methods to an Interface in Go

golang_interface_extension.go

// This package demonstrates a technique for (very nearly) adding methods
// to an interface.  The code works and runs as is.
package main

import "fmt"

// A simple custom type
type Cell int

// CellSetMinimal is a (very) minimal interface to be easily implemented
// by custom collections.
type CellSetMinimal interface {
	Do(func(Cell))
}

// CellSet is somewhat more fleshed out interface to be implemented by
// more ambitious custom collections.
type CellSet interface {
	CellSetMinimal
	Len() int
	Has(Cell) bool
}

// CellSetWrapper wraps any implementation of CellSetMinimal to provide
// a default, though pessimal, implementation of CellSet.
type CellSetWrapper struct {
	CellSetMinimal
}

func (w CellSetWrapper) Len() (ln int) {
	w.Do(func(_ Cell) {
		ln += 1
	})
	return
}
func (w CellSetWrapper) Has(cell Cell) (yes bool) {
	w.Do(func(c Cell) {
		yes = yes || (cell == c)
	})
	return
}

// CellSlice is a sample implementation of CellSetMinimal
type CellSlice []Cell

func (cs CellSlice) Do(do func(Cell)) {
	for _, c := range cs {
		do(c)
	}
}

func main() {
	var cs CellSlice
	cs = append(cs, Cell(42))
	// Note: braces not parentheses on the next line!
	w := CellSetWrapper{cs}
	fmt.Println("cs.Len() =", w.Len())
	fmt.Println("cs.Has(42) =", w.Has(Cell(42)))
}