First-class support of interface slices!

interface-slice.go

package main

import (
	"fmt"
	"strconv"
)

type FancyInt int

func (x FancyInt) String() string {
	return strconv.Itoa(int(x))
}

type FancyRune rune

func (x FancyRune) String() string {
	return string(x)
}

// Literally any object with String() method
type Stringy interface {
	String() string
}

// String, made of string representations of items given.
func Join(items []Stringy) (joined string) {
	for _, item := range items {
		joined += item.String()
	}

	return
}

func main() {
	numbers := []FancyInt{1, 2, 3, 4, 5}
	runes := []FancyRune{'a', 'b', 'c'}

	// You can't do this!
	//
	// fmt.Println(Join(numbers))
	// fmt.Println(Join(runes))
	//
	// prog.go:40: cannot use numbers (type []FancyInt) as type []Stringy in argument to Join
	// prog.go:41: cannot use runes (type []FancyRune) as type []Stringy in argument to Join
	//
	// Instead, you are supposed to do this:
	//

	properNumbers := make([]Stringy, len(numbers))
	for i, number := range numbers {
		properNumbers[i] = number
	}

	properRunes := make([]Stringy, len(runes))
	for i, r := range runes {
		properRunes[i] = r
	}

	fmt.Println(Join(properNumbers))
	fmt.Println(Join(properRunes))
}