More play with Go; sorting with a comparison function. It's interesting to note that we have to do type conversions that we wouldn't have to do if we were to go along the grain of the sort.Sort() function.

sort_func.go

package main

import (
	"fmt"
	"sort"
)

type ComparableSlice struct {
	elts []interface{}
	cmp func(interface{}, interface{}) bool
}

func (c ComparableSlice) Len() int {
	return len(c.elts)
}

func (c ComparableSlice) Less(i, j int) bool {
	return c.cmp(c.elts[i], c.elts[j])
}

func (c ComparableSlice) Swap(i, j int) {
	c.elts[i], c.elts[j] = c.elts[j], c.elts[i]
}

func SortComparables(elts []interface{},
	cmp func(interface{}, interface{}) bool) {
	sort.Sort(ComparableSlice{elts, cmp})
}

//////////////////////////////////////////////////////////////////////
// Let's try using this:

func RuneLessThan(o1 interface{}, o2 interface{}) bool {
	return o1.(rune) < o2.(rune)
}

func main() {
	msg := "Hello world!"

	comparables := make([]interface{}, len(msg))
	for i, v := range msg {
		comparables[i] = v
	}

	SortComparables(comparables, RuneLessThan)

	sortedRunes := make([]rune, len(msg))
	for i, v := range comparables {
		sortedRunes[i] = v.(rune)
	}

	fmt.Printf("result: %#v\n", string(sortedRunes))
}