How to Sort Custom Types in Go

To sort custom types in Go, implement the sort.Interface methods (Len, Less, Swap) on your type and call sort.Sort.

import "sort"

type Item struct {
    Name string
    Value int
}

type ByValue []Item

func (s ByValue) Len() int           { return len(s) }
func (s ByValue) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }
func (s ByValue) Less(i, j int) bool { return s[i].Value < s[j].Value }

items := []Item{{"A", 2}, {"B", 1}}
sort.Sort(ByValue(items))

Go sorts custom data by asking your type three questions: how many items exist, how to swap two items, and which of two items comes first. You answer these by defining three small functions, then tell Go to sort your list. It's like giving a librarian a specific rulebook for how to arrange your unique collection of books.