A quick untyped map & filter example for Go. Author unknown, source: http://play.golang.org/p/yU6KUS1y5w

mapfilter.go

package main

import "fmt"

type MapFunc func(interface{}) interface{}
type FilterFunc func(interface{}) bool
type Collection []interface{}

type User string
type Host string

func main() {
	c := Collection{User("A"), Host("Z"), User("B"), User("A"), Host("Y")}
	fmt.Println(c.Filter(ByUser("A")).Map(RenameUser("A", "C")))
}

func (c Collection) Map(fn MapFunc) Collection {
	d := make(Collection, 0, len(c))
	for _, c := range c {
		d = append(d, fn(c))
	}
	return d
}

func (c Collection) Filter(fn FilterFunc) Collection {
	d := make(Collection, 0)
	for _, c := range c {
		if fn(c) {
			d = append(d, c)
		}
	}
	return d
}

func ByUser(u User) FilterFunc {
	return func(v interface{}) bool {
		if w, ok := v.(User); ok && w == u {
			return true
		}
		return false
	}
}

func RenameUser(oldu, newu User) MapFunc {
	return func(v interface{}) interface{} {
		if w, ok := v.(User); ok && w == oldu {
			return newu
		}
		return v
	}
}