http://neverwork.in/go-k-combinations/

k-combinations.go

package main

import (  
    "fmt"
)

func main(){  
    for combo := range GenerateCombinations([]string{"1","2","3","4"}, 2) {
        fmt.Println(combo)
    }
}

func GenerateCombinations(data []string, length int) <-chan []string {  
    c := make(chan []string)
    // Starting a separate goroutine that will create all the combinations,
    // feeding them to the channel c
    go func(c chan []string) {
        defer close(c) // Once the iteration function is finished, we close the channel
        Combos(c, []string{}, data, length) // We start by feeding it an empty string
    }(c)
    return c // Return the channel to the calling function
}
// Combos adds a element to the combination to create a new combination.
// This new combination is passed on to the channel before we call Combos once again
// to add yet another element to the new combination in case length allows it
func Combos(c chan []string, combo []string, data []string, length int) {  
    // Check if we reached the length limit
    // If so, we just return without adding anything
    if length <= 0 {
        return
    }
    var newCombo []string
    for _, ch := range data {
        newCombo = append(combo, ch)
        //remove this conditional to return all sets of length <=k
        if(length == 1){
            output := make([]string, len(newCombo))
            copy(output, newCombo)
            c <- output
        }
        Combos(c, newCombo, data, length-1)
    }
}