Any sensible solution to the lack of array/slice covariance in Go?

The problem I've just faced is what to do in the following case:

func printItems(header string, items []interface{}, fmtString string) {
  // ...
}

func main() {
  var iarr = []int{1, 2, 3}
  var farr = []float{1.0, 2.0, 3.0}
  printItems("Integer array:", iarr, "")
  printItems("Float array:", farr, "")
}

Go has no generics and doesn't allow to use collection covariance:

prog.go:26: cannot use iarr (type []int) as type []interface { } in function argument      
prog.go:27: cannot use farr (type []float) as type []interface { } in function argument

Ideas?

I'm surprised nobody mentioned using an interface to solve the problem, which is a very idiomatic approach, if a little clunky:

package main

import "fmt"

type List interface {
	At(i int) interface{}
	Len() int
}
    
func printItems(header string, items List) {
	for i := 0; i < items.Len(); i++ {
		fmt.Print(items.At(i), " ")
	}
	fmt.Println()
}

type IntList []int
type FloatList []float64

func (il IntList)   At(i int) interface{} { return il[i] }
func (fl FloatList) At(i int) interface{} { return fl[i] }

func (il IntList)   Len() int { return len(il) }
func (fl FloatList) Len() int { return len(fl) }

func main() {
	var iarr = []int{1, 2, 3}
	var farr = []float64{1.0, 2.0, 3.0}
	printItems("Integer array:", IntList(iarr))
	printItems("Float array:", FloatList(farr))
}

By defining the size and indexing of the list for each type, you can access them "generically". Of course, generics would still be nice so you don't have to do this.

There's not really a way to do this right now without either

1. Making your []int and []float both into []interface{}.
2. Making printItems accept interface{} instead of []interface{} and then use reflection, similar to what the fmt package does.

Neither solution is pretty.

An example of using reflection:

package main

import (
    "fmt"
    "reflect"
    "strings"
    "container/vector"
)

func printItems(header string, items interface{}, fmtString string) {
    value, ok := reflect.NewValue(items).(reflect.ArrayOrSliceValue)
    if !ok {
        panic("Not an array or slice")
    }

    stringBuilder := new(vector.StringVector)
    stringBuilder.Push(header)

    n := value.Len()
    for i := 0; i < n; i++ {
        stringBuilder.Push(fmt.Sprintf(fmtString, value.Elem(i).Interface()))
    }

    fmt.Println(strings.Join(*stringBuilder, ""))
}

func main() {
    var iarr = []int{1, 2, 3}
    var farr = []float{1.0, 2.0, 3.0}

    printItems("Integer array:", iarr, " %d,")
    printItems("Float array:", farr, " %.1f,")
}

package main

import "fmt"

func printItems(header string, items interface{}, fmtString string) {
  if intItems, ok := items.([]int); ok {
	fmt.Println(header, intItems)
  } else if floatItems, ok := items.([]float64); ok {
	fmt.Println(header, floatItems)
  }
}

func main() {
  var iarr = []int{1, 2, 3}
  var farr = []float64{1.0, 2.0, 3.0}
  printItems("Integer array:", iarr, "")
  printItems("Float array:", farr, "")
}

IMHO, more elegant then solution using reflect.

package main

func printItems(header string, items interface{}, fmtString string) {
  // ...
}

func main() {
  var iarr = []int{1, 2, 3}
  var farr = []float{1.0, 2.0, 3.0}
  printItems("Integer array:", iarr, "")
  printItems("Float array:", farr, "")
}

Take a look at similar functions, like fmt.Printf(), in the core Go package documentation and source code.