Duck typing in Go - interface

Question

I want to write a Join function that takes arbitrary objects with a String() method:
package main
import (
"fmt"
"strings"
)
type myint int
func (i myint) String() string {
return fmt.Sprintf("%d", i)
}
func main() {
parts := []myint{myint(1), myint(5), myint(6)}
fmt.Println(Join(parts, ", "))
}
func Join(parts []fmt.Stringer, sep string) string {
stringParts := make([]string, len(parts))
for i, part := range(parts) {
stringParts [i] = part.String()
}
return strings.Join(stringParts , sep)
}
http://play.golang.org/p/EhkbyibzHw
[]myint cannot be passed to Join, because it is not of type fmt.Stringer, so I have to write:
parts := []fmt.Stringer{myint(1), myint(5), myint(6)}
But what if I need parts for another operation where the values have to be ints? Should I cast a slice of myint to a slice of Stringer then?

Answer 1

Q: Should I cast a slice of myint to a slice of Stringer then?
There are no casts in Go - only conversions. But conversion from a slice of concrete, non interface type to a slice of interface type is not possible, except by doing it explicitly in a loop. The reason is that, as for example in your case, the []myint backing array has a different memory layout than []fmt.Stringer has. (Related FAQ)
On a side note, you don't have to write:
parts := []myint{myint(1), myint(5), myint(6)}
This should work identically:
parts := []myint{1, 5, 6}

Answer 2

Change the Join function to
func Join(sep string, parts ...fmt.Stringer) string {
}
makes it a lot easier:
package main
import (
"fmt"
"strings"
)
type myint int
func (i myint) String() string {
return fmt.Sprintf("%d", i)
}
type myfloat float32
func (f myfloat) String() string {
return fmt.Sprintf("%0.2f", f)
}
func main() {
fmt.Println(Join(", ", myint(3), myfloat(3.5543)))
}
func Join(sep string, parts ...fmt.Stringer) string {
stringParts := make([]string, len(parts))
for i, part := range parts {
stringParts[i] = part.String()
}
return strings.Join(stringParts, sep)
}