How to convert type of map in golang

Question

The function B return type map[T][]T like this:

type T interface{}

func B() map[T][]T {
  result := make(map[T][]T)
  return result
}

And now I have a function A call function B like this:

func A() map[string][]string {
  res := B()
  return res.(map[string][]string) //I'm sure the type is map[string][]string, so I use assertion, but it doesn't works
}

So, how can I do this cover type of map?

Answer 1

A different approach is to return T not map[T][]Tplay:

type T interface{}

func B() T {
    result := map[string][]string{
        "test": {"test", "test"},
    }
    return T(result)
}

func A() map[string][]string {
    res := B()
    if v, ok := res.(map[string][]string); ok {
        return v
    }
    return nil
}

func main() {
    fmt.Println("Hello, playground", A())
}

// Edit, converter function : http://play.golang.org/p/cW_PNTqauV

func makeMap() map[T][]T {
    return map[T][]T{
        "test": {"test", "test"},
        "stuff": {"stuff", 1212, "stuff"},
        1: {10, 20},
    }
}

func convertMap(in map[T][]T) (out map[string][]string) {
    out = make(map[string][]string, len(in))
    for k, _ := range in {
        if ks, ok := k.(string); ok {
            v := in[k] // this way we won't use a copy in the for loop
            out[ks] = make([]string, 0, len(v))
            for i := range v {
                if vs, ok := v[i].(string); ok {
                    out[ks] = append(out[ks], vs)
                } else {
                    fmt.Printf("Error: %v (%T) is not a string.\n", v[i], v[i])
                }

            }
        } else {
            fmt.Printf("Error: %v (%T) is not a string.\n", k, k)
        }

    }
    return
}

func main() {
    fmt.Println(convertMap(makeMap()))
}

Answer 2

You can't. Those are completely different types. You have to copy and type cast item by item: http://play.golang.org/p/uhLPytbhpR

import "fmt"

type T interface{}

func B() map[T][]T {
    result := make(map[T][]T)
    return result
}

func A() map[string][]string {
    res := B()
    result := make(map[string][]string)
    for k,v := range res {
        key := k.(string)   
        value := make([]string, 0, len(res))
        for i := 0; i<len(value); i +=1 {
            value[i] = v[i].(string)
        }
        result[key]= value
    }
    return result
}

func main() {
    fmt.Println("Hello, playground", A())
}