map[string]interface{} is not the same as map[string]string. Type interface{} is not the same as type string.

If they are both map[string]string:

package main

import "fmt"

func main() {

    v := map[string]string{"hello": "world"}

    checkCast(v)

}

func checkCast(v interface{}) {

    _, isCorrectType := v.(map[string]string)

    if !isCorrectType {

        fmt.Printf("incorrect type")

        return

    }

}

Output:

[no output]

The statement v.(map[string]string) is a type assertion, not a cast.

The Go Programming Language Specification

Type assertions

For an expression x of interface type and a type T, the primary expression

x.(T)

asserts that x is not nil and that the value stored in x is of type T. The notation x.(T) is called a type assertion.

Go has conversions.

The Go Programming Language Specification

Conversions

Conversions are expressions of the form T(x) where T is a type and x is an expression that can be converted to type T.

•  
  is there a way to cast in Go? – Karan Jul 4 '14 at 19:56
• 1
  @Karan: See my revised answer. Go has conversions. – peterSO Jul 4 '14 at 20:05