Why can't I use an empty interface that holds a concrete type of string as a return value in a function declared to return string?


I am a newcomer of go and recently I am following the instruction of A Tour of Go.
I am reading the chapter on Interface and I am really confused by the concept.

The code is as follows

package main

import "fmt"

func main() {

func testTypeAssertion() string {
    var i interface{}="hello"
    fmt.Printf("type of i is %T",i)
    return i
//  return "hello"

In this case, it will cause error

# example
./prog.go:12:2: cannot use i (type interface {}) as type string in return argument: need type assertion

But if I comment return i and uncomment return "hello", it goes like this

type of i is string

So why exactly do we need a type assertion here?
What’s the type of i exactly?

I believe this question is different from cannot use type interface {} as type person in assignment: need type assertion. Because in that question the poster is trying to assign an empty interface value to a variable that has a concrete self-defined type person. In my question, I am trying to figure out why an interface holding a concrete string value cannot be a return value of a function whose return value type is exactly string.

Thanks to mkopriva‘s answer in the comment section and bugstop‘s answer. I will accept that it’s caused by the different usage of static type and dynamic type. By the way, Reality‘s answer is very interesting and really helped me to understand the whole concept!


it’s an interface value i with a dynamic type string.

It may be useful to read about:

Answered By – bugstop

Answer Checked By – Mary Flores (GoLangFix Volunteer)

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