Why Does Type Assertion Fail But Conversion Work with Custom Types in Go?

Named Type Assertions and Conversions in Go

Consider the following scenario where a custom type is defined to redefine a pre-defined type:

<code class="go">type Answer string</code>

Now, let's attempt to use this custom type in a function that accepts the original pre-defined type:

<code class="go">func acceptMe(str string) {
    fmt.Println(str)
}

func main() {
    type Answer string
    var ans Answer = "hello"

    // Type assertion fails
    acceptMe(ans.(string)) 
    // Conversion works
    acceptMe(string(ans))
}</code>

The type assertion ans.(string) fails with the error: invalid type assertion: ans.(string) (non-interface type Answer on left). On the other hand, the conversion string(ans) succeeds. This behavior raises the question:

Why does the type assertion fail, but the conversion work?

To understand this, let's clarify that type assertions only work for interfaces. An interface can have arbitrary underlying types, making type assertions and type switches essential. Type assertions return a boolean value along with the result, indicating the success of the assertion.

In contrast, your custom type Answer has a fixed underlying type, which is string. Since you already know the exact types, you don't need type assertions because the conversion from Answer to string will always succeed. The syntax for this conversion is string(ans).

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