Golang cast type alias

type somethingFuncy func(int) bool

func funcy(i int) bool {
    return i%2 == 0

var a interface{} = funcy

func main() {

    _ = a.(func(int) bool)  // Works

    fmt.Println("Awesome -- apparently, literally specifying the func signature works.")

    _ = a.(somethingFuncy)  // Panics

    fmt.Println("Darn -- doesn't get here. But somethingFuncy is the same signature as func(int) bool.")


For type assertions (which you use) only the actual type matters. So somethingFuncy is only equal to somethingFuncy and not to func(int) bool.


To start with, this has nothing to do with casting. There is no casting in go. There are type assertions and type conversions.

You’re dealing with a type assertion and are assuming that the same conditions hold as for type conversions. I made the same mistake while reading your question but actually there is a huge difference in behaviour.

Suppose you have two types, say int and type MyInt int. These are convertible as they both share the same underlying type (one of the conversion rules), so this works (play):

var a int = 10
var b MyInt = MyInt(a)

Now, suppose a is not of type int but of type interface{} (play):

var a interface{} = int(10)
var b MyInt = MyInt(a)

The compiler will tell you:

cannot convert a (type interface {}) to type MyInt: need type assertion

So now we’re not doing conversions anymore but assertions. We need to do this (play):

var a interface{} = int(10)
var b MyInt = a.(MyInt)

Now we have the same problem as in your question. This assertion fails with this panic:

panic: interface conversion: interface is int, not main.MyInt

The reason for this is stated in the type assertions section of the spec:

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. More precisely, if T is not an interface type, x.(T) asserts that the dynamic type of x is identical to the type T.

So int must be identical to MyInt. The rules of type identity state that (amongst other rules):

Two named types are identical if their type names originate in the same TypeSpec.

As int and MyInt have different declarations (TypeSpecs) they’re not equal and the assertion fails. When you assert a to int, the assertion works. So what you’re doing is not possible.


The actual check happens in this code, which simply checks if both types are the same, as expected.