How to cast to a type alias in Go?

See this playground snippet.

Relevant code:

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.")
}

The first cast works, by explicitly declaring the type. But the second cast panics. Why? Is there a clean way to cast to a longer func signature?

tl;dr

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

Explanation

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.

Bonus:

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

Update 2017:

With type assertions in Go 1.9, you can simply add = where you define the type.

type somethingFuncy = func(int) bool

This tells the compiler that somethingFuncy is an alternate name for func(int) bool.

Just to complete nemo's awesome answer, note that while you can't jump directly from an interface (e.f., interface{}) of a given dynamic type (e.g., int) to another type (e.g., type MyInt int), you can do the two steps one after the other:

• assert that the dynamic type of your variable is what you expect it to be;
• convert the result of that assertion to the type of your choice.

Note that since the underlying type is, as its name implies, dynamic, it's a good idea to test if the type assertion succeeded or failed. On the other hand, the type conversion correctness is enforced by the compiler.

Here is your playground snippet slightly modified: http://play.golang.org/p/FZv06Zf7xi

I believe that type alias is what you want. Proposal is accepted and should be in Go 1.9. Ie.

TypeSpec = identifier [ "=" ] Type .

References
https://github.com/golang/go/issues/18130<br> https://github.com/golang/proposal/blob/master/design/18130-type-alias.md