Idiomatic Go: using strings or type assertion?

Consider:

type Maker interface {
    Name() string
    Type() string
}

type BallMaker interface {
    R() int
    Circ() int
}

type SquareMaker interface {
    Width() int
    Perimeter() int
}

If I want to check what my Maker is:

if maker.Type() == ''BallMaker" {
    //make balls
}

Or

s, ok := maker.(SquareMaker)
if ok {
    // make squares
}

Which one is idiomatic?

I would always choose the type assertion. For one, there's no risk of mistyping the magic string.

the short answer is that your question lacks sufficient context. neither are really idiomatic go because a problem like this would not be modeled as multiple interface types that don't satisfy the same interface.

taking a stab at an approximation of your problem, I'd consider doing something like this: https://play.golang.org/p/s9GPE3UId2N

The answer is "it depends", really, especially when the compiler will restrict when you can use the second one.