Complex conditional statement in golang - if-statement

Question

I recently started learning golang and Revel. Im trying to understand what exactly the below if statement does. Seems like it is doing a type check, but i dont see what the conditional achieves. Appreciate if anyone can tell me whats happening here. thanks
if str, ok := obj.(string); ok {
return len(str) > 0
}

Answer 1

It tries to convert obj (which is of some abstract interface probably) into a string, checks if that worked, and only enters if it turned out okay.
Written more sparsely it can be viewed as:
// do a type assertion/conversion of obj to a string.
// If obj isn't really a string, ok will be false
str, ok := obj.(string)
// this will only run if we're talking about a string
if ok {
return len(str) > 0
}
What go does is safe casting from some interface to the real type. If you do this without the ok part, your program will panic if obj isn't a string. i.e this code will crash your program if obj isn't a string:
str := obj.(string)
return len(str) > 0
You can read more about type assertions in the docs:
http://golang.org/ref/spec#Type_assertions

Answer 2

This is called a type assertion. Your variable obj is an interface{}, in other words, its real type can change from one execution to another. A type assertion is used to determine the real type of an interface{}. There are two ways to do so:
str = obj.(string)
This one is unsecure: if ever obj is not a string, the program will panic. The other one is the one you used in your code. If obj is not a string, the ok boolean will be false.
For instance:
func f(v interface{}) {
if _, ok := v.(string); ok {
fmt.Println("v is a string!")
} else if _, ok := v.(float64); ok {
fmt.Println("v is a float64!")
} else {
fmt.Println("v is something else...")
}
}
f(17.42) // "v is a float64!"
f("foobar") // "v is a string!"
f([]string{"hello", "world"}) // "v is something else..."