Placeholder in inner functions in scala

Question

Here is the simple code:

(0 to 20).foreach(print(math.pow(2, _)))

I was wondering, why it doesn't work, but this similar code

(0 to 20).foreach(x => print(math.pow(2, x)))

do work. What's the problem with using the placeholder inside the inner function?

Answer 1

Scala uses underscores to create an anonymous function with the smallest expression that isn't the identity function.

So the compiler first tries:

(0 to 20).foreach(print(x => math.pow(2, x => x)))

Nope, that's the identity function, so it goes out one set of parentheses and tries:

(0 to 20).foreach(print(x => math.pow(2, x)))

That is a nontrivial anonymous function, so it stops there.