Why does leaving the dot out in foldLeft cause a compilation error?

Question

Can anyone explain why I see this compile error for the following when I omit the dot notation for applying the foldLeft function?(version 2.9.2)

Answer 1

Omitting the dot is possible because of scala's syntactic support for the infix notation, which expects 3 parts:

leftOperand operator rightOperand. 

But because foldLeft had two list of parameters, you end up with 4 parts at the syntactic level: l foldLeft (1) (_ * _)

Which does not fit infix notation, hence the error.