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.

Answer 2

It's because foldLeft is curried. As well as using the dot notation, you can also fix this by adding parentheses:

scala> (l foldLeft 1)(_ * _)
res3: Int = 6

Oh - and regarding your comment about not being able to invoke foldLeft(l), you can, but you need to partially apply it like this:

scala> (l foldLeft 1) _
res3: ((Int, Int) => Int) => Int = <function1>