Why does this wildcarded function tells me it has the wrong number of parameters?

Question

The offending code was:

:47: error: wrong number of parameters; expected = 2
            terms.foldLeft(r.unitA)(r.add(_, _.eval(x)))
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Answer 1

From what I've read on this type of issue, when you use "_" as a place holder for an anonymous parameter of a function, the scope of that function is the innermost parenthesis containing it. So when you wrapped your two placeholders with r.add(), the scope of the params is lost. Check out this link and see if it helps explain the rules better.

http://www.scala-lang.org/node/2916