Question

This produces an anonymous function, as you would expect (f is a function with three arguments):

f(_, _, _)

What I don’t understand is why this doesn’t compile, instead giving a “missing parameter type” error:

f(_, _, 27)

Instead, I need to specify the types of the underscores explicitly. Shouldn’t Scala be able to infer them given that it knows what the function f’s parameter types are?

Answer 1

References below are to the Scala Language Specification

Consider the following method:

def foo(a: Int, b: Int) = 0

Eta Expansion can convert this to a value of type (Int, Int) => Int. This expansion is invoked if:

a) _ is used in place of the argument list (Method Value (§6.7))

val f = foo _

b) the argument list is omitted, and expected type of expression is a function type (§6.25.2):

val f: (Int, Int) => Int = foo

c) each of the arguments is _ (a special case of the ‘Placeholder Syntax for Anonymous Functions’ (§6.23))

val f = foo(_, _)   

The expression, foo(_, 1) doesn’t qualify for Eta Expansion; it just expands to (a) => foo(a, 1) (§6.23). Regular type inference doesn’t attempt to figure out that a: Int.