CODEWITHSUNDEEP
scalacompiler-errors
Andrey Tyukin
Andrey Tyukin

Reputation: 44937

Counterintuitive suggestion in the "missing argument list"-error message for methods with multiple argument lists?

Consider the following code snippet:

def foo(a: Int)(b: Int) = a + b
foo

It does not compile, and produces the following error message:

error: missing argument list for method foo
Unapplied methods are only converted to functions when 
  a function type is expected.
You can make this conversion explicit by writing 
`foo _` or `foo(_)(_)` instead of `foo`.

The foo _ hint works. But if I write the expression

foo(_)(_)

as suggested by the previous error message, I get a new error message:

error: missing parameter type for expanded 
function ((x$1: <error>, x$2: <error>) => foo(x$1)(x$2))

This seems rather counterintuitive.

Under what circumstances is the foo(_)(_)-hint supposed to be helpful, what exactly does it tell me?

(noise removed; the more I kept editing the question, the less sense did it make; Kolmar is right)

Upvotes: 1

Views: 799

Answers (1)

Kolmar
Kolmar

Reputation: 14224

The type of foo(_)(_) is (Int, Int) => Int. So it works if you specify that type or use it in the context that expects this type:

scala> foo(_: Int)(_: Int)
res1: (Int, Int) => Int = $$Lambda$1120/1321433666@798b36fd

scala> val f: (Int, Int) => Int = foo(_)(_)
f: (Int, Int) => Int = $$Lambda$1121/1281445260@2ae4c424

scala> def bar(f: (Int, Int) => Int): Int = f(10, 20)
bar: (f: (Int, Int) => Int)Int

scala> bar(foo(_)(_))
res2: Int = 30

Upvotes: 2

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