why-curry.scala

scala> def apply[A, B](a: A)(f: A => B) = f(a)
apply: [A,B](a: A)(f: (A) => B)B

scala> apply(1)(_ * 2)
res0: Int = 2

scala> def apply[A, B](a: A, f: A => B) = f(a)
apply: [A,B](a: A,f: (A) => B)B

scala> apply(1, _ * 2)                        
<console>:7: error: missing parameter type for expanded function ((x$1) => x$1.$times(2))
       apply(1, _ * 2)
                ^

scala> apply(1, (_: Int) * 2)
res2: Int = 2

tks! it makes me fully understood!