Question

I’m a bit confused by the difference in type-checking between curried and uncurried generic functions:

scala> def x[T](a: T, b: T) = (a == b)
x: [T](a: T, b: T)Boolean
scala> def y[T](a: T)(b: T) = (a == b)
y: [T](a: T)(b: T)Boolean

My intuition was that both x(1, "one") and y(1)("one") should give type errors, but I was wrong:

scala> x(1, "one")
res71: Boolean = false
scala> y(1)("one")
<console>:9: error: type mismatch;
 found   : java.lang.String("one")
 required: Int
              y(1)("one")
                   ^

At first I thought there was some sort of implicit casting going on, but that didn’t seem to be the case:

scala> x(1 :Int, "one" :String)
res73: Boolean = false

So what’s going on? What should my intuition be?

Answer 1

I think that in the first case it is upcasting (downcasting?) both arguments such that T:Any. In the second, it is currying for Int, and then failing on the String.

This seems to bear me out:

scala> y(1)_
res1: Int => Boolean = <function1>