Usefulness (as in practical applications) of Currying v.s. Partial Application in Scala
I'm trying to understand the advantages of currying over partial applications in Scala. Please consider the following code:
def sum(f: Int => Int) = (a: Int, b: Int) => f(a) + f(b)
def sum2(f: Int => Int, a: Int, b: Int): Int = f(a) + f(b)
def sum3(f: Int => Int)(a: Int, b: Int): Int = f(a) + f(b)
val ho = sum({identity})
val partial = sum2({ identity }, _, _)
val currying = sum3({ identity })
val a = currying(2, 2)
val b = partial(2, 2)
val c = ho(2, 2)
So, if I can calculate partially applied function that easy, what are the advantages of currying?
Answer
Currying is mostly used if the second parameter section is a function or a by name parameter. This has two advantages. First, the function argument can then look like a code block enclosed in braces. E.g.
using(new File(name)) { f =>
...
}
This reads better than the uncurried alternative:
using(new File(name), f => {
...
})
Second, and more importantly, type inference can usually figure out the function's parameter type, so it does not have to be given at the call site.
For instance, if I define a max function over lists like this:
def max[T](xs: List[T])(compare: (T, T) => Boolean)
I can call it like this:
max(List(1, -3, 43, 0)) ((x, y) => x < y)
or even shorter:
max(List(1, -3, 43, 0)) (_ < _)
If I defined max as an uncurried function, this would not work, I'd have to call it like this:
max(List(1, -3, 43, 0), (x: Int, y: Int) => x < y)
If the last parameter is not a function or by-name parameter, I would not advise currying. Scala's _ notatation is amost as lightweight, more flexible, and IMO clearer.