2

What is the difference between def f(x: Int)(y: Int) = x + y and def f(x: Int) = (y: Int) => x + y?

The REPL doesn’t seem happy when I treat the former the same as the latter:

scala> def f(x: Int)(y: Int) = x + y
f: (x: Int)(y: Int)Int

scala> f(42)
<console>:9: error: missing arguments for method f;
follow this method with `_' if you want to treat it as a partially applied function
              f(42)
               ^

scala> def f(x: Int) = (y: Int) => x + y
f: (x: Int)Int => Int

scala> f(42)
res2: Int => Int = <function1>

What are the exact differences and when should I use which form?

  • Well, one is a function with two arguments and one returns a function that takes one argument. No? – Boris the Spider Oct 24 '13 at 20:07
  • The way I like to think of it is that the first is a method with two unary arglists returning an `Int` (so the result of `f(0)` is still a method `(y: Int)Int`) whereas the latter is a method with one unary arglist returning a unary function `Int => Int`. This explains why only the former complains of the missing underscore; `f(0)` in this case does not evaluate to a function. – Hugh Oct 25 '13 at 00:37

2 Answers2

3

The difference is

def f(x: Int)(y: Int) = x + y

is a curried function. A function with two parameterlists. You can use it with just one argument, but you need to specify which one. 

f(42) _ // this is short for f(42)(_: Int)

will generate a partial applied function, where the value of x is 42. You could also do:

f(_: Int)(42) // note: the first parameter must be used this way

This will set the value of y to 42.
Calling a curried function with just a few parameters will generate a partial applied function.

def f(x: Int) = (y: Int) => x + y

is a partial applied function. Here you have a function which takes one argument and returns a function which takes an argument itself.

tgr
  • 3,557
  • 4
  • 33
  • 63
1

With first syntax, you need to add _ as suggested by the compiler

scala> f(42) _
res1: Int => Int = <function1>

cf : Why and when do I need to follow a method name with _?

Community
  • 1
  • 1
Yann Moisan
  • 8,161
  • 8
  • 47
  • 91