Currying: Encyclopedia II - Currying - Examples

Currying - Examples

Suppose that plus is a function taking two arguments x and y and returning x + y. In the ML programming language we would define it as follows:

   plus = fn(x, y) => x + y

and plus(1, 2) returns 3 as we expect.

The curried version of plus takes a single argument x and returns a new function which takes a single argument y and returns x + y. In ML we would define it as follows:

   curried_plus = fn(x) => fn(y) => x + y

and now when we call curried_plus(1) we get a new function that adds 1 to its argument:

   plus_one = curried_plus(1)

and now plus_one(2) returns 3 and plus_one(7) returns 8.

When declaring functions in the strictly-typed OCaml programming language, the type returned by a function shows the Curried form of the function. Typing the function into the OCaml interpreter displays the type immediately:

   # let plus x y = x + y ;;
   val plus : int -> int -> int = <fun>

Here is a way to do currying in C++ (from this comment on codepost.org):

The plus function:

   int plus(int x, int y) {
       return x + y;
   }

The curried version of plus:

   class curried_plus {
   private:
       int x;
   public:
       curried_plus(int _x) : x(_x) { ; }
       int operator () (int y) const {
           return plus(x, y);
       }
   };

and the usage:

   curried_plus plus_one(1);

now plus_one(2) returns 3.

Adapted from the Wikipedia article "Examples", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki