std.functional.curry isn't

[Bill Baxter]

On Thu, Jun 24, 2010 at 5:06 PM, Graham Fawcett <fawcett at uwindsor.ca> wrote:
> Hi folks,
>
> The template "std.functional.curry(alias fun, alias arg)" claims to
> "curry fun by tying its first argument to a particular value."
>
> That is not currying; it is partial application. In particular, it is a
> kind of partial application called a "left section," because the argument
> provided is partially applied as the left-hand argument of the binary
> function.
>
> Partial application takes a binary function and a value, and returns a
> unary function.
>
> Currying takes a binary function and returns a unary function, which
> returns a unary function.
>
> Let [A,B]-> C be the type of binary functions taking arguments of types A
> and B, and returning a value of type C. A unary function from B to C can
> be written [B]->C.
>
> left section:  [ ([A,B]->C), A ] -> ([B] -> C).
> right section: [ ([A,B]->C), B ] -> ([A] -> C).
> curry:         [ ([A,B]->C) ] -> ([A] -> ([B] -> C)).
>
> The example in the std.functional documentation:
>
> int fun(int a, int b) { return a + b; }
> alias curry!(fun, 5) fun5;
> assert(fun5(6) == 11);
>
> If this were a real curry, you would write it like this:
>
> int fun(int a, int b) { return a + b; }
> assert(curry!(fun)(5)(6) == 11);
>
> Confusing curring and partial application is a very common mistake. But
> please rename this template in std.functional. It makes the
> std.functional module look amateurish to have such an error.
>
> I'd vote for "template partial(alias fun, alias arg)" and "template
> partialR(alias fun, alias arg)" for left and right sections. It's
> unlikely that you really need a "curry" template at all.
>
> Regards,
> Graham
>

Agreed.  I brought up the same thing back when curry() was just an
example in the documentation of variadic templates.

--bb