Operators as function arguments?

[Chris Nicholson-Sauls]

Falk Henrich wrote:
> Hi!
> 
> I'm trying to do some functional programming in D and came across the
> following problems:
> 
> Can I somehow use the built-in operator ~ as an argument to a function? The
> doc [1] seems to say no as operator overloading is implemented by adding a
> non-static member function to some class.
> 
> It would avoid lots of repetitive definitions of anonymous delegates. The
> following example uses reduce from [2]:
> 
> int[][] a; int[] b;
> b = reduce(a, (int[] x, int[] y){return x~y;});
> 
> Now it would be useful to do
> 
> b = reduce(a,~);
> 
> It would be even nicer to define "flatten" using templates to avoid code
> duplication for different types of arrays. I tried to define
> 
> T[] cat(T)(T[] a, T[] b) { return a ~ b; }
> 
> but then
> 
> b = reduce(a, &cat);
> 
> yields
> 
> cat(T) is not an lvalue
> 
> The only workaround seems to be to explicitly instantiate the template
> function:
> 
> alias cat!(int) catInt;
> b = reduce(a, &catInt);
> 
> But this is extremely clumsy. Any idea?
> 
> Falk
> 
> 
> [1] http://www.digitalmars.com/d/operatoroverloading.html
> [2] http://www.prowiki.org/wiki4d/wiki.cgi?DanielKeep/functools

None, frankly.  Although, with the upcoming macros feature, you might be able to do 
something like this:

macro binop (Type, Op) {
   (Type x, Type y) { return x Op y; }
}

b = reduce(a, binop(int[], ~));

Although, heck, you might be able to write reduce itself as a macro passing the operator 
you want to it then.

macro reduce (Val, Op) {
   //...
}

b = reduce(a, ~);

Which, come to think of it, is basically what you wanted in the first place!...except that 
it isn't, because macros are always inlined.  Best I could come up with off the top my 
head, though, and reliant on as yet unreleased features.

-- Chris Nicholson-Sauls