suggestion: default values

[Leandro Lucarella]

David B. Held, el 19 de abril a las 23:42 me escribiste:
> bobef wrote:
> >It would be nice if there was some character like $ or something, if used as function argument, the default value for this argument to be used. What I mean is this:
> >void myfun(int a,char[] b,bool d=false,int e=55){}
> >void main()
> >{
> >    myfun(5,"5",$,$); // which should act like myfun(5,"5",false,55);
> >}
> 
> While some languages provide named parameters, they tend to be langs which encourage functions with many arguments.  If you find you are creating functions like this, you 
> should first ask yourself if this is the best way.  If you decide that it is, one pattern to use is analogous to the Builder pattern:
> 
> struct myFunArgs
> {
>     int a;
>     char[] b;
>     bool d = false;
>     int e = 55;
> }
> 
> void myfun(myFunArgs args) { ... }
> 
> myFunArgs args;
> args.a = 5; args.b = "5";
> myfun(args);
> 
> This is the *very* Poor Man's named parameters mechanism, but if you have enough parameters to need named ones, then this probably isn't at all burdensome (and may increase 
> clarity).
> 
> Another alternative, if you are creating a library function that you want to be *very* friendly, you can create a variadic type-safe template function and parse the args 
> inside the function (though this doesn't work very well with args of the same type).  Of course, you can use the Perl convention of name, value pairs to make it completely 
> general, but that's not very elegant or idiomatic D style.

Another way is to use partial function aplication:

void myfun(int a,char[] b,bool d=false,int e=55){}

void partial_myfun(int a,char[] b,int e=55)
{
	myfun(a, b, false, e);
}

partial_myfun(5, "5", 65); // myfun(5, "5", false, 65);

I think template black-magic could help to make this a little shorter to
use (maybe boost has something like this?) and well, with a little
reflection (for default value extraction), you can have a complete
automated partial function application, I think.

-- 
LUCA - Leandro Lucarella - Usando Debian GNU/Linux Sid - GNU Generation
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