"Named parameter builder" pattern for template parameters

[monarch_dodra via Digitalmars-d]

On Friday, 21 November 2014 at 14:46:00 UTC, ketmar via
Digitalmars-d wrote:
> On Fri, 21 Nov 2014 13:39:42 +0000
> monarch_dodra via Digitalmars-d <digitalmars-d at puremagic.com> 
> wrote:
>
>> D has phenomenal meta programming possibilities, and I see more
>> and more templates taking more and more parameters. So I 
>> thought
>> to myself, why not have a template builder pattern?
> i did something similar in my iv.writer (ctfe format string 
> parser).
> once i got about ten argumnets to each template (and growing), 
> i moved
> everything to structure.
>
>   struct WrData {
>     ... fields ...
>
>     auto set(string name, T) (in T value)
>     if (__traits(hasMember, this, name))
>     {
>       __traits(getMember, this, name) = value;
>       return this;
>     }
>   }
>
> and then i can pass the struct around, changing only the fields 
> i want,
> in "pseudo-functional" style:
>
>   template MyTpl!(WrData data) {
>     ...
>     MyTpl!(data.set!"field0"(value0).set!"field1"(value1));
>   }
>
> that was a real saver. but now i have to wait for GDC update, 
> 'cause
> 2.065 frontend is not able to use structs as template args.

Hum, use a standard structure combined with CTFE to achieve the
same goal. Smart. BTW, if you use opDispatch, you could probably
have it as:
MyTpl!(data.set_field0(value0).set_field1(value1));

Which may or may not read nicer, depending on how you look at it.

With proper template overloading, you should even be able to
implement this as a backwards compatible template.