Partially instantiating templates?

Magnus Lie Hetland magnus at
Mon Jan 31 05:47:27 PST 2011

On 2011-01-31 12:55:07 +0100, Simen kjaeraas said:

> ElementType!Range minArg( alias fun, Range )( Range range, out 
> ReturnType!fun ) {
>      ...
> }

Aaaah. I guess I tried ElementType(Range), forgetting to make it a 
compile-time parameter. Thanks. (Hadn't seen ReturnType; makes sense :)

> Might I also ask why you use an out parameter instead of a tuple return?

Well... I had a tuple return at first, but one of the advantages of 
returning multiple values that I'm accustomed to is the ability to 
assign to multiple variables, such as

  arg, val = minArg(...)

(Yeah, I'm a Python guy... ;)

As far as I can see, you can't do that here? Using result[0] and 
result[1] or the like, or assigning separately to two variables just 
seemed more cumbersome. Then again, I could use a tuple with named 
members, I guess. In your opinion, what would be the prettiest (in D 
terms) way of doing this?

> D currently does not support template currying to any good degree.

OK. Well, I guess I don't really need it. Still trying to get a feel 
for what's "normal" D :)

> However, there is at least one library out there that does that for you:
> In the file templates.d, there is the template CurryTemplate, which
> rewrites a template to a set of nested templates. This would allow you
> to partially instantiate a template, and add more parameters as you go.

I see. I actually don't mind writing nested templates myself -- but for 
some reason I couldn't get them to work properly. (D kept complaining 
about declarations vs instances, and the like; I guess I'll have a look 
at how dranges does it.)

>> I've been able to make either one of these two pieces of functionality 
>> work with some twiddling and nesting (i.e., *either* instantiating 
>> optArg into minArg/maxArg, *or* instantiating explicitly defined 
>> minArg/maxArg into specialized functions) but combining them has so far 
>> eluded me (unless I start fiddling with strinc constants and mixin(), 
>> which seems excessively hacky for such a simple thing).
> dranges' templates.CurryTemplate should take care of some of your problems.
> Not sure if it will fix them all.

OK, thanks.

By the way, if you have suggestions for other more "D-like" ways of 
encapsulating this functionality (basically a linear scan for an 
element that yields a max/min value for a given expression), I'd be 
interested to hear that too. The best way to solve a problem is often 
to rephrase it :)

Magnus Lie Hetland

More information about the Digitalmars-d-learn mailing list