Variadic templates

[Kirk McDonald]

Kirk McDonald wrote:
> Walter Bright wrote:
>> See http://www.digitalmars.com/d/variadic-function-templates.html
>>
>> Why now? Because it's such a pain to do template programming without 
>> them, and because I wanted to have a good signals and slots 
>> implementation. That was the last piece needed to make S&S work right 
>> (unless I'm way off track with it).
>>
>> There's a lot of unexplored territory with the tuples, they should be 
>> able to do a lot more than the current rather limited ability.
> 
> This is great! I've already thought of a number of ways to vastly 
> improve the metaprogramming stuff I'm using in Pyd. (Sigh... and I just 
> re-wrote all of that once already.) Pyd is a great use-case for an MPL, 
> and I've been compiling a pretty decent one while writing it (with great 
> work from h3r3tic and Don and others). I think it's about to get a whole 
> lot more interesting...
> 

In trying to work with the variadic templates, I've discovered a few 
weaknesses and bugs.

1) Index in static foreach is not const

Take this template function:

void foo(T ...)(T t) {
     foreach (i, arg; t) {
         t[i] = something_else;
     }
}

 >dmd test
test.d(35): Integer constant expression expected instead of cast(int)(i)

Tuples can only be indexed by const (compile-time) index values. The 
index form of foreach results in runtime indexes. This inconsistency 
prevents the user from altering the tuple from inside a loop. This 
prevents some exceedingly useful constructions.

Just as good would be the ability to declare "arg" as inout, as in:

void foo(T ...)(T t) {
     foreach (inout arg; t) {
         arg = something_else;
     }
}

DMD currently barfs on this.

2) Tuple aliasing is buggy

I'll let the example speak for itself.

template Foo(T ...) {
     alias T Foo;
}

void func(T ...)(T t) {
     foreach (arg; t) {
         writefln(arg);
     }
}

void main() {
     alias Foo!(int, real, char[]) F;
     func!(F)(12, 3.8, "hello");
}

 >dmd test2
Assertion failure: 'global.errors' on line 2752 in file 'template.c'

abnormal program termination

I had other complaints, but these are the only ones that occur to me at 
the moment.

-- 
Kirk McDonald
Pyd: Wrapping Python with D
http://pyd.dsource.org