mixin extension

[Hasan Aljudy]

Passing a block of code to a template is a nice and original idea.
Overall, I like it, and it gets my vote.


Daniel Keep wrote:
> Howdy.
> 
> Matthias Spycher wrote:
> 
>>Here's an idea to extend mixins in a manner that would allow you to mix
>>code around a block in D.
>>
>>If you had:
>>
>>template Trace(f:char[])
>>{
>>  printf("Entering %s", f);
>>}
>>{
>>  printf("Exiting %s", f);
>>}
> 
> 
> How about this instead:
> 
> template Trace(f:char[], block inner_block)
> {
>     writefln("Entering %s", f);
>     inner_block;
>     writefln("Exiting %s", f);
> }
> 
> That way, you can also support more complicated constructs like this:
> 
> template Repeat(int times, block inner_block)
> {
>     for( int i=0; i<times; i++ )
>         inner_block;
> }
> 
> Of course, the problem with this is that templates only allow for
> declarations, not arbitrary statements.  Perhaps we could then add the
> following, which would mix a block into the instantiating scope:
> 
> template Repeat(int times, block inner_block)
> {
>     block Repeat
>     {
>         for( int i=0; i<times; i++ )
>             inner_block;
>     }
> }
> 
> 
>>Note the two blocks associated with a single template declaration. You
>>might mix code around a third block with:
>>
>>void test()
>>{
>>  mixin Trace!("test") {
>>    do_something();
>>    more_here();
>>  }
>>}
>>
> 
> 
> Personally, I'd like to be able to drop the "mixin" keyword.  I realise
> that semantically, it makes sense since you're mixing the contents of
> the template in, but without it, it just looks cooler :)
> 
> void test()
> {
>     Trace!("test")
>     {
>         do_something();
>         more_here();
>     }
> }
> 
> 
>>resulting in the equivalent of:
>>
>>void test()
>>{
>>  printf("Entering %s", f);
>>  do_something();
>>  more_here();
>>  printf("Exiting %s", f);
>>}
>>
>>Ideally, such a construct could be used in conjunction with a
>>conditional version statement:
>>
>>void test()
>>{
>>  version (Log) mixin Trace!("test") {
>>    do_something();
>>    more_here();
>>  }
>>}
>>
>>which when logging is disabled would evaluate to:
>>
>>void test()
>>{
>>  do_something();
>>  more_here();
>>}
>>
>>Is this feasible? Are there better ways?
>>
>>Matthias
> 
> 
> I'll steal a Pythonism, and vote +1.  This would be *really* handy, and
> it would allow for the creation of almost arbitrary control structures!
> 
> For the longest time, I've had evil thoughts of making a D preprocessor
> that only operated on complete, valid parse trees.  It would basically
> be a D compiler that read in D, modified it in some way, then spat it
> back out.  With that, you could make structures like this:
> 
> fori( int i; 10 )
>     block;
> 
> Which would be "expanded" by the preprocessor as:
> 
> for( int i=0; i<10; i++ )
>     block;
> 
> But using the above idea, you could just write this as a template:
> 
> template fori(alias variable, int limit, block inner)
> {
>     block fori
>     {
>         for( variable = 0; variable < limit; variable++ )
>             inner;
>     }
> }
> 
> Of course, this would be helped if we could drop in arbitrary symbols or
> declarations, but I can live without that :)
> 
> 	-- Daniel "Must... have... meta... programming..." Keep
>