Is metaprogramming useful?

[Georg Wrede]

Andrey Khropov wrote:
> Georg Wrede wrote:
> 
> 
>>It's not metaprogramming itself, it's the bad implementations that make it
>>hard.
>>
>>If we had Perfect Metaprogramming(TM) in D, then I could do the following:
>>
>>  I'm coding some stuff and I notice that what I'd really want
>>  is a new keyword, "unless", that would make it so much easier
>>  for me to write this application clearly. I decide to create it.
>>
>>  I want to use it like this
>>
>>    unless (fullMoonTonight) { doRegularStuff() }
>>
>>  So, to create such a thing in D, I'd write something like
>>
>>    define("unless", "(", BooleanExpression, ")", BlockStatement)
>>    {
>>      if(!BooleanExpression) BlockSTatement;
>>    }
>>
>>Now, if I'd made errors in writing the meta code, then the compiler would
>>error me, of course. No biggie. And since the D compiler would understand
>>what's going on (as opposed to the C preprosessor or compiler), the error
>>messages would be what we're used to in D.
>>
>>Later, when I actually use the "unless" construct, again the error messages
>>would be normal because D now understands what "unless" is all about.
> 
> 
> Well, I'm sorry I'm saying that again but it's almost exactly the way Nemerle
> does it.

Hmm. I'n not sorry at all! :-)

There's confluence in the air....

> And it actually has that unless macro :-) 
> Here's the actual code from the compiler svn:
> 
> -------------------------------------------------------------------
> macro @unless (cond, body)
> syntax ("unless", "(", cond, ")", body) 
> {
>     <[ match ($cond) { | false => $body : void | _ => () } ]>
> }
> -------------------------------------------------------------------
> 
> but it's defined using pattern matching.
> 
> anyway it can be defined your way:
> 
> -------------------------------------------------------------------
> macro @unless (cond, body)
> syntax ("unless", "(", cond, ")", body) 
> {
>     <[ when( !($cond) ) $body ]>
> }
> -------------------------------------------------------------------
> 
> ('when' is used in Nemerle for 'if without else') 
> 
> 
>>---
>>
>>People say GC has to be slow, but that is mostly because it used to only
>>exist in languages that were slow to begin with (and badly implemented).
> 
> 
> Sad to say, but GC in D is still conservative and hence slow :-(
>