plans for macros

Steven Schveighoffer schveiguy at
Thu May 15 08:32:09 PDT 2008

"janderson" wrote
> Steven Schveighoffer wrote:
>> "janderson" wrote
>>> Steven Schveighoffer wrote:
>>>> I just found a very good use for macros, and I was wondering how they 
>>>> could be used to help in this situation.
>>>> If I have a log object, and that log object is supposed to evaluate its 
>>>> arguments only if the logging level allows it, checked at runtime.
>>>> So this is the ideal usage in the calling function:
>>>> if(log.isEnabledAtLevel(Information))
>>>>   log.output(someExpensiveStringBuild());
>>>> This is ideal because it only outputs at the appropriate level, and it 
>>>> only evaluates the expensive function if the log level is enabled.
>>>> However, this is very verbose, and is prone to errors.  Many log 
>>>> systems use the following method:
>>>> log.outputInformation(someExpensiveStringBuild());
>>>> Which does the if-statement for you.  However, they warn you to write 
>>>> your logging code in the first form if the code to build the output is 
>>>> expensive to avoid building the output even when it is not output.  But 
>>>> D has a better way:
>>>> class Log
>>>> {
>>>> void outputInformation(lazy string x)
>>>> {
>>>>     if(isEnabledAtLevel(Information))
>>>>       output(x);
>>>> }
>>>> }
>>>> Now, we can still use the second form, even when building the string is 
>>>> expensive.  But there are issues with this solution.  For one, lazy 
>>>> evaluation adds delegate functions wherever the logging is required, 
>>>> adding to runtime and code bloat.  Second, variadic functions would be 
>>>> nice for logging, especially with formatting, but the only way to do 
>>>> lazy variadic functions is with template tuples, so there is another 
>>>> lot of generated code, and is even more inefficient.
>>>> But a macro would solve the problem quite nicely.  A macro would 
>>>> evaluate the if statement in the calling function, and so would prevent 
>>>> evaluation of the expensive string building unless necessary, AND would 
>>>> require no delegates to do it.
>>>> The question I have is, when macros are implemented, can I have a 
>>>> 'class scoped' macro?  That is, a macro that knows what context it is 
>>>> supposed to be in, and is passed a 'this' pointer?  And will macros 
>>>> support variadic arguments?
>>>> For example, I'd like to have a macro to output formatted log 
>>>> information only if the log is enabled, but I want to call it like a 
>>>> member function of the log.
>>>> -Steve
>>> I'm not sure if this solves your problem.  Here's an interesting syntax 
>>> I discovered in 1.01 (haven't checked other versions).
>>> void LogIt(alias func)()
>>> {
>>>   if (true)
>>>   {
>>>     printf(func());
>>>   }
>>> }
>>> LogIt!( { char[] test = "test"; return test.ptr; } )();
>>> LogIt!( { return "test"; } )();  //You couldn't do this.
>>> Unfortunately I don't want to update my compiler at this time to see if 
>>> this would work in new versions.
>>> I also wonder if it could be simpled by wrapping it in something else -> 
>>> thoughts?  Its a pretty cool technique, essentially a inlined function 
>>> pointer.
>>> If alias could be replaced with the word lazy string and have D add the 
>>> extra sugar we'd be set.
>> Lazy evaluation is already supported, and already adds the extra sugar 
>> (not sure if 1.01 does though).
> For templates?  1.01 does support lazy for as function parameters but not 
> as template parameters.

Why do you need a template?  A template seems like a step backwards in ease 
of use.  I'd rather have just a lazy parameter where calling it is as easy 


>> The problem I'm trying to solve is lazy evaluation of variadic arguments. 
>> And in general, lazy evaluation is not as efficient as a macro would 
>> be --  there would be no automatic delegate generated, especially if 
>> variadic arguments need a delegate per argument, which would generate n 
>> delegates.
>> I really think macros are the best solution to this problem, but I was 
>> wondering how easy it would be to make macros look like member functions 
>> of a class, and if they will support variadic arguments.
>> -Steve
> Templates are just as efficient as macros, particularly if u use the mixin 
> syntax (ie force inlining of the template function itself).

mixin would probably work, but again, the syntax is not as appealing as a 



mixin!(log.formatInfo, ...);

(don't know if this is right, I don't use mixins a lot)


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