Re: [boost] Interest in DSL framework

From: Andreas Pokorny (dieMumiee_at_[hidden])
Date: 2006-08-14 17:26:54

Hi, 

"Andy Little" wrote 
> "Joel de Guzman" wrote
> > IIUC, the "rule" specifies the result type (not deduces the
> > result type). I think it's not a macro at all; it would rather
> > be something like: "unspecified_type", or
> > "you-define-your-type-here-and-ill-do-the-rest"
> 
> Well again I am probably not understanding it fully, but from  one of
> Andreas
> posts higher in the thread:
> 
> get_result_type<add_tag,LeftT,RightT>
> 
> presumably could be implemented as:
> 
> template <typename LeftT, typename RightT>
> struct get_result_type<add_tag,LeftT,RightT>
> {
>     typedef BOOST_TYPEOF_TPL(LeftT() + RightT()) type;
> };

That would be a hen-egg situation. The result_type of the 
operator is defined by the rules, not the other way around. 
Like the Joel said, the encoding of the expression tree into 
operator and function return types is up to the user.

The frontend of the DSLs built with this framework consist of
overloaded operators (already provided by the framework), and 
small user defined inline functions (probably created using a framework macro). 
These functions invoke the rule system with the operands, and 
an operator tag which identifies the function or operator. The 
result type is deduced by finding a matching rule, the result
(value) is returned after calling a static method inside the 
rule found.

So the user might even refuse to encode the expression inside a rule,
but evaluate the expression directly and return the result of the
evaluation.  

Regards
Andreas Pokorny  
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