From: Andreas Pokorny (dieMumiee_at_[hidden])
Date: 2006-08-14 17:26:54
"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
> posts higher in the thread:
> 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
So the user might even refuse to encode the expression inside a rule,
but evaluate the expression directly and return the result of the
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