Eric Niebler wrote:
Joel Falcou wrote:
> Hello,
> i have a DSL where terminal are defined like : terminal< simd<proto::_> >
> What I want is that any binary operator that apply on those terminal should
> be available only if both lhs and rhs parts have the same underlying 
> type. E.G :
> simd<float> + simd<float> is ok while
> (simd<char> + simd<char>)*simd<float> is not ok.
> How I can enforce this check ? Is having a transform that take an 
> expression and returns
> the underlying type OR mpl::void_ if an error occurs the good solution ?

This question has come up a couple of times. The solution isn't very 
pretty ... write a grammar that allows expressions with incompatible 
terminals, and then write a separate transform that checks the terminals 
for compatibility. This issue came up during Proto's review and I posted 
some example code here (see the SameTerminals transform):

I wish I knew of a better solution.
The canonical way to incorporate typing (including much richer type systems than this) in a formal grammar is through van Wijngaarden grammars. See It is possible to generate both the parser and type analysis from such a grammar, and compilers that do so still exist.

Sadly, there is no Boost VWG library. Perhaps you'll write one?