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): http://lists.boost.org/Archives/boost/2008/03/135169.php 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 http://en.wikipedia.org/wiki/Algol68. 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?