Re: [Boost-users] [Proto] Extracting types of sub-expression in transform

Subject: Re: [Boost-users] [Proto] Extracting types of sub-expression in transform
From: Ivan Godard (igodard_at_[hidden])
Date: 2008-10-18 22:57:44

  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?