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

Subject: Re: [Boost-users] [Proto] Extracting types of sub-expression in transform
From: Eric Niebler (eric_at_[hidden])
Date: 2008-10-18 12:54:34

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.

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com