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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):

I wish I knew of a better solution.

Eric Niebler
BoostPro Computing

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