Proto for Boost mailing list page: Re: [proto] Nested Transforms

Subject: Re: [proto] Nested Transforms
From: Nate Knight (Nate.Knight_at_[hidden])
Date: 2011-02-28 11:40:24

On Feb 26, 2011, at 4:55 AM, Eric Niebler wrote:

> On 2/26/2011 8:18 AM, Nate Knight wrote:
>> 
>> int main()
>> {
>>    la::state_type x = {1.0,2.0};
>>    la::state_type const& xc = x;
>> 
>>    // this compiles
>>    la::EvalOpt()( lit(x)[1] );
>> 
>>    // this fails to compile
>>    //la::EvalOpt()( lit(xc)[1] );
>> 
>>    // this fails to compile
>>    //_default<>()( la::Optimize()( boost::proto::lit(x)[1] ) );
>> 
>>    // this compiles
>>    _default<>()( as_expr( la::Optimize()( boost::proto::lit(x)[1] ) ) );
>> }
> 
> 
> I think I know what's causing this. Can you try compiling with
> BOOST_PROTO_STRICT_RESULT_OF?
> 

Eric,

Thanks for the information.  Compiling with BOOST_PROTO_STRICT_RESULT_OF
allows the first commented line to compile.  I guess we'll wait to hear from Joel about 
the impact of this change on the run times of his library.  

The second commented line does not compile.  This seems to be because there are no
'const Expr&' overloads of boost::proto::transform::operator().  Is this correct? The pertinent 
part of the compiler error (from clang) is 

boost/proto/transform/impl.hpp:96:9: note: candidate function 
    [with Expr =
      boost::proto::exprns_::expr<boost::proto::tag::subscript, boost::proto::argsns_::list2<const boost::proto::utility::literal<std::tr1::array<double, 2> &,
      boost::proto::domainns_::default_domain> &, const boost::proto::exprns_::expr<boost::proto::tag::terminal, boost::proto::argsns_::term<const int &>, 0> &>, 2>]
    not viable: no known conversion from 'typename boost::proto::detail::apply_transform<transform_type (const expr<subscript, list2<const literal<array<double, 2>
      &, default_domain> &, expr<terminal, term<const int &>, 0> >, 2> &)>::result_type' (aka 'boost::proto::exprns_::expr<boost::proto::tag::subscript,
      boost::proto::argsns_::list2<const boost::proto::utility::literal<std::tr1::array<double, 2> &, boost::proto::domainns_::default_domain> &, const
      boost::proto::exprns_::expr<boost::proto::tag::terminal, boost::proto::argsns_::term<const int &>, 0> &>, 2>') to
      'boost::proto::exprns_::expr<boost::proto::tag::subscript, boost::proto::argsns_::list2<const boost::proto::utility::literal<std::tr1::array<double, 2> &,
      boost::proto::domainns_::default_domain> &, const boost::proto::exprns_::expr<boost::proto::tag::terminal, boost::proto::argsns_::term<const int &>, 0> &>, 2>
      &' for 1st argument
        BOOST_PROTO_TRANSFORM_(PrimitiveTransform, X)

Nate