Subject: Re: [Boost-users] proto: analytical-only math functions
From: Eric Niebler (eric_at_[hidden])
Date: 2009-02-09 12:25:26
Hicham Mouline wrote:
>
> Re my constants definitions (my constant terminals are defined as
> proto::terminal< constant_tag<24> >::type c24; ), the
> allowed syntax is :
> c24 = const_expression; // const_expression cannot include c24
>
> a loose EBNF for my const_expression is:
>
> const_expression :: integer-literal e.g. 7
> | floating-literal e.g. 5.4
> | integer-type runtime variable e.g. int x, long y (known
> only at runtime)
> | fp runtime variable e.g. float r, double q (known only
> at runtime)
> | my DSEL constants
> | - const_expression
> | const_expression + const_expression
> | const_expression - const_expression
> | const_expression * const_expression
> | const_expression / const_expression
> | const_expression % const_expression
> | basic_function( const_expression )
> | function( const_expression, ..., const_expression ) //
> respect the dimensionality
>
> I don't see I am missing anything obvious?
I don't know, that's really for you to say as the DSEL designer.
> The concept of const_expression here is not meant that the value of the
> expression is known at compile-time.
> It only means that on taking the derivatives of my functions wrt to my DSEL
> variables, the const_expression disappears.
>
> In order to exclude the constant being defined from the = RHS, I need to
> define the grammar wholly for LHS and RHS of =
> at the same time, I couldn't define RHS grammar alone because I need to
> exclude c24, right?
Not necessarily. I would just define the RHS grammar without worrying
about excluding any particular constants. Then give your constants an
overloaded operator= that uses a compile-time assertion to make sure the
RHS doesn't contain the constant currently being assigned. Something
like (untested) ...
template<class ID>
struct constant
: proto::extends<
typename proto::terminal<constant_impl<ID> >::type
, constant<ID>
>
{
// For this constant, this grammar defines the valid
// right-hand side expressions.
struct RHS
: proto::or_<
proto::and_<
proto::terminal<_>
, proto::not_<proto::terminal<constant_impl<ID> > >
>
, proto::nary_expr<_, proto::vararg<RHS> >
>
{};
template<typename E>
/* ... whatever you want this to return ... */
operator=(E const &e) const
{
// Make sure E doesn't contain the constant
// currently being assigned.
BOOST_MPL_ASSERT((proto::matches<E, RHS>));
/* ... whatever else you want here ... */
}
};
HTH,
-- Eric Niebler BoostPro Computing http://www.boostpro.com