Re: [Boost-users] [phoenix][lambda] lambda to phoenix translation: get arguments from expression

Subject: Re: [Boost-users] [phoenix][lambda] lambda to phoenix translation: get arguments from expression
From: Joel de Guzman (joel_at_[hidden])
Date: 2010-05-29 01:57:57

On 5/29/2010 11:29 AM, alfC wrote:
> Hi I have a phoenix expression of  type
>
> actor<
> 	composite<
> 		divides_eval,
> 		vector<
> 			LambdaExp,
> 			composite<
> 				minus_eval,
> 				vector<
> 					argument<0>,
> 					value<double>
> 				>
> 			>
> 		>
> 	>
>> f_expr
>
> with boost.lambda I could extract the different components (Lambda and
> the actual value stored by value<double>) from f_expr as:
> get<0>(f_expr.args)  //for lambda
> and
> get<1>(get<1>(f_expr.args).args) //for lambda
>
> what is the equivalent for Boost.phoenix?,
> it seems that the phoenix actor doesn't have the member "args".
> something like at_c<1>(at_c<1>(f_expr)) seems to be in the right
> direction but it returns something of type value<double>  from which I
> don't know how to extract the value
> in the same way at_c<1>(f_expr) returns the denominator, but it is not
> an actor, so it is not a lambda expression.
>
> the corresponding lambda type was:
> lambda_functor<lambda_functor_base<
> 		arithmetic_action<divide_action>,
> 		tuple<
> 			lambda_functor<//
> 				LambdaExp
> 			>,//
> 			lambda_functor<lambda_functor_base<
> 					arithmetic_action<minus_action>,
> 					tuple<
> 						lambda_functor<
> 							placeholder<1>
> 						>,
> 						double const //<-- attention adds const to avoid unreadable
> errors
> 					>
> 			>  >
> 		>
>>> f_expr

Check out the extension mechanism (somewhere near the end) where this is
documented. See composites. Essentially, while lambda uses tuples, phoenix
uses fusion containers -- more or less the same, but with more horsepower
(i.e. algorithms and iterators).

Regards,
-- 
Joel de Guzman
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