Re: [Boost-users] [Proto] How to access the grammar of an expression?

Subject: Re: [Boost-users] [Proto] How to access the grammar of an expression?
From: Roland Bock (rbock_at_[hidden])
Date: 2010-09-24 13:49:36

On 09/24/2010 07:35 PM, Eric Niebler wrote:
> On 9/24/2010 11:15 AM, Roland Bock wrote:
>    
>> On 09/24/2010 04:52 PM, Thomas Heller wrote:
>>      
>>> you can compute that type information directly in your proto transform:
>>>
>>> struct sorted_argument_function:
>>>       proto::and_
>>>       <
>>>          proto::function<fun_terminal, proto::_, proto::_>,
>>>          proto::if_
>>>          <
>>>             grammar_less<
>>>                proto::_value(proto::_child0(proto::_child1))
>>>              , proto::_value(proto::_child0(proto::_child2))>()
>>>          >
>>>       >
>>>    {};
>>>        
>> OK, admittedly, that looks a lot nicer, but in case I call something like
>>
>> fun(b(0), 42));
>>
>> the error messages are just horrible in both solutions...
>>      
> The proto::and_ has two branches: proto::function and proto::if_.
>
> The proto::if_ is assuming:
>
> 1. That there are at least three child nodes
> 2. That child1 and child2 themselves have a child
> 3. That the child nodes of child1 and child2 are terminals
>
> The proto::function has only verified the first of these assumptions.
> You need to change it to validate the other two as well:
>
>
>      struct function_argument
>        : proto::function<proto::terminal<proto::_>, proto::_>
>      {};
>
>      struct sorted_argument_function
>        : proto::and_
>          <
>              proto::function
>              <
>                  fun_terminal,
>                  function_argument,
>                  function_argument
>              >,
>              proto::if_
>              <
>                  grammar_less
>                  <
>                     proto::_value(proto::_child0(proto::_child1))
>                   , proto::_value(proto::_child0(proto::_child2))
>                  >()
>              >
>          >
>      {};
>
> HTH,
>    
Yes, that helps a lot! Very cool.

Sigh, seeing it, it suddenly seems so obvious, at least after Thomas 
showed the

proto::_value(proto::_child0(proto::_child1))

Thanks and regards,

Roland