Re: [Boost-users] [proto] Manipulating _data

Subject: Re: [Boost-users] [proto] Manipulating _data
From: Eric Niebler (eric_at_[hidden])
Date: 2010-02-02 05:27:15

Hi David,

On 2/2/2010 3:45 PM, David A. Greene wrote:
> So I've had fun tinkering with my lambda-like language.
> I moved on to try to introduce scoping, but I'm having
> some trouble.  I've attached a simplified example.
>
> Compiling as-is, the test outputs what I would expect.
> It seems like the LookupSymbol transform works
> brilliantly, so symbol lookup is a go.
>
> However, convincing the grammar to match an expression
> that invokes the AddScope transform seems more difficult.
> Commenting the #if clause results in an error about converting
> a Wrapper<...>  to a shared_ptr<Base>  which I can only guess
> means the grammar is not matching the proto expression.

You guess correctly, and you could have verified that for yourself with 
a compile-time assertion, which I've added for you. See attached.

> I've tried all sorts of different things.  I don't think the
> UnaryPassThrough transform should be neccessary,

It's not; I've removed it.

> but
> maybe it does't hurt either.  With or without it, the error
> is the same.
>
> What am I missing here?

You're going to kick yourself. The problem is exactly the same as last 
time. The expression is a subscript expression, but you don't have a 
case that handles subscript expressions. You seem to be confusing 
terminals within sub-expressions with the operators that bind 
sub-expressions together. Pretty much the only change I had to make was 
to change this:

template<>
struct ConstructStatementGrammarCases::case_<keyword::scope>

to this:

template<>
struct ConstructStatementGrammarCases::case_<boost::proto::tag::subscript>

If I can make a suggestion.... proto::switch_ is an optimization. Leave 
it for later. Just use proto::or_ for now, and when you get everything 
working, you can easily change it to use proto::switch_ to improve 
compile times. I think with proto::or_, you'll have an easier time 
keeping terminals and operators straight.

I'm attaching a corrected example. There seems to be a logic flaw 
though; outer scopes are not getting searched. That should be a simple fix.

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com