Re: [boost] [Proto]Rationale for grammar/transform bundling (was Re: [Proto] Review result

From: Eric Niebler (eric_at_[hidden])
Date: 2008-04-16 19:09:50

Larry Evans wrote:
> There was also an issue with separation of transforms from grammars.
> This mixing made it hard for me to understand transforms.  I believe
> separating the two would make understanding for other novice users
> easier also.  Of course I'm not even real sure that separation is
> possible, as I intimated with the '*highly* speculative' qualifier
> in my review.  However, since then I've been trying to separate the
> two.  This lead me to understand *maybe* why bundling the two is
> necessary.  The pass_through transform docs contain:
> 
>    , boost::result_of
>      < Grammar::proto_childN(Expr::proto_childN, State, Data)
>      >::type
> 
> Now if Grammar::proto_childN is *not* derived from pass_through,
> then that would make the above an object CTOR call.

No, you are confusing how boost::result_of and proto::when interpret 
function types. result_of has a protocol, documented in TR1, for 
calculating the return type of a function or function object given a 
function type as above. proto::when has a different convention, which 
doesn't come into play here.

> Unless
> the N-th child has a 3 element CTOR, then this will fail to
> compile. OTOH, if then N-th child is derived from pass_through
> (IOW, if the grammar and transform are bundled), then this
> will compile.
> 
> Eric, is that about right?

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com