Re: [boost] scoped models (was Re: [fusion] Support for classic boost::tuple)

From: David Abrahams (dave_at_[hidden])
Date: 2006-10-02 12:57:23

Jeremy Siek <jeremy.siek_at_[hidden]> writes:

> It's early in the morning, but I'll do my best...
>
> In this outer scope, the type 'r' models Sequence<int>, so
> the call to f type checks.
>
> Inside of function f, 'S1' models Sequence<int> and 'r' models  
> Sequence<int*>.

Cool, that's the key thing.

Now, it's great that you can get there by using two scopes.  You could
imagine wanting to do the same thing from within a single function,
but I don't think that's so easy.

> The call to transform will not type check because the requirement
> Sequence<int*> S1 is not satisfied.

Because I got the requirements on transform backwards.  Shoulda been:

     template <class T, Sequence<T> S2, Sequence<T*> S1,
     UnaryFunction<T,T*> F>
     void transform(S1 const& src, S2& dst, F f)

I fear we're OT for Boost, now, though.  Maybe we should take this private.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com