Re: [Boost-users] [fusion] const and transform algorithm

6 Jan 2009

      ----- Original Message ----- 
From: "Joel de Guzman" 
To: 
Sent: Tuesday, January 06, 2009 11:26 PM
Subject: Re: [Boost-users] [fusion] const and transform algorithm
...
vicente.botet wrote:
...
Hi,
What is the reason fusion::transform taskes a const Sequence?
template<
    typename Sequence,
    typename F
    >
typename result_of::transform::type transform(
    Sequence const& seq, F f);
Does this means that the transform do not change the sequence itself o also its elements?
Yes. Fusion transform returns a lazy (transform) view. This view
is non-mutating. It is not like STL's transform which mutates
the target container in place. Fusion's is purely functional and
lazily evaluated.
Can for_each mutate the sequence elements? if yes, would the following get a transformation I'm locking for?

vector,shared_pair > t;
boost::fusion::for_each(t, intrusive_get());
vector res=unzip<1>(boost::fusion::for_each(t, intrusive_get()));

where 
shared_pair would be a sequence something like a shared_ptr and
unzip would be the reverse function of zip. BTW do you think that the counterpart function of zip has a place on Fusion?
...
...
boost::tuple = ??(boost::fusion::transform(t, get()));
boost::as_vector? Is there a as_tuple function? Should this force the evaluation of the get function?
As I said, Fusion transform is purely functional. The result of a transform
is a view. The function will only be called when you iterate over the
view. The transformation does not change/mutate the sequence itself.
If that's what you want, you can assign the view back to the original
container. For example:
vector v;
    v = transform(v, f);
I was lock for in the documentation and I have not found this assignation. Where in the documentation can I found that I can do that?

If for_each can change/mutate the sequence itself, why don't have a mutating_transform that takes a Sequence& and not a Sequence const& 

Just a last question. How move semantics maps with purely functional Sequences? Do you plan to add move semantics for the elements of the tuple? The concept of MovableSequence has a sens for you?

Thanks,
Vicente