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

vicente.botet wrote:

----- Original Message ----- From: "Joel de Guzman" To: Sent: Tuesday, January 06, 2009 11:26 PM Subject: Re: [Boost-users] [fusion] const and transform algorithm

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vicente.botet wrote:

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Hi,

What is the reason fusion::transform taskes a const Sequence? template< typename Sequence, typename F

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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?

Yes. if yes, would the following get a

transformation I'm locking for?

I don't know. I'm not sure what you are looking for.

vector,shared_pair

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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?

I am not sure what it is you want to do. Could you explain in english instead of non-functioning code?

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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?

See the sequence/views concepts and the docs for the containers. For example (http://tinyurl.com/84vdvp): Notation: v Instance of vector s A Forward Sequence Expression Semantics: v = s Assigns to a vector, v, from a Forward Sequence, s.

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&

Because it is not needed.

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?

Makes sense. Last BoostCon, we had a workshop about implementing Fusion for 0x incuding move. We'll have it when it is available. Regards, -- Joel de Guzman http://www.boostpro.com http://spirit.sf.net