Answering my own question: The separation of concerns can still be ensured in this case, but using a form of nested bind, aka function composition. for_each( mapInstance , bind( Layout::addWidget, layout , bind( toWidget(), _1 ) ) ); Bind fortunately allows for the creation of hetero-argument-typed function objects when the right-most function object in the functional composition is also hetero-argument-typed. (this is a dense sentence, but it is precise in its meaning). Keep up your experimentation with this powerful library! The rewards are truly worth it. ;-) David On Thu, May 7, 2009 at 10:21 AM, David Sankel <camior@gmail.com> wrote:

Hello,

I'm using boost fusion to do some interesting robotics work. This has come up on the list before, but I'd like to point out my scenario where a mutable transform function would be useful.

I'm writing a generic interface generator for various datatypes. For a given instance of the type, the interface generator return a widget that can modify the instance.

I'd like my implementation for map types to look like this:

   //...    for_each( transform( mapInstance                       , toWidget()                       )            , bind( Layout::addWidget, layout, _1 )            );    //...

It has a very nice separation of concerns: 1) the "transform toWidget" function, and 2) the "for_each addWidget" which acts much like a fold.

However, my toWidget function requires a non-const reference so this doesn't work without a hacky cast to remove the const.

Hackery aside, the only way I know how do to this is using a for_each who's function argument explicity does both the conversion to a widget (with toWidget()) and the folding to the layout. Those two concepts seem separate to me.

Maybe there's a better way or maybe I'm not understanding the drawbacks of exposing a mutable transpose function.

Thanks,

David

-- David Sankel Sankel Software www.sankelsoftware.com

-- David Sankel Sankel Software www.sankelsoftware.com