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
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
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
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David Sankel