Re: Discussion: reconsider lens-like exports from containers

That transformation is effectively what 'fusing' does. It CPSes (er.. Yonedas, actually) the functor `f` as forall r. (a -> r) -> f r, and accumulates the changes by composing in more functions, so that it can all be discharged with one fmap. You should be able to achieve the same result by hand with a manual worker/wrapper transformation. -Edward On Thu, Apr 28, 2016 at 8:20 AM, David Feuer wrote:

I was thinking switching to CPS might help, passing the context down on the way to the leaf, so the tree is built "inside" the functor. This is pretty easy for nodes and digits, but I'm struggling with the polymorphic recursion in the spine right now. I think I'll probably figure it out eventually.

On Apr 27, 2016 6:12 PM, "Edward Kmett" wrote:

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For cases were the Functor winds up particularly complicated or won't be known at compile time so the inlining can't help, it turns out you can usually work around this with the 'fusing' combinator from the lens library. This works around the repeated stacks of `fmap` that get built up otherwise, transforming everything into one gigantic fmap all in one go.

There is a similar `confusing` combinator for working with traversals. The implementation lives up to the name.

-Edward

On Wed, Apr 27, 2016 at 5:19 PM, wrote:

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On Tue, Apr 26, 2016, at 15:03, David Feuer wrote:

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Way back when, Shachaf Ben-Kiki suggested an efficient implementation of `at` for Data.Map in https://mail.haskell.org/pipermail/libraries/2013-May/019761.html but that was never approved for inclusion.

Funny thing, I just tried doing that not to long ago with `at`:

https://github.com/haskell/containers/pull/192

In the end, the results were mixed. It was a small optimization in some situations, and a moderate pessimization in others (esp. when the Functor becomes too complex for GHC to optimize).

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