Hi, I thought a lot about everywhere' when writing uniplate. I have an equivalent of everywhere (transform), but no transform'. In the end I decided that it was virtually impossible to use everywhere' correctly, and that almost every call of everywhere' was either somewhere an everywhere would have worked equally well, or a bug. I then went through all the Haskell source code I could find, found about 6 calls to everywhere', and all of them would have performed the same job as everywhere. So, my suggestion is that everywhereM' probably isn't very useful, but descend/descendM (or compos) is a useful top-down traversal scheme. Thanks, Neil On Mon, Apr 12, 2010 at 8:35 PM, José Pedro Magalhães wrote:

Hi Maciej,

Data.Generics.Schemes is now on the syb package, and I'm its maintainer. But I'm not so sure it is wise to start adding more traversal schemes of this form, since there might be many of them: if we include everywhereM', we certainly have to include the bottom-up everywhereM too. Then there are the possible Applicative variants, etc...

In general, since these traversal functions are small and everywhere and everywhere' already show the pattern of how to write them, maybe it's best that they are defined by the user, unless there is some general consensus that they would be widely useful.

Cheers, Pedro

On Mon, Apr 12, 2010 at 20:56, Maciej Podgurski wrote:

...

Hello,

as an analogue to everywhere', I suggest adding everywhereM' to Data.Generics.Schemes. The implementation would be

  everywhereM' :: Monad m => GenericM m -> GenericM m   everywhereM' f x = do { x' <- f x;                           gmapM (everywhereM' f) x' }

Best wishes,

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