Hi Neil, for a library I use a version of everywhereM', similar to Maciej's one, too. I need to traverse an AST in top-down manner and replace some terms by others, like in the following simplified example: B (A (B x)) -> f x A (B x) -> g x B x -> h x x -> x Here a bottom-up traversal would produce wrong results. (The functions f, g, h don't make use of A and B, so I didn't think about if everywhereM' would completely fail if they would.) This said, if I understand your last post correctly, the use of everywhereM' would be a bug here in your eyes. Do you agree? Because I don't think so. I haven't done a lot of testing yet, but each case where a replacement has to be performed works as expected. And even if it's not a bug, do you suggest to drop everywhereM' and use uniplate instead, just to play it safe? Regards, Matthias Am 18.04.2010 09:38 schrieb Neil Mitchell:
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