> Or a variant of Functor constructor class that I have proposed some time > ago on comp.lang.functional: > > class FunctorM t where > fmapM :: Monad m => (a -> m b) -> (t a -> m (t b)) > fmapM_ :: Monad m => (a -> m b) -> (t a -> m ()) > fmapM_ f t = fmapM f t >> return () > > instance FunctorM [] where > fmapM = mapM > fmapM_ = mapM_
I am sorry, I misunderstood the problem.
I think you responded usefully to the spirit of my question, if not the letter. I think I may be able to use these ideas.
Your FunctorM class looks rather like the kind of idea I was wrestling with when I posted my question.
If I define a monad to accumulate the desired result, and a monadic form of the transformation/collector function, then I think I can define and use fmapM to return a transformed expression (tree, or whatever) and pick the accumulated value from the monad.
I'm trying to decide whether to try this (which has the advantage, AFAICT, of not using any language extension) or go for the a gmap based approach (which may, in the longer run, be more flexible). Mainly, I'm trying to figure which is easier in the near term.
#g
------------------- Graham Klyne <[EMAIL PROTECTED]> PGP: 0FAA 69FF C083 000B A2E9 A131 01B9 1C7A DBCA CB5E
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