Proposal: Non-allocating way to iterate over a Data.Map: traverseWithKey_

Ryan Newton rrnewton at
Tue Aug 6 15:51:12 CEST 2013

On Sat, Aug 3, 2013 at 5:31 PM, Simon Peyton-Jones <simonpj at>

> > Fixing this involves **nested** CPR analysis, which I am working on at
the moment.
Sounds neat!  Is nested CPR analysis on a branch?

> This one I do not understand. Could you pull out the two alternative ways
> of phrasing this algorithm into a standalone form, in which one allocates
> more than t’other?  Then I could investigate more easily.
Milan pasted the relevant code (thanks) for traverseWithKey_ vs.
foldWithKey which I reattach at the bottom of this email.

On Sun, Aug 4, 2013 at 7:48 AM, Sjoerd Visscher <sjoerd at> wrote:
> Would it help if traverse_ was implemented with foldMap using the
> Traverse_ newtype? In which case maybe we should fix Data.Foldable!

That sounds good to me!  And straightforward.  Any objections?

Milan, why does it bother you that there is no specified order?  I'm
perfectly happy as long as its deterministic on a particular machine.  (I
have never been sure whether pure code in Haskell must be deterministic
across multiple machines... numCapabilities was a counter example for a
long time.)  Aren't we already used to using Data.Map.toList and
Data.Set.toList where order is not specified?  Further, other languages
(like Scheme) have maps/folds that do not specify order of side effects.


P.S.: Relevant code:

traverseWithKey_ :: Applicative t => (k -> a -> t b) -> Map k a -> t ()
  traverseWithKey_ f = go
    where go Tip = pure ()
          go (Bin _ k v l r) = f k v *> go l *> go r

  foldrWithKey :: (k -> a -> b -> b) -> b -> Map k a -> b
  foldrWithKey f z = go z
    where go z' Tip = z'
          go z' (Bin _ kx x l r) = go (f kx x (go z' r)) l

and we call them like this:
  let actionTraverse _k 1000000 = putStrLn "Hit 1000000"
      actionTraverse _k _v = return ()
  in traverseWithKey_ actionTraverse map

and this:
  let actionFoldr _k 1000000 z = putStrLn "Hit 1000000" *> z
      actionFoldr _k _v z = z
  in foldrWithKey actionFoldr (pure ()) map
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