alterF: No need to delete when the key is absent?!

[sjakobi]

unordered-containers/Data/HashMap/Internal.hs

Lines 1279 to 1286 in 6f1a92f

	alterF f = \ !k !m ->
	let
	!h = hash k
	mv = lookup' h k m
	in (<$> f mv) $ \fres ->
	case fres of
	Nothing -> delete' h k m
	Just v' -> insert' h k v' m

When fres is Nothing, we delete' k from m no matter whether m contained k in the first place. We could skip the deletion though if mv is Nothing.

I'm surprised that the code doesn't already do that. Are there reasons against doing it?

[treeowl]

You're totally right. Someone goofed, and it was probably me. lens says

instance (Eq k, Hashable k) => At (HashMap k a) where
  at k f m = f mv <&> \r -> case r of
    Nothing -> maybe m (const (HashMap.delete k m)) mv
    Just v' -> HashMap.insert k v' m
    where mv = HashMap.lookup k m
  {-# INLINE at #-}

and that looks right (though we want to use the primed versions). Have you checked IntMap for this?

[sjakobi]

Thanks for the quick response, @treeowl, and no worries! :)

IntMap seems to be fine – it already uses the maybe m ... delete pattern both for the lazy and he strict version.

[sjakobi]

Fix is up in #288.