On 22/04/2010, at 08:47, Louis Wasserman wrote:
Additionally, it takes relatively little effort to ensure that keys, elems, toList, etc. deforest under folds. Therefore, we add the following rewrite rules:
{-# RULES
"foldr/Data.Map.elems" forall f z m . foldr f z (elems m) = foldr f z m; "foldl/Data.Map.elems" forall f z m . foldl f z (elems m) = foldl f z m; "foldr/Data.Map.keys" forall f z m . foldr f z (keys m) = foldrWithKey (\ k _ -> f k) z m; "foldl/Data.Map.keys" forall f z m . foldl f z (keys m) = foldlWithKey (\ z k _ -> f z k) z m; "foldr/Data.Map.toAscList" forall f z m . foldr f z (toAscList m) = foldrWithKey (curry f) z m; "foldl/Data.Map.toAscList" forall f z m . foldl f z (toAscList m) = foldlWithKey (curry . f) z m; #-}
Why not implement elems etc. in terms of build? This would make them play nicely with foldr/build fusion at no cost. Roman