> {-# 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; #-}

• I've occasionally had problems getting inlining to actually happen, which is irritating.
• Maintaining portability requires almost as much code for the preprocessor as the rules I wrote above, and is finicky at best.
• Perhaps my biggest concern with that approach is that it doesn't play nicely with foldl, which came up in the application that motivated this proposal!  The rewrite rule approach I outline above is perfectly compatible with foldr/build rewriting, but also permits foldls to function nicely.