Can you achieve the same performance gain by adding some rewrite rules?

Perhaps you could try to spot (>>=) malloc (\p -> newForeignPtr p free) Hmmm. Actually I'd like to do both: add the functions, because they encapsulate a common case and guarantee a speed improvement if you use them, and add the rewrite rule because it might catch some cases in existing code. The problem with relying on rewrite rules exclusively is that they tend to be a bit fragile and can fail to trigger without you noticing. Cheers, Simon