Re: "do" notation and ">>"

I don't believe that it will break many programs. How many programs produce large *input independent* output, that is not already literally in the source, in a caf with a long life-time?

That sounds like a description of all the animation programs in Paul Hudak's School of Expression book and there's plenty more examples like that. [I haven't tested whether these programs do leak space with the modified compiler - my point is that there is a large class of programs with exactly the characteristics you describe.] -- Alastair Reid ps I think your CAF restriction is a bit of a red herring - Koen's modification to make his example leak in GHC (which GCs CAFs) shows that the leak happens as long as the relevant thunk isn't collected.