On Wed, Dec 21, 2011 at 7:31 AM, Joachim Breitner
this problem comes up in various places; forcing the whole thunk is of course an option, but would it be possible – theoretical, with the GHC runtime, or even with current tools – to have a function evaluateUntilItDoesNotNeed :: a -> b -> a such that f x `evaluateUntilItDoesNotNeed` x will not retain a reference to x, but is otherwise as lazy as possible?
If thunks retain references to their free variables, and these can be introspected, then it should be possible to keep seq’ing those thunks that refer to x, until the expression is either fully evaluated or no referenced thunk references x.
Sounds complicated. What about map f xs `evaluateUntilItDoesNotNeed` xs If it evaluated only the head of 'map f xs' then the result would not depend on 'xs' anymore, but it would depend on the tail of 'xs'. Cheers, -- Felipe.