Foldl' doesn't leak, but it sure looks like it does, if you think the folded function is strict in the accumulator argument, but isn't.
For example, this attempt at computing the average of a list
uncurry (/) . foldl' (\(s,l) x -> (s+x,l+1)) (0,0)
leaks, because during the recursion, there is no reason to scrutinize the intermediate pairs. This version is fine:
uncurry (/) . foldl' (\(!s,!l) x -> (s+x,l+1)) (0,0)
I don't think it does. 'Strict' doesn't add an implicit ! to nested patterns (so (s,l) becomes !(s,l), not (!s,!l)), and 'StrictData' doesn't change anything about the tuple, which comes from a different module. (Disclaimer: I haven't actually used 'Strict', this is my interpretation of the user guide.)
