On Sat 23/05/09 4:15 PM , Alexander Dunlap alexander.dunlap@gmail.com sent:

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Does anybody know of a pragma or another way to make a function *non-strict* even if it does always evaluate its argument? In other words, is there a way to selectively disable the strictness optimization?

parallelize a b | False = (undefined, undefined)                 | otherwise = a `par` (b `pseq` (a, b))

might do, unless strictness analysis is smart enough to know that the False guard is always, well, False. _______________________________________________

GHC.Prim.lazy?

It's GHC.Exts.lazy nowadays, and it doesn't have any effect. Neither does the `| False' guard. The only way I found to disable the eager argument evaluation is to pass them in as functions:

parallelize :: Num t => (t -> a) -> (t -> b) -> (a, b) parallelize a b = let a' = a 1 b' = b 1 in (a' `par` (b' `pseq` (a', b')))

Then it can be imported and called like this:

test n1 n2 = let (p1, p2) = parallelize (\n0-> product $ factors $ product [n0..n1]) (\n0-> product $ factors $ product [n0..n2])

This solution is incredibly fragile. If the declared type of parallelize is modified by replacing t by Integer, the evaluation becomes eager again. Also, the argument functions can't simply take () for argument which would make this solution reasonable. If this is all the strictness optimizer's fault, I'm awed by how difficult it is to trick it into not doing its job.