A similar use-case and same solution with IORefs:
http://hpaste.org/diff/80055/80058 Guess which one threw a
stackoverflow and which one ran indefinitely when given a few hundred
million lines of input.
On 7 January 2013 07:35, Albert Y. C. Lai
On 13-01-07 12:12 AM, Thomas Hartman wrote:
I have a space leak in a function that increments a number inside IORef or STRef (either lazy or strict).
IORef and STRef operations do not automatically evaluate contents. "writeIORef r (x + 1)" simply stores a pointer to the expression (thunk) "x + 1" into the mutable cell. readIORef just reports back a pointer. modifyIORef just calls readIORef and writeIORef. No evaluation throughout.
"modifyIORef incr" where
incr !x = x + 1
does not make a difference because it is just "writeIORef r (incr x))", i.e., simply stores a pointer to the expression (thunk) "incr x" into the mutable cell. The whole process doesn't even care about how many bangs are in incr.
(It is illuminating to consider how "const True (incr x)" does not evaluate x. A pointer to True and a pointer to "incr x" are passed to const, then const throws away the latter without even looking. See also "const True undefined". One day, you will thank "writeIORef r undefined"; I certainly did.)
Same for both Data.STRef.Strict and Data.STRef.Lazy. They do not mean what you think. Here is what they mean:
Data.STRef.Strict means what Control.Monad.ST.Strict means Data.STRef.Lazy means what Control.Monad.ST.Lazy means
Control.Monad.ST.Strict means that the following hangs:
x = head (runST list) where list :: ST s [Bool] list = do {xs <- list; return (True : xs)}
Control.Monad.ST.Lazy means that the above terminates and gives the answer True.
(Up to this point, same story for Control.Monad.State.Strict and Control.Monad.State.Lazy.)
I still have not understood Control.Monad.ST.Lazy enough to articulate its full semantics, but I have some more examples to show what it does:
By understanding what "Lazy" in Control.Monad.ST.Lazy means, you also see what "Strict" does *not* mean.
In IO or Control.Monad.ST.Strict, use
let y = x+1 in y `seq` write[IO/ST]Ref r y
to expedite the evaluation of x+1. Using the same idea, you may write your own modify[IO/ST]RefNOW to evaluate while updating.
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