[Haskell-cafe] help diagnosing space leak with IORef/STRef, just incrementing a million times.

Albert Y. C. Lai trebla at vex.net
Mon Jan 7 07:35:32 CET 2013

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 

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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