Hmmm, I generally try to avoid using seq. It's a hack and often one can
refrain from using it. Here's how I would solve the problem:
First of all your monad constains an int which is passed around. Typically
we want this to be computed strictly. The way to do this is to add a
strictness annotation. A naive try would be this:
newtype M a = M (Int -> (a,!Int))
Unfortunately we cannot have stricness annotation in this way. So to
achieve this effect we make a new data type for pairs with a strict int:
data PairInt a = P a !Int
Secondly, your bind is lazy. This can be usedful but usually one wants it
to be strict because then we get tail-recursion. The lack of
tail-recursion is the second problem in this example. Strictness is
achieved be converting let to case like this:
M f >>= k = M $ \i -> case f i of
P x i2 ->
case k x of
M f2 -> f2 i2
This version doesn't have any problems with spaceleaks. It can now swallow
pretty large numbers, I've tried with 1000000.
I hope this sheds some light on the problem.
/Josef
On Fri, 15 Nov 2002, Magnus Lindberg wrote:
> Hi again!
> Thanks for your help, Malcolm. I added `seq' at two places to prevent
> laziness and now I never run out of memory.
>
> instance Monad M where
> return x = M $ \i -> (x,i)
> M f >>= k = M $ \i -> let (x,i2) = i `seq' f i
> M f2 = x `seq' k x
> in f2 i2
>
> (however, I think that just one of the two `seq' should do but if I
> remove any single `seq' then I still run out of memory :( )
>
> Kind regards
> Magnus
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