Bug in Control.Monad.State

Simon Marlow simonmar at microsoft.com
Wed Nov 23 05:47:02 EST 2005

Apologies for being a bit blunt, but can't this problem be solved by
introducing a LazyStateT in addition to StateT?  This follows the
pattern of ST (default is strict, we also have Control.Monad.ST.Lazy).


On 23 November 2005 09:59, Udo Stenzel wrote:

> Hello Yitzchak,
>>> If an operation is too strict...  you can always
>>> wrap the offending data in another constructor.
>> I have seen this claim several times, but I am
>> not yet convinced.
>> In my case, the unwanted strictness is coming from
>> the bind method of a monad from the standard
>> libraries. I don't see how to get rid of that
>> strictness by wrapping something in a constructor.
> as Iavor already pointed out, theres the fine difference between being
> string in some implementation detail (the pair of state and result) or
> in the state itself, which I missed.
> I was thinking of the state component, and I maintain that it should
> be strict.  Rationale: if the state is some primitive type, you want
> it strict anyway.  If it isn't, the strictness doesn't cost much
> anyway. If it's still wrong, wrapping in a constructor helps.
> Your problem however is the strict match against a pair you don't even
> know is there.  Indeed, a strict match against a single constructor
> datatype is probably always pointless.  So it's really a bug in the
> library, after all.
>> Here is an example. How would you fix this by
>> wrapping something in a constructor?
> You can't, since you don't have access to the offending pair.
>> createItems :: RandomGen g => State g [Item]
>> createItems =
>>   liftM catMaybes $ runListT $
>>   flip evalStateT initialState $ runMaybeT $
>>   do
>>     item <- liftRandom $ repeatM randomItem
>>     updateState item
>>     needMoreItems >>= guard
>>     return item
>>   where
>>     liftRandom = lift . lift . lift
> ...but this is probably broken anyway.  After (repeatM randomItem)
> presumably the state (the RandomGen) is _|_, but the type of
> createItems would suggest it is still usable.  I wouldn't do that. 
> Other than that it's a bit hard to see what you're trying to
> accomplish here. 
> Udo.

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