Re: [Haskell-cafe] MonadRandom-computation that does not terminate

2011-01-12 Thread Neil Brown 

On 11/01/11 23:19, Tim Baumgartner wrote:

Hi,

I'm having difficulties with this function I wrote:

iterateR :: (MonadRandom m) = (a - m a) - a - m [a]
iterateR g s = do
  s' - g s
  return (s:) `ap` iterateR g s'

I'm running the computation with evalRandIO and surprisingly the first 
call of main in ghci succeeds, but the second does not terminate. 
Reproducible.

Any clues what I'm doing wrong here?


If we unfold ap we get:

iterateR g s = do
  s' - g s
  f - return (s:)
  x - iterateR g s'
  return (f x)

What happens here depends on exactly how the monad is defined, but for 
many monads that will form an infinite loop that prevents a value being 
returned.  In the case of RandT from MonadRandom, it is not possible to 
execute the action after the iterateR call finishes without knowing the 
final state from the call, which requires evaluating the infinite loop 
of monadic actions.  Does that help?


Thanks,

Neil.

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Re: [Haskell-cafe] MonadRandom-computation that does not terminate

2011-01-12 Thread Tim Baumgartner 
2011/1/12 Neil Brown nc...@kent.ac.uk

 On 11/01/11 23:19, Tim Baumgartner wrote:

 Hi,

 I'm having difficulties with this function I wrote:

 iterateR :: (MonadRandom m) = (a - m a) - a - m [a]
 iterateR g s = do
  s' - g s
  return (s:) `ap` iterateR g s'

 I'm running the computation with evalRandIO and surprisingly the first
 call of main in ghci succeeds, but the second does not terminate.
 Reproducible.
 Any clues what I'm doing wrong here?


 If we unfold ap we get:


 iterateR g s = do
  s' - g s
  f - return (s:)
  x - iterateR g s'
  return (f x)

 What happens here depends on exactly how the monad is defined, but for many
 monads that will form an infinite loop that prevents a value being returned.
  In the case of RandT from MonadRandom, it is not possible to execute the
 action after the iterateR call finishes without knowing the final state from
 the call, which requires evaluating the infinite loop of monadic actions.
  Does that help?


Yes, this helps definitely. So if I understand you right, the infinite loop
was not entered immediately because of lazyness? That's funny somehow.

Thanks a lot
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Re: [Haskell-cafe] MonadRandom-computation that does not terminate

2011-01-12 Thread Brent Yorgey 
On Wed, Jan 12, 2011 at 12:19:50AM +0100, Tim Baumgartner wrote:
 Hi,
 
 I'm having difficulties with this function I wrote:
 
 iterateR :: (MonadRandom m) = (a - m a) - a - m [a]
 iterateR g s = do
   s' - g s
   return (s:) `ap` iterateR g s'

As a side note, the MonadRandom constraint is funny, since this
function doesn't depend on any sort of randomness.  You may consider
changing the constraint to simply Monad m = ...

-Brent

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[Haskell-cafe] MonadRandom-computation that does not terminate

2011-01-11 Thread Tim Baumgartner 
Hi,

I'm having difficulties with this function I wrote:

iterateR :: (MonadRandom m) = (a - m a) - a - m [a]
iterateR g s = do
  s' - g s
  return (s:) `ap` iterateR g s'

I'm running the computation with evalRandIO and surprisingly the first call
of main in ghci succeeds, but the second does not terminate. Reproducible.
Any clues what I'm doing wrong here?

Thanks in advance,
Tim
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