[Haskell] Control.Monad.Writer as Python generator

[ChrisK]

Thanks for the Cont example, David.  But...

The MonadCont is clever and it works ... but then fails -- ghci does 
not garbage collect and it blows up.
With the MonadCont version I can count up to 10^7 zeros:

*Main> length $ take (10^7) zerosInf
10000000
(26.20 secs, 0 bytes)

But this increases RSIZE of ghc-6.4 to 165MB.  The 10^8 version goes to 
swap space and I had to kill it.  My original MonadWriter version does 
not increase RSIZE when run (constant space), so the garbage collection 
must be working, and it is O(N) in the # of zeros counted:

*Main> length $ take (10^7) zerosInf
10000000
(1.22 secs, 0 bytes)
*Main> length $ take (10^8) zerosInf
100000000
(10.05 secs, 0 bytes)
*Main> length $ take (10^9) zerosInf
1000000000
(109.83 secs, 6 bytes)

-- 
Chris


On Apr 14, 2005, at 1:05 AM, David Menendez wrote:

> ChrisK writes:
>
>> I was thinking to myself:
>> What in Haskell would give me a "yield" command like a Python
>> generator?
>>
>> And the answer was "tell" in Control.Monad.Writer -- and I wrote some
>> simple examples (see below).
>
> Another possibility would be a continuation monad.
>
>     import Control.Monad.Cont
>
>     yield :: a -> Cont [a] ()
>     yield x = Cont (\c -> x : c ())
>
>     asGenerator :: Cont [a] v -> [a]
>     asGenerator (Cont f) = f (const [])
> -- 
> David Menendez <zednenem at psualum.com> | "In this house, we obey the 
> laws
> <http://www.eyrie.org/~zednenem>      |        of thermodynamics!"