[Haskell-cafe] how to write a loop in haskell way

[ender]

2010/12/19 Henning Thielemann <lemming at henning-thielemann.de>:
>
> On Sun, 19 Dec 2010, ender wrote:
>
>>>  do
>>>  alloca $ \value -> do
>>>  poke value (500000::Int)
>>>  allocaArray 4 $ \part_stack -> do
>>>  alloca $ \part_ptr -> do
>>>  poke part_ptr part_stack
>>>  let loop = do
>>>           val <- peek value
>>>           if val == 0 then return () else do
>>>           p <- peek part_ptr
>>>           poke p (val `rem` 10000)
>>>           poke part_ptr (p `plusPtr` 1)
>>>           poke value (val `quot` 10000)
>>>           loop
>>>  loop
>>
>> and I really think that's not a "haskell way", it's just translate c
>> code into haskell code byte by byte
>> My question is: how to translate above c code into haskell in "haskell
>> way"
>
> If the count of loop runs does not depend on results of the loop body, then
> 'mapM' and 'mapM_' applied to the list of increasing pointers are your
> friends. In your case, the loop aborts when 'val' becomes zero. I'm
> certainly thinking too complicated, but you might use MaybeT IO () (e.g.
> from transformers package) and abort 'mapM_' with 'mzero' when 'val' becomes
> zero. (MaybeT IO a) is like an IO monad with an early exit (somehow an
> exception) option.
>
Hi Henning:
   Thanks for your quick reply. So recursive and monad is the proper
way to simulate loop,right?

Thanks and BR