[Haskell] Monadic Loops

[Gustavo Villavicencio]

An interesting paper on Monads and Recursion is

"Merging Monads and Folds for Functional Programming"
Erik Meijer and Johan Jeuring

best regards,
gustavo

Peter Robinson said:
> On Thursday 17 June 2004 12:39, Johannes Waldmann wrote:
>> > while test body = do
>> >                   (cond,res) <- body
>> >                   if (test cond) then do rs <- while test body
>> >                                       return (res:rs)
>> >                   else return [res]
>>
>> do you need the monad here? what monad is it?
>>
>> the problem could to be that the "return $ res: rs"
>> can happen only after it is certain
>> that  "while test body" succeeds.
>> so you won't even see the very first cons cell
>> before the last one is evaluated.
> See also
> http://haskell.org/hawiki/TailRecursive
>
>>
>> could you produce a (lazy) list of results instead?
>> the garbage collector might be able to collect
>> the list cells that are no longer needed
>>
>> possibly, a lazy state monad would help
>> (if the computation of "while test body" cannot fail)
>>
>> > Is there a better way to implement (possibly infinite) loops in
>> Haskell?
> You could have a look at "Tackling the awkward squad: monadic
> input/output,  concurrency, exceptions, and foreign-language calls in
> Haskell" (Simon Peyton  Jones)
> http://research.microsoft.com/users/simonpj/papers/marktoberdorf/
> where quite a few "control structures" are described.
> Cheers,
> Peter
>>
>> generally, don't program your own recursions -
>> use pre-defined combinators instead.
>> (I like to think of this as a "higher analogon"
>> of "don't use goto - use block structures" from imperative
>> programming)
>>
>> if you need monads, have a look at sequence, sequence_, mapM, mapM_
>> http://www.haskell.org/onlinereport/monad.html
>> if you can do with lists, then use iterate, fold etc.
>> http://www.haskell.org/onlinereport/list.html
>>
>> best regards,
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