[Haskell-beginners] Maybe monad and computations

[Emmanuel Touzery]

Hmm I've been looking some more. Turns out I'm pretty sure... What I've
been looking for is liftMx.

In this case, liftM2.

liftM2 ... doA doB

where "..." represents the rest of the computation as it did in the example
snippet I sent in my first email.

In the case of the Maybe monad, "..." will be called with the "Just" values
and only if both doA and doB are not Nothing.

and for "n" operations, there is "ap", I'll look at that one too.

emmanuel


On Fri, Sep 6, 2013 at 10:43 PM, Emmanuel Touzery <etouzery at gmail.com>wrote:

> Thank you (and the others) for tips. I'll check the suggestions but yes
> sequence requires that all items have the same type so that sequence2 would
> work better but feels un-idiomatic.
> I needed such code several times in several contexts, so i assumed the
> answer would be obvious. Maybe i'm structuring my code the wrong way for
> haskell, i'll rethink those functions. Thanks for now!
>
> Emmanuel
> On Sep 6, 2013 10:37 PM, "Nicholas Vanderweit" <nick.vanderweit at gmail.com>
> wrote:
>
>> It seems like he wants something like sequence, but for tuples. Say:
>>
>> sequence2 :: Monad m => (m a, m b) -> m (a, b)
>> sequence2 (ma, mb) = ma >>= \a -> mb >>= \b -> return (a, b)
>>
>> But without knowing the use case it's hard to know whether or not this
>> could be done simply with "sequence" and mapM*
>>
>>
>> Nick
>>
>>
>> On Fri, Sep 6, 2013 at 2:28 PM, Michael Steele <mikesteele81 at gmail.com>wrote:
>>
>>> The `sequence`, `mapM`, and `mapM_` functions may be what you are
>>> looking for.
>>>
>>> let allTogether = sequence [doA, doB, doC, doZ]
>>> case allTogether of
>>>   Nothing -> ...
>>>   Just as -> ...
>>>
>>>
>>> Use `maybe` and `fromMaybe` to avoid case statements:
>>>
>>> fromMaybe "failure!" $ do
>>>     as <- sequence [doA, doB, doC, doZ]
>>>     return $ "Success: " ++ show (length as)
>>>
>>> The `catMaybes` function can be used to execute each computation in
>>> the list even if some of them return nothing.
>>>
>>> On Fri, Sep 6, 2013 at 12:49 PM, Emmanuel Touzery <etouzery at gmail.com>
>>> wrote:
>>> > Hello,
>>> >
>>> >  I'm often using the Maybe monad to combine Maybe computations one
>>> after the
>>> > other without conditionals.
>>> >
>>> >  But I'm not sure how to do it, when I have several operations which
>>> return
>>> > Maybe and I want all the indiviual values in the end, not a
>>> combination of
>>> > the values.
>>> >
>>> >  Currently I do:
>>> >
>>> > let allTogether = do
>>> >   ma <- doA
>>> >   mb <- doB
>>> >   return (ma, mb)
>>> > case allTogether of
>>> >   Nothing -> ...
>>> >   Just (a, b) -> ...
>>> >
>>> >  This way in this case I have one case instead of two, however I'm sure
>>> > there must be a nicer way to achieve this result?
>>> >
>>> >  Thank you!
>>> >
>>> > Emmanuel
>>> >
>>> > _______________________________________________
>>> > Beginners mailing list
>>> > Beginners at haskell.org
>>> > http://www.haskell.org/mailman/listinfo/beginners
>>> >
>>>
>>>
>>>
>>> --
>>> -- Michael Steele
>>>
>>> _______________________________________________
>>> Beginners mailing list
>>> Beginners at haskell.org
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>>
>>
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