Re: [Haskell-beginners] Maybe monad and computations

6 Sep 2013


      Hi,
  In that case you can use <$> and <*>.
  Your code will look like, .... <$> doA <*> doB  where ... is the
rest of the computation

Thanks
Divyanshu Ranjan

On Sat, Sep 7, 2013 at 2:34 AM, Emmanuel Touzery  wrote:
...
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 
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" 
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 
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 
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
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-- Michael Steele
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Re: [Haskell-beginners] Maybe monad and computations

divyanshu ranjan