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Today's Topics:
1. Re: Maybe monad and computations (Daniel Trstenjak)
2. Re: Maybe monad and computations (Emmanuel Touzery)
3. Re: Maybe monad and computations (Daniel Trstenjak)
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Message: 1
Date: Sat, 7 Sep 2013 09:49:16 +0200
From: Daniel Trstenjak <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of
primarilybeginner-level topics related to Haskell
<[email protected]>
Cc: The Haskell-Beginners Mailing List - Discussion of
primarilybeginner-level topics related to Haskell
<[email protected]>
Subject: Re: [Haskell-beginners] Maybe monad and computations
Message-ID: <[email protected]>
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> I thought I understood it, but looking at the difference in behaviour in the
> examples I posted, maybe I don't.
I would try to add a type signature for liftM2, perhaps you're than getting a
more helpful error message.
Greetings,
Daniel
------------------------------
Message: 2
Date: Sat, 7 Sep 2013 10:01:14 +0200
From: Emmanuel Touzery <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Maybe monad and computations
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<CAC42Reni0ENzM-Y9Muy3=30MXLPv46+a+FRgJS=ar1dmzav...@mail.gmail.com>
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I got an helpful answer on stackoverflow that probably explains it all.
Otherwise if i add the type, i'll have to bind it to a concrete monad due
to the lack of typeclasses, that's why i didn't put a type. Thanks again.
On Sep 7, 2013 9:50 AM, "Daniel Trstenjak" <[email protected]>
wrote:
> > I thought I understood it, but looking at the difference in behaviour in
> the examples I posted, maybe I don't.
>
> I would try to add a type signature for liftM2, perhaps you're than
> getting a more helpful error message.
>
> Greetings,
> Daniel
> _______________________________________________
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Message: 3
Date: Sat, 7 Sep 2013 10:28:38 +0200
From: Daniel Trstenjak <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of
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Subject: Re: [Haskell-beginners] Maybe monad and computations
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Am 07.09.2013 um 10:01 schrieb Emmanuel Touzery <[email protected]>:
> I got an helpful answer on stackoverflow that probably explains it all.
> Otherwise if i add the type, i'll have to bind it to a concrete monad due to
> the lack of typeclasses, that's why i didn't put a type. Thanks again.
>
Ok, that explains a lot.
But if you need this kind of operation a lot, you could still have something
like:
liftMaybe2 :: (a -> b -> c) -> Maybe a -> Maybe b -> Maybe c
liftMaybe2 f (Just a) (Just b) = Just $ f a b
liftMaybe2 _ _ _ = Nothing
Greetings,
Daniel
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