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Today's Topics:
1. [m [a]] -> m [a] (Guy)
2. Re: [m [a]] -> m [a] (Daniel Fischer)
3. Re: [m [a]] -> m [a] (Guy)
4. Re: [m [a]] -> m [a] (Daniel Fischer)
5. Re: Question: Data Type for user selection (Brent Yorgey)
6. Re: [m [a]] -> m [a] (Brent Yorgey)
7. Re: How to find a "best alternative" (Henk-Jan van Tuyl)
8. Re: [m [a]] -> m [a] (Tony Morris)
----------------------------------------------------------------------
Message: 1
Date: Wed, 17 Aug 2011 14:20:55 +0300
From: Guy <[email protected]>
Subject: [Haskell-beginners] [m [a]] -> m [a]
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Is there a function which can do this, where m is not mplus?
I would like all the lists to be joined inside the monad with ++.
------------------------------
Message: 2
Date: Wed, 17 Aug 2011 13:32:00 +0200
From: Daniel Fischer <[email protected]>
Subject: Re: [Haskell-beginners] [m [a]] -> m [a]
To: [email protected]
Cc: Guy <[email protected]>
Message-ID: <[email protected]>
Content-Type: Text/Plain; charset="iso-8859-1"
On Wednesday 17 August 2011, 13:20:55, Guy wrote:
> Is there a function which can do this, where m is not mplus?
>
> I would like all the lists to be joined inside the monad with ++.
sequence gives you m [[a]], then an fmap concat (or liftM concat, since
fmap requires an additional Functor constraint) flattens the resulting list
to m [a].
------------------------------
Message: 3
Date: Wed, 17 Aug 2011 15:05:43 +0300
From: Guy <[email protected]>
Subject: Re: [Haskell-beginners] [m [a]] -> m [a]
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
On 17/08/2011 14:32, Daniel Fischer wrote:
> On Wednesday 17 August 2011, 13:20:55, Guy wrote:
>> Is there a function which can do this, where m is not mplus?
>>
>> I would like all the lists to be joined inside the monad with ++.
>
> sequence gives you m [[a]], then an fmap concat (or liftM concat, since
> fmap requires an additional Functor constraint) flattens the resulting list
> to m [a].
Thanks. I was hoping that there would be some kind of fold that could do this;
I take it there isn't one?
------------------------------
Message: 4
Date: Wed, 17 Aug 2011 14:33:20 +0200
From: Daniel Fischer <[email protected]>
Subject: Re: [Haskell-beginners] [m [a]] -> m [a]
To: [email protected]
Cc: Guy <[email protected]>
Message-ID: <[email protected]>
Content-Type: Text/Plain; charset="iso-8859-1"
On Wednesday 17 August 2011, 14:05:43, Guy wrote:
> On 17/08/2011 14:32, Daniel Fischer wrote:
> > On Wednesday 17 August 2011, 13:20:55, Guy wrote:
> >> Is there a function which can do this, where m is not mplus?
> >>
> >> I would like all the lists to be joined inside the monad with ++.
> >
> > sequence gives you m [[a]], then an fmap concat (or liftM concat,
> > since fmap requires an additional Functor constraint) flattens the
> > resulting list to m [a].
>
> Thanks. I was hoping that there would be some kind of fold that could do
> this; I take it there isn't one?
>
Well, sequence is a fold, and concat is a fold. You can fuse them
foldr (\x xs -> x >>= \ys -> xs >>= \zs -> return (ys ++ zs)) (return [])
but is that really more readable?
------------------------------
Message: 5
Date: Wed, 17 Aug 2011 09:42:37 -0400
From: Brent Yorgey <[email protected]>
Subject: Re: [Haskell-beginners] Question: Data Type for user
selection
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
On Tue, Aug 16, 2011 at 04:44:15PM +0200, Ertugrul Soeylemez wrote:
> Brent Yorgey <[email protected]> wrote:
>
> > > That's a bit of a contradiction, because you are using existentials
> > > yourself in your GADT.
> >
> > No, he isn't.
> >
> > data BasicSelect a where
> > SelectionNum :: Num a => a -> BasicSelect a
> > SelectionStr :: Show a => a -> BasicSelect a
> >
> > 'a' shows up in the result type of both constructors, so there is no
> > existential quantification going on here.
>
> Oh, right. How would one express this as an ADT? Seems impossible to
> me.
You cannot, with just Haskell 2010. Strangely, if you try this:
data BasicSelect a = Num a => SelectionNum a
| Show a => SelectionStr a
you get this error (ghc 7.0.3):
Data constructor `SelectionNum' has existential type variables, or a
context
(Use -XExistentialQuantification or -XGADTs to allow this)
In the definition of data constructor `SelectionNum'
In the data type declaration for `BasicSelect'
And enabling ExistentialQuantification makes the error go away! So
apparently the ExistentialQuantification flag also enables type class
constraints on data constructors, even when no existential
quantification is involved. Odd.
-Brent
------------------------------
Message: 6
Date: Wed, 17 Aug 2011 09:47:04 -0400
From: Brent Yorgey <[email protected]>
Subject: Re: [Haskell-beginners] [m [a]] -> m [a]
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
On Wed, Aug 17, 2011 at 01:32:00PM +0200, Daniel Fischer wrote:
> On Wednesday 17 August 2011, 13:20:55, Guy wrote:
> > Is there a function which can do this, where m is not mplus?
> >
> > I would like all the lists to be joined inside the monad with ++.
>
> sequence gives you m [[a]], then an fmap concat (or liftM concat, since
http://spl.smugmug.com/Humor/Lambdacats/lift-concat/960794992_KUg77-M-1.jpg
------------------------------
Message: 7
Date: Wed, 17 Aug 2011 16:13:59 +0200
From: "Henk-Jan van Tuyl" <[email protected]>
Subject: Re: [Haskell-beginners] How to find a "best alternative"
To: [email protected], "Martin Drautzburg"
<[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; charset=iso-8859-15; format=flowed;
delsp=yes
On Sun, 14 Aug 2011 21:11:21 +0200, Martin Drautzburg
<[email protected]> wrote:
> Hello all,
>
> I want to write transformations on musical motives and I came upon the
> following algorithmic question:
>
I think you have a better chance of an answer on the Haskell-cafe mailing
list, as there more people reading that list. There are several Haskellers
with music expertise.
Regards,
Henk-Jan van Tuyl
--
http://Van.Tuyl.eu/
http://members.chello.nl/hjgtuyl/tourdemonad.html
--
------------------------------
Message: 8
Date: Thu, 18 Aug 2011 09:50:25 +1000
From: Tony Morris <[email protected]>
Subject: Re: [Haskell-beginners] [m [a]] -> m [a]
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=ISO-8859-1
mapM join
On 08/17/2011 09:20 PM, Guy wrote:
> Is there a function which can do this, where m is not mplus?
>
> I would like all the lists to be joined inside the monad with ++.
>
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
--
Tony Morris
http://tmorris.net/
------------------------------
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