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Today's Topics:
1. Re: Typeclasses and "inheritance" (Chadda? Fouch?)
2. uses of random number generators (Michael P Mossey)
3. Re: uses of random number generators (Brandon S. Allbery KF8NH)
4. Re: uses of random number generators (Aditya Mahajan)
5. Question about class declaration
(Rafael Gustavo da Cunha Pereira Pinto)
6. Re: Question about class declaration (Brent Yorgey)
7. Re: Question about class declaration (Felipe Lessa)
8. Re: Question about class declaration
(Rafael Gustavo da Cunha Pereira Pinto)
9. Re: Typeclasses and "inheritance" (Patrick LeBoutillier)
----------------------------------------------------------------------
Message: 1
Date: Thu, 23 Jul 2009 22:26:07 +0200
From: Chadda? Fouch? <[email protected]>
Subject: Re: [Haskell-beginners] Typeclasses and "inheritance"
To: Patrick LeBoutillier <[email protected]>
Cc: [email protected]
Message-ID:
<[email protected]>
Content-Type: text/plain; charset=UTF-8
On Thu, Jul 23, 2009 at 10:18 PM, Chaddaï
Fouché<[email protected]> wrote:
> Your instance would look like that :
>
>> instance IPAddr IPv4Addr IPv4Host IPv4Mask where
>> Â type Host IPv4Addr = IPv4Host
>> Â type Mask IPv4Addr = IPv4Mask
>> Â host (IPv4Addr h _) = h
>> Â mask (IPv4Addr _ m) = m
Oops... I forgot to trim the extra parameters !
> instance IPAddr IPv4Addr where
> type Host IPv4Addr = IPv4Host
> type Mask IPv4Addr = IPv4Mask
> host (IPv4Addr h _) = h
> mask (IPv4Addr _ m) = m
One advantage compared to the multiparameter + functional dependencies
solution is that you can write :
(IPAddr a) => ...
In your context rather than introducing h and m when they're not needed.
If you need to write "Host a" several time in a function, you can put
the following in your context :
(IPAdrr a, Host a ~ h) => ...
and use h for Host a thereafter.
--
Jedaï
------------------------------
Message: 2
Date: Thu, 23 Jul 2009 22:35:04 -0700
From: Michael P Mossey <[email protected]>
Subject: [Haskell-beginners] uses of random number generators
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Hello,
I have an application for random numbers. So far, in my journey with Haskell, I
have learned basics and a few things about monads, and I was hoping I could get
some guidance how to employ random # gens.
I need to run some simulated experiments to calculate some statistics by a
Monte
Carlo method.
There are several components to the experiment, but one component is:
- Given a hat with 5 red squares and 5 blue squares, draw one square at a time,
note its color, for a total of 8 squares. Remember the order of the 8 squares.
Another function will take two inputs each of which is "red" or "blue". If they
are the same color, return the other color. Otherwise "toss a coin" to
determine
which color to return.
-- this uses Bool rather than "red" "blue" for obvious reasons.
balanceColors :: Bool -> Bool -> Bool
balanceColors c1 c2 =
if c1 == c2
then not c1
else -- flip coin --
Can I get a few examples or pointers? I believe I will have to run this in a
State monad or the IO monad, will I not?
Thanks,
Mike
------------------------------
Message: 3
Date: Fri, 24 Jul 2009 01:42:48 -0400
From: "Brandon S. Allbery KF8NH" <[email protected]>
Subject: Re: [Haskell-beginners] uses of random number generators
To: Michael P Mossey <[email protected]>
Cc: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset="us-ascii"
On Jul 24, 2009, at 01:35 , Michael P Mossey wrote:
> Can I get a few examples or pointers? I believe I will have to run
> this in a State monad or the IO monad, will I not?
IO is only needed to get an initial seed. The standard random number
functions are designed for use with State, but you might also want to
look at http://hackage.haskell.org/package/MonadRandom .
--
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] [email protected]
system administrator [openafs,heimdal,too many hats] [email protected]
electrical and computer engineering, carnegie mellon university KF8NH
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Message: 4
Date: Fri, 24 Jul 2009 02:05:16 -0400
From: Aditya Mahajan <[email protected]>
Subject: [Haskell-beginners] Re: uses of random number generators
To: [email protected]
Message-ID:
<[email protected]>
Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed
On Thu, 23 Jul 2009, Michael P Mossey wrote:
> Hello,
>
> I have an application for random numbers. So far, in my journey with Haskell,
> I have learned basics and a few things about monads, and I was hoping I could
> get some guidance how to employ random # gens.
>
> I need to run some simulated experiments to calculate some statistics by a
> Monte Carlo method.
>
> Can I get a few examples or pointers? I believe I will have to run this in a
> State monad or the IO monad, will I not?
Have a look at the monte-carlo monad:
http://hackage.haskell.org/cgi-bin/hackage-scripts/package/monte-carlo
For example usages see
http://quantile95.com/2008/12/31/monte-carlo-poker-odds/ and
http://randomdeterminism.wordpress.com/2009/01/06/monty-hall-problem-using-monte-carlo-simulations/
Aditya
------------------------------
Message: 5
Date: Fri, 24 Jul 2009 09:41:34 -0300
From: Rafael Gustavo da Cunha Pereira Pinto
<[email protected]>
Subject: [Haskell-beginners] Question about class declaration
To: beginners <[email protected]>
Message-ID:
<[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
Hi folks,
Once in a while I stumble upon declarations like this:
class EditAction e a | e -> a where
apply :: a -> e -> a
Why is the vertical bar there? I understand that => defines a context, but
can't find what is the "|" for.
Thanks
Rafael Gustavo da Cunha Pereira Pinto
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Message: 6
Date: Fri, 24 Jul 2009 08:44:32 -0400
From: Brent Yorgey <[email protected]>
Subject: Re: [Haskell-beginners] Question about class declaration
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
On Fri, Jul 24, 2009 at 09:41:34AM -0300, Rafael Gustavo da Cunha Pereira Pinto
wrote:
> Hi folks,
>
> Once in a while I stumble upon declarations like this:
>
> class EditAction e a | e -> a where
> apply :: a -> e -> a
>
>
>
> Why is the vertical bar there? I understand that => defines a context, but
> can't find what is the "|" for.
It's a functional dependency: the e -> a means that the type e
determines the type a, that is, there can only be one instance of
EditAction for any particular type e.
-Brent
------------------------------
Message: 7
Date: Fri, 24 Jul 2009 09:57:47 -0300
From: Felipe Lessa <[email protected]>
Subject: Re: [Haskell-beginners] Question about class declaration
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
On Fri, Jul 24, 2009 at 08:44:32AM -0400, Brent Yorgey wrote:
> On Fri, Jul 24, 2009 at 09:41:34AM -0300, Rafael Gustavo da Cunha Pereira
> Pinto wrote:
> > class EditAction e a | e -> a where
> > apply :: a -> e -> a
>
> It's a functional dependency: the e -> a means that the type e
> determines the type a, that is, there can only be one instance of
> EditAction for any particular type e.
Alternatively you could write that class using a type family:
class EditAction e where
type Text e :: *
apply :: Text e -> e -> Text e
(I don't know if "Text e" is a good name, but Haskell-wise this
should do the same trick as the fundep class.)
One example of an instance (out of the top of my head):
data Editor = Editor {text :: String, ...}
data EditorAction = Prepend String | ...
instance EditAction EditorAction where
type Text EditorAction = Editor
apply editor (Prepend s) = editor {text = s ++ text editor}
HTH,
--
Felipe.
------------------------------
Message: 8
Date: Fri, 24 Jul 2009 10:53:08 -0300
From: Rafael Gustavo da Cunha Pereira Pinto
<[email protected]>
Subject: Re: [Haskell-beginners] Question about class declaration
To: [email protected]
Message-ID:
<[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
So, it is a language extension as I predicted. A cool one, by the way.
Thank you all!
On Fri, Jul 24, 2009 at 09:44, Brent Yorgey <[email protected]> wrote:
> On Fri, Jul 24, 2009 at 09:41:34AM -0300, Rafael Gustavo da Cunha Pereira
> Pinto wrote:
> > Hi folks,
> >
> > Once in a while I stumble upon declarations like this:
> >
> > class EditAction e a | e -> a where
> > apply :: a -> e -> a
> >
> >
> >
> > Why is the vertical bar there? I understand that => defines a context,
> but
> > can't find what is the "|" for.
>
> It's a functional dependency: the e -> a means that the type e
> determines the type a, that is, there can only be one instance of
> EditAction for any particular type e.
>
> -Brent
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
--
Rafael Gustavo da Cunha Pereira Pinto
Electronic Engineer, MSc.
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Message: 9
Date: Fri, 24 Jul 2009 12:16:26 -0400
From: Patrick LeBoutillier <[email protected]>
Subject: Re: [Haskell-beginners] Typeclasses and "inheritance"
To: Chadda? Fouch? <[email protected]>
Cc: [email protected]
Message-ID:
<[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
Hi,
On Thu, Jul 23, 2009 at 4:26 PM, Chaddaï Fouché <[email protected]>wrote:
> On Thu, Jul 23, 2009 at 10:18 PM, Chaddaï
> Fouché<[email protected]> wrote:
> > Your instance would look like that :
>
> Oops... I forgot to trim the extra parameters !
>
> > instance IPAddr IPv4Addr where
> > type Host IPv4Addr = IPv4Host
> > type Mask IPv4Addr = IPv4Mask
> > host (IPv4Addr h _) = h
> > mask (IPv4Addr _ m) = m
>
This is great stuff and exactly what I wanted. Thank you very much guys!
I kept on hacking at it and so far I have this code:
http://hpaste.org/fastcgi/hpaste.fcgi/view?id=7428
which unfortunately doesn't compile:
Net/IP.hs:44:39:
Couldn't match expected type `Word (Host a)'
against inferred type `Word (Mask a)'
In the second argument of `($)', namely `(bits h) .&. (bits m)'
In the first argument of `makeIPAddr', namely
`(fromBits $ (bits h) .&. (bits m))'
In the first argument of `($)', namely
`makeIPAddr (fromBits $ (bits h) .&. (bits m))'
In the declaration of the class IPAddr, is there any way to force that the
IPHost and IPMask types are made up from the same IPBits type? Basically I
would like the compiler to enforce that Word (Host a) and Word (Mask a) be
the same type for a specific instance of IPAddr.
Note: I'm not sure how practical all this is going to be in the end (perhaps
a bit to convoluted), but it's an excellent learning exercise for me.
Thanks a lot,
Patrick
>
> One advantage compared to the multiparameter + functional dependencies
> solution is that you can write :
> (IPAddr a) => ...
> In your context rather than introducing h and m when they're not needed.
>
> If you need to write "Host a" several time in a function, you can put
> the following in your context :
> (IPAdrr a, Host a ~ h) => ...
> and use h for Host a thereafter.
>
> --
> Jedaï
>
--
=====================
Patrick LeBoutillier
Rosemère, Québec, Canada
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