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Today's Topics:
1. Re: Overloading resolution with numbers (damodar kulkarni)
2. Re: Combining the Rand and State monads (Brent Yorgey)
3. Re: Combining the Rand and State monads (Amy de Buitl?ir)
----------------------------------------------------------------------
Message: 1
Date: Thu, 5 Apr 2012 20:29:41 +0530
From: damodar kulkarni <[email protected]>
Subject: Re: [Haskell-beginners] Overloading resolution with numbers
To: [email protected]
Cc: Amy de Buitl?ir <[email protected]>
Message-ID:
<cad5hsyqczirf6ztdppyew9jwhfm7ojnsvmptuudydqkohcp...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
See this:
Prelude> :t 2.3
2.3 :: Fractional a => a
Hope this helps.
On Thu, Apr 5, 2012 at 7:24 PM, <[email protected]> wrote:
> On Thu, Apr 05, 2012 at 01:18:39PM +0000, Amy de Buitl?ir wrote:
> > <j.romildo <at> gmail.com> writes:
> > > Why does (read "2" + 1) works, but (read "2.3" + 1) fail at runtime?
> >
> > Try this:
> >
> > read "2.3" + 1 :: Float
> >
> > Or this:
> >
> > read "2.3" + 1.0
> >
> > The reason that your version didn't work is because GHCi is guessing
> that you
> > want the read operation to parse an Integer, since you're adding it to 1.
>
> This is explanation does not seem to be enough once we consider the type
> of the literal 1:
>
> Prelude> :t 1
> 1 :: Num a => a
>
> That is, the literal 1 is overloaded to any numeric type. It is not
> necessarily an Integer.
>
> Romildo
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
--
Thanks and regards,
-Damodar Kulkarni
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Message: 2
Date: Thu, 5 Apr 2012 13:52:47 -0400
From: Brent Yorgey <[email protected]>
Subject: Re: [Haskell-beginners] Combining the Rand and State monads
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=iso-8859-1
On Thu, Apr 05, 2012 at 10:42:20AM +0000, Amy de Buitl?ir wrote:
> I'm trying to develop a very simple simulation framework. A simulation
> consists
> of a list of models. Models generate output events based on an input event.
> Here
> is what I have currently (it works fine).
>
>
> modelT :: Model g
> modelT _ = do
> s <- get
> put $ s ++ " R"
> n <- HOW DO I GET A RANDOM NUMBER????
> return [n]
I assume you are using the MonadRandom package? Getting a random
number is as simple as doing something like "getRandomR ('A','Z')" (or
using any other method from the MonadRandom class [1]). There is an
instance MonadRandom m => MonadRandom (StateT s m), so calls to
getRandom, getRandomR, etc. are automatically lifted into your Model
monad.
Your problem seems to be the type you have given to modelT: it claims
that it will work for *any* type g but this is not so; g must
represent a pseudorandom number generator. This works:
modelT :: RandomGen g => Model g
modelT _ = do
s <- get
put $ s ++ " R"
n <- getRandomR ('A', 'Z')
return [n]
-Brent
[1]
http://hackage.haskell.org/packages/archive/MonadRandom/0.1.6/doc/html/Control-Monad-Random-Class.html
------------------------------
Message: 3
Date: Thu, 5 Apr 2012 21:55:11 +0000 (UTC)
From: Amy de Buitl?ir <[email protected]>
Subject: Re: [Haskell-beginners] Combining the Rand and State monads
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
Brent Yorgey <byorgey <at> seas.upenn.edu> writes:
> Your problem seems to be the type you have given to modelT: it claims
> that it will work for *any* type g but this is not so; g must
> represent a pseudorandom number generator. This works:
D'oh! A dumb mistake on my part. Thank you, Brent.
------------------------------
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