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Today's Topics:
1. Installing Haskell on a Mac (Torsten Otto)
2. Re: Installing Haskell on a Mac (Antoine Latter)
3. State monad (Britt Anderson)
4. Re: State monad (Mihai Maruseac)
5. Re: State monad (Mihai Maruseac)
6. Re: State monad (Britt Anderson)
7. Re: State monad (Ozgur Akgun)
8. Monad Transformer Type Signatures (Britt Anderson)
9. Re: Monad Transformer Type Signatures (Brent Yorgey)
----------------------------------------------------------------------
Message: 1
Date: Mon, 14 Feb 2011 15:28:43 +0100
From: Torsten Otto <t-otto-n...@gmx.de>
Subject: [Haskell-beginners] Installing Haskell on a Mac
To: beginners <beginners@haskell.org>
Message-ID: <92f44889-644a-468e-80a1-533cdc763...@gmx.de>
Content-Type: text/plain; charset=us-ascii; format=flowed; delsp=yes
Hi!
Two students in my class tried to install Haskell on their private
MacBooks (one Leopard, one SnowLeopard). We used the package at
http://hackage.haskell.org/platform/mac.html
. Neither of them got to install it as the button "Install" is greyed
and not clickable.
What is the best way to install GHC on a current Mac? It would not
necessarily have to be the latest greatest.
Regards,
Torsten
------------------------------
Message: 2
Date: Mon, 14 Feb 2011 09:28:25 -0600
From: Antoine Latter <aslat...@gmail.com>
Subject: Re: [Haskell-beginners] Installing Haskell on a Mac
To: Torsten Otto <t-otto-n...@gmx.de>
Cc: beginners <beginners@haskell.org>
Message-ID:
<AANLkTi=YaiPtonGN=jy2sduhz_dhzopqr9jvnrh-v...@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
On Mon, Feb 14, 2011 at 8:28 AM, Torsten Otto <t-otto-n...@gmx.de> wrote:
> Hi!
>
> Two students in my class tried to install Haskell on their private MacBooks
> (one Leopard, one SnowLeopard). We used the package at
> http://hackage.haskell.org/platform/mac.html. ?Neither of them got to
> install it as the button "Install" is greyed and not clickable.
>
> What is the best way to install GHC on a current Mac? It would not
> necessarily have to be the latest greatest.
>
The most common cause of this error is not having Xcode installed. It
should be on one of the discs that came with the Mac, but it is also
available from Apple if you make a (free) login to their dev site:
http://developer.apple.com/technologies/xcode.html
Antoine
> Regards,
> Torsten
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://www.haskell.org/mailman/listinfo/beginners
>
------------------------------
Message: 3
Date: Mon, 14 Feb 2011 13:11:19 -0500
From: Britt Anderson <britt.uwater...@gmail.com>
Subject: [Haskell-beginners] State monad
To: beginners@haskell.org
Message-ID:
<aanlktimw-2maaiebbg66fy_mr0xed1tu3cmvpfnjq...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
Many of the tutorials on the state monad seem to suggest that you can use
State as a data constructor, but I can't get this to work. E.g. in ghci
after :m Control.Monad.State I can
let f = (\x -> (x,x))
let y = state f
but if I,
let z = State f
I get an error message: Not in scope: data constructor `State'.
Can someone please explain?
Thanks, Britt
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Message: 4
Date: Mon, 14 Feb 2011 20:18:16 +0200
From: Mihai Maruseac <mihai.marus...@gmail.com>
Subject: Re: [Haskell-beginners] State monad
To: Britt Anderson <britt.uwater...@gmail.com>
Cc: beginners@haskell.org
Message-ID:
<aanlktinxyhzbe7zy-r1hpy_t4jefa1yjlkf0+aa6r...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
On Mon, Feb 14, 2011 at 8:11 PM, Britt Anderson
<britt.uwater...@gmail.com> wrote:
> Many of the tutorials on the state monad seem to suggest that you can use
> State as a data constructor, but I can't get this to work. E.g. in ghci
> after :m Control.Monad.State I can
>
> let f = (\x -> (x,x))
> let y = state f
>
> but if I,
>
> let z = State f
>
> I get an error message: Not in scope: data constructor `State'.
>
> Can someone please explain?
>
> Thanks, Britt
>
>
GHCi doesn't load all Haskell modules by default. It only loads
Prelude. You'll have to load them by hand. Use
:m +Control.Monad.State
--
Mihai
------------------------------
Message: 5
Date: Mon, 14 Feb 2011 20:19:13 +0200
From: Mihai Maruseac <mihai.marus...@gmail.com>
Subject: Re: [Haskell-beginners] State monad
To: Britt Anderson <britt.uwater...@gmail.com>
Cc: beginners@haskell.org
Message-ID:
<aanlktikukccx09t+buntrcon8dmc5dh6z-qsp0kok...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
On Mon, Feb 14, 2011 at 8:18 PM, Mihai Maruseac
<mihai.marus...@gmail.com> wrote:
> On Mon, Feb 14, 2011 at 8:11 PM, Britt Anderson
> <britt.uwater...@gmail.com> wrote:
>> Many of the tutorials on the state monad seem to suggest that you can use
>> State as a data constructor, but I can't get this to work. E.g. in ghci
>> after :m Control.Monad.State I can
>>
>> let f = (\x -> (x,x))
>> let y = state f
>>
>> but if I,
>>
>> let z = State f
>>
>> I get an error message: Not in scope: data constructor `State'.
>>
>> Can someone please explain?
>>
>> Thanks, Britt
>>
>>
>
> GHCi doesn't load all Haskell modules by default. It only loads
> Prelude. You'll have to load them by hand. Use
>
> :m +Control.Monad.State
>
Also, see if you have the mtl library installed (or Transformers).
--
Mihai
------------------------------
Message: 6
Date: Mon, 14 Feb 2011 13:23:43 -0500
From: Britt Anderson <britt.uwater...@gmail.com>
Subject: Re: [Haskell-beginners] State monad
To: beginners@haskell.org
Message-ID:
<aanlktimpt255extafh2ccez461wr6lt-wzxszemjs...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
Actually, I can load the module and I have mtl. I just don't understand why
I can use State as a data constructor, but the inverse of runState, i.e.
state works just fine. Has this library/package been reconfigured from when
most people were writing tutorials?
On Mon, Feb 14, 2011 at 1:19 PM, Mihai Maruseac <mihai.marus...@gmail.com>wrote:
> On Mon, Feb 14, 2011 at 8:18 PM, Mihai Maruseac
> <mihai.marus...@gmail.com> wrote:
> > On Mon, Feb 14, 2011 at 8:11 PM, Britt Anderson
> > <britt.uwater...@gmail.com> wrote:
> >> Many of the tutorials on the state monad seem to suggest that you can
> use
> >> State as a data constructor, but I can't get this to work. E.g. in ghci
> >> after :m Control.Monad.State I can
> >>
> >> let f = (\x -> (x,x))
> >> let y = state f
> >>
> >> but if I,
> >>
> >> let z = State f
> >>
> >> I get an error message: Not in scope: data constructor `State'.
> >>
> >> Can someone please explain?
> >>
> >> Thanks, Britt
> >>
> >>
> >
> > GHCi doesn't load all Haskell modules by default. It only loads
> > Prelude. You'll have to load them by hand. Use
> >
> > :m +Control.Monad.State
> >
>
> Also, see if you have the mtl library installed (or Transformers).
>
> --
> Mihai
>
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Message: 7
Date: Mon, 14 Feb 2011 18:44:32 +0000
From: Ozgur Akgun <ozgurak...@gmail.com>
Subject: Re: [Haskell-beginners] State monad
To: Britt Anderson <britt.uwater...@gmail.com>
Cc: beginners@haskell.org
Message-ID:
<AANLkTi=qo1frtzdkgfaxyyrwy57xsdyfxqcs5xbkt...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
In the latest version of mtl, there is no State data constructor.
http://hackage.haskell.org/packages/archive/mtl/latest/doc/html/Control-Monad-State-Lazy.html#g:2
State<http://hackage.haskell.org/packages/archive/transformers/latest/doc/html/Control-Monad-Trans-State-Lazy.html#t:State>is
defined to be a type alias to the application of Identity monad to the
StateT monad transformer.
As you've already discovered, you need to use
state<http://hackage.haskell.org/packages/archive/transformers/latest/doc/html/Control-Monad-Trans-State-Lazy.html#v:state>
instead.
HTH,
Ozgur
On 14 February 2011 18:11, Britt Anderson <britt.uwater...@gmail.com> wrote:
> Many of the tutorials on the state monad seem to suggest that you can use
> State as a data constructor, but I can't get this to work. E.g. in ghci
> after :m Control.Monad.State I can
>
> let f = (\x -> (x,x))
> let y = state f
>
> but if I,
>
> let z = State f
>
> I get an error message: Not in scope: data constructor `State'.
>
> Can someone please explain?
>
> Thanks, Britt
>
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://www.haskell.org/mailman/listinfo/beginners
>
>
--
Ozgur Akgun
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------------------------------
Message: 8
Date: Mon, 14 Feb 2011 14:10:13 -0500
From: Britt Anderson <britt.uwater...@gmail.com>
Subject: [Haskell-beginners] Monad Transformer Type Signatures
To: beginners@haskell.org
Message-ID:
<AANLkTi=ivhOyQCE2Pj5wh_c=lg4c+pd71rwpdvo+s...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
I am looking at code from a tutorial that has the following type
signature for a function:
getScreen :: MonadReader AppConfig m => m Surface
AppConfig is a data structure that was defined previously. My question
is how am I to interpret this type signature? Is it any m belonging to
the MonadReader AppConfig class? or any AppConfig m belonging to the
MonadReader class? Since I can't see any instance of the MonadReader
Class being defined for MonadReader AppConfig I am puzzled.
I would appreciate some clarification or a pointer to a prior
discussion of this issue. I have tried to look on my own.
Thank you.
------------------------------
Message: 9
Date: Mon, 14 Feb 2011 14:24:18 -0500
From: Brent Yorgey <byor...@seas.upenn.edu>
Subject: Re: [Haskell-beginners] Monad Transformer Type Signatures
To: beginners@haskell.org
Message-ID: <20110214192418.ga29...@seas.upenn.edu>
Content-Type: text/plain; charset=us-ascii
On Mon, Feb 14, 2011 at 02:10:13PM -0500, Britt Anderson wrote:
> I am looking at code from a tutorial that has the following type
> signature for a function:
>
> getScreen :: MonadReader AppConfig m => m Surface
>
> AppConfig is a data structure that was defined previously. My question
> is how am I to interpret this type signature? Is it any m belonging to
> the MonadReader AppConfig class?
Yes. And there will be an instance for MonadReader AppConfig m
exactly when m carries around an implicit "read-only" AppConfig value,
available for querying with the 'ask' and 'asks' functions.
> Since I can't see any instance of the MonadReader
> Class being defined for MonadReader AppConfig I am puzzled.
Typically what you will see is some concrete monad later being defined
in terms of ReaderT AppConfig, which will automatically give it an
appropriate MonadReader instance, allowing users of the monad to query
the AppConfig.
> I would appreciate some clarification or a pointer to a prior
> discussion of this issue. I have tried to look on my own.
I recommend reading Cale Gibbard's article "How to use monad
transformers"... except at the moment I can't seem to find it
anywhere. =(
-Brent
------------------------------
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