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Today's Topics:
1. Developing Web Applications with Haskell and Yesod (KMandPJLynch)
2. Checking if a Stack is Empty (doaltan)
3. Re: Checking if a Stack is Empty (Brandon Allbery)
----------------------------------------------------------------------
Message: 1
Date: Tue, 12 Mar 2013 10:47:30 -0400
From: KMandPJLynch <[email protected]>
Subject: [Haskell-beginners] Developing Web Applications with Haskell
and Yesod
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
Good morning,
I sent a previous email to you in regard to this - if my request is rejected,
will I be notified?
I'm reading the book "Developing Web Applications with Haskell and Yesod".
It is a very interesting read and I'm hoping to be able to be able to put up a
simple web app using it as a result.
I was wondering if anyone has had experience with it.
Thank you
On Mar 12, 2013, at 9:21 AM, [email protected] wrote:
> Send Beginners mailing list submissions to
> [email protected]
>
> To subscribe or unsubscribe via the World Wide Web, visit
> http://www.haskell.org/mailman/listinfo/beginners
> or, via email, send a message with subject or body 'help' to
> [email protected]
>
> You can reach the person managing the list at
> [email protected]
>
> When replying, please edit your Subject line so it is more specific
> than "Re: Contents of Beginners digest..."
>
>
> Today's Topics:
>
> 1. Re: Writing a custom pop function for a stack data type (doaltan)
> 2. Re: Writing a custom pop function for a stack data type
> (Brent Yorgey)
> 3. Re: Suspend/resume computation using Cont monad and callCC
> (Ertugrul S?ylemez)
> 4. Re: Writing a custom pop function for a stack data type
> (Emanuel Koczwara)
> 5. Re: Performance problem with Haskell/OpenGL/GLFW
> (Jesper S?rnesj?)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Tue, 12 Mar 2013 11:06:35 +0000 (GMT)
> From: doaltan <[email protected]>
> Subject: Re: [Haskell-beginners] Writing a custom pop function for a
> stack data type
> To: divyanshu ranjan <[email protected]>
> Cc: "[email protected]" <[email protected]>
> Message-ID:
> <[email protected]>
> Content-Type: text/plain; charset="iso-8859-1"
>
> Actually I'm getting the error with this :
>
> data Stack = Empty | Elem Char Stack deriving Show
>
>
> pophead :: Stack -> Char
> pophead Empty = Empty
> pophead (Elem x stack) = x
>
>
>
> ________________________________
> From: divyanshu ranjan <[email protected]>
> To: doaltan <[email protected]>; The Haskell-Beginners Mailing List -
> Discussion of primarily beginner-level topics related to Haskell
> <[email protected]>
> Sent: Tuesday, 12 March 2013, 12:53
> Subject: Re: [Haskell-beginners] Writing a custom pop function for a stack
> data type
>
>
> You have declared new data type mystack not Stack, so haskell compiler could
> not find Stack data type and its constructors. Secondly data type in Haskell
> need to be start with?capital?letters like
>
> data Mystack = Empty | Elem Char Mystack deriving Show
> then correct Function?definition is?
> pophead :: Mystack -> Char
>
> Regards
> Divyanshu?
>
>
>
> On Tue, Mar 12, 2013 at 4:12 PM, doaltan <[email protected]> wrote:
>
> Hi I have such a stack data structure:?
>> datamystack =Empty |Elem Char mystack derivingShow
>>
>> I'm trying to get the head of the stack using this:
>> pophead :: Stack -> Char
>> pophead Empty = Empty
>> pophead (Element x stack) = x
>> And I'm getting this error for the last sentence of the function :
>> Not in
> scope: data constructor `Stack'
>> Can you tell me how to fix it?
>> Thanks.
>>
>> _______________________________________________
>> Beginners mailing list
>> [email protected]
>> http://www.haskell.org/mailman/listinfo/beginners
>>
>>
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> ------------------------------
>
> Message: 2
> Date: Tue, 12 Mar 2013 07:13:01 -0400
> From: Brent Yorgey <[email protected]>
> Subject: Re: [Haskell-beginners] Writing a custom pop function for a
> stack data type
> To: [email protected]
> Message-ID: <[email protected]>
> Content-Type: text/plain; charset=iso-8859-1
>
> On Tue, Mar 12, 2013 at 11:06:35AM +0000, doaltan wrote:
>> Actually I'm getting the error with this :
>>
>> data Stack = Empty | Elem Char Stack deriving Show
>>
>>
>> pophead :: Stack -> Char
>> pophead Empty = Empty
>> pophead (Elem x stack) = x
>
> This code will result in a type error, but not the one you said. Try
> compiling this exact code and see what error you get.
>
> -Brent
>
>>
>>
>>
>> ________________________________
>> From: divyanshu ranjan <[email protected]>
>> To: doaltan <[email protected]>; The Haskell-Beginners Mailing List -
>> Discussion of primarily beginner-level topics related to Haskell
>> <[email protected]>
>> Sent: Tuesday, 12 March 2013, 12:53
>> Subject: Re: [Haskell-beginners] Writing a custom pop function for a stack
>> data type
>>
>>
>> You have declared new data type mystack not Stack, so haskell compiler could
>> not find Stack data type and its constructors. Secondly data type in Haskell
>> need to be start with?capital?letters like
>>
>> data Mystack = Empty | Elem Char Mystack deriving Show
>> then correct Function?definition is?
>> pophead :: Mystack -> Char
>>
>> Regards
>> Divyanshu?
>>
>>
>>
>> On Tue, Mar 12, 2013 at 4:12 PM, doaltan <[email protected]> wrote:
>>
>> Hi I have such a stack data structure:?
>>> datamystack =Empty |Elem Char mystack derivingShow
>>>
>>> I'm trying to get the head of the stack using this:
>>> pophead :: Stack -> Char
>>> pophead Empty = Empty
>>> pophead (Element x stack) = x
>>> And I'm getting this error for the last sentence of the function :
>>> Not in
>> scope: data constructor `Stack'
>>> Can you tell me how to fix it?
>>> Thanks.
>>>
>>> _______________________________________________
>>> Beginners mailing list
>>> [email protected]
>>> http://www.haskell.org/mailman/listinfo/beginners
>>>
>>>
>
>> _______________________________________________
>> Beginners mailing list
>> [email protected]
>> http://www.haskell.org/mailman/listinfo/beginners
>
>
>
>
> ------------------------------
>
> Message: 3
> Date: Tue, 12 Mar 2013 12:53:37 +0100
> From: Ertugrul S?ylemez <[email protected]>
> Subject: Re: [Haskell-beginners] Suspend/resume computation using Cont
> monad and callCC
> To: [email protected]
> Message-ID: <[email protected]>
> Content-Type: text/plain; charset="us-ascii"
>
> Dmitriy Matrosov <[email protected]> wrote:
>
>> I have two functions f and g, and i want them to execute in following
>> order: first function f runs, then suspends and passes control to
>> function g. Function g runs, then suspends and "unpauses" function f.
>> Function f finishes and passes control to function g, which also
>> finishes. Here is illustration ('o' means start of function, dot means
>> suspend and pass control to other function, 'x' means end of
>> function):
>>
>> [...]
>>
>> I want to implement this using Cont monad and callCC.
>
> Not directly answering your question, but what you need is called
> coroutines, and there are better monads for that purpose. This is how
> the Cont monads are defined:
>
> newtype Cont r a = Cont ((a -> r) -> r)
>
> But what you really need here is called a Coroutine monad:
>
> newtype Coroutine f a = Coroutine (Either (f (Coroutine f a)) a)
>
> Don't worry about that scary type, because if you look closely you will
> find that this is just Free as defined in the 'free' package:
>
> data Free f a
> = Free (f (Free f a))
> | Pure a
>
> This is how it works: The computation either results in a value (Pure)
> or it returns a way to continue the computation wrapped in `f` (Free):
>
> Free (Identity (Pure 15))
>
> This computation suspends with the continuation "Pure 15". If you
> continue it, it will result in 15. Of course there are some helper
> functions to ease defining continuations:
>
> liftF (Identity 15)
>
> So first you need a functor. The monad-coroutine package has coined the
> term "suspension functor" for this particular purpose. It captures the
> nature of the suspension. As you saw the Identity functor allows you to
> suspend and resume:
>
> type Suspend = Identity
>
> suspend :: Free Suspend ()
> suspend = liftF (Suspend ())
>
> or even more generally:
>
> suspend :: (Applicative f) => Free f ()
> suspend = liftF (pure ())
>
> You can use this in a computation:
>
> doStuff
> suspend
> doOtherStuff
> suspend
> return 15
>
> This returns to the controller and allows it to resume the computation
> if it wishes to:
>
> loop :: Free Suspend Integer -> IO Integer
> loop (Pure x) = return x
> loop (Free (Identity k)) = do
> putStrLn "Suspended."
> loop k
>
> You can also define an abortion functor (predefined in
> Data.Functor.Constant from the "transformers" package):
>
> newtype Constant r a = Constant r
> deriving (Functor)
>
> abort :: r -> Free (Constant r) a
> abort = Free . Constant
>
> You will find that in a loop you don't receive a continuation, but
> instead an abortion value, much like in a Cont computation that ignores
> its continuation:
>
> loop :: Free (Constant Integer) Integer -> IO Integer
> loop (Pure x) = putStrLn "Completed" >> return x
> loop (Free (Constant x)) = do
> putStrLn ("Aborted with: " ++ show x)
> return x
>
> Another possibility is a functor to request values of a certain type:
>
> type Request = (->)
>
> request :: Free (Request e) a
> request = Free Pure
>
> Now the controlling loop has to supply values when requested to do so:
>
> comp :: Free (Request String) Integer
> comp = do
> x <- fmap read request
> y <- if x /= 15
> then fmap read request
> else return 5
> return (x + y)
>
> loop :: Free (Request String) Integer -> IO Integer
> loop (Pure x) = return x
> loop (Free k) = do
> putStrLn "Gimme something:"
> getLine >>= loop . k
>
> Optionally add a prompt:
>
> data Prompt e a = Prompt String (e -> a)
> deriving (Functor)
>
> prompt :: String -> Free (Prompt e) e
> prompt p = Free (Prompt p Pure)
>
> loop :: Free (Prompt String) Integer -> IO Integer
> loop (Pure x) = return x
> loop (Free (Prompt p k)) = do
> putStrLn p
> getLine >>= loop . k
>
> With a type system extension you can even request arbitrary IO actions:
>
> data Run a = forall b. Run (IO b) (b -> a)
>
> requestIO :: IO a -> Free Run a
> requestIO c = Free (Run c Pure)
>
> loop :: Free Run Integer -> IO Integer
> loop (Pure x) = return x
> loop (Free (Run c k)) = do
> putStrLn "IO action requested."
> c >>= loop . k
>
> And you can yield values:
>
> type Yield = (,)
>
> yield :: v -> Free (Yield v) ()
> yield x = Free (x, Pure ())
>
> loop :: Free (Yield String) Integer -> IO Integer
> loop (Pure x) = return x
> loop (Free (str, k)) = do
> putStrLn ("Yielded: " ++ str)
> loop k
>
> Or both request and yield (comonad-transformers package):
>
> type MySusp v e = Coproduct (Yield v) (Request e)
>
> yield :: v -> Free (MySusp v e) ()
> yield x = Free . Coproduct . Left $ (x, Pure ())
>
> request :: Free (MySusp v e) e
> request = Free . Coproduct . Right $ Pure
>
> loop :: Free (MySusp String String) Integer -> IO Integer
> loop (Pure x) = return x
> loop (Free (Coproduct f)) =
> case f of
> Left (x, k) -> do
> putStrLn ("Yielded " ++ x)
> loop k
> Right k -> do
> putStrLn "Requested."
> getLine >>= loop . k
>
> There are many more ways to use Free, but this should give you the basic
> building blocks.
>
> I hope it helps.
>
>
> Greets,
> Ertugrul
>
> --
> Not to be or to be and (not to be or to be and (not to be or to be and
> (not to be or to be and ... that is the list monad.
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>
> ------------------------------
>
> Message: 4
> Date: Tue, 12 Mar 2013 13:46:08 +0100
> From: Emanuel Koczwara <[email protected]>
> Subject: Re: [Haskell-beginners] Writing a custom pop function for a
> stack data type
> To: doaltan <[email protected]>, The Haskell-Beginners Mailing List
> - Discussion of primarily beginner-level topics related to Haskell
> <[email protected]>
> Message-ID: <1363092368.2892.7.camel@emanuel-Dell-System-Vostro-3750>
> Content-Type: text/plain; charset="UTF-8"
>
> Hi,
>
> Dnia 2013-03-12, wto o godzinie 11:06 +0000, doaltan pisze:
>> Actually I'm getting the error with this :
>>
>>
>> data Stack = Empty | Elem Char Stack deriving Show
>>
>>
>> pophead :: Stack -> Char
>> pophead Empty = Empty
>> pophead (Elem x stack) = x
>>
>>
>
> pophead should return Char. You can't return Stack if you defined the
> type of pophead as Stack -> Char. You can try to use Maybe here and
> return Maybe Char (Just x or Nothing) or you can use error function to
> raise an error.
>
> You can use _ instead of stack like this:
>
> pophead (Elem x _) = x
>
> Emanuel
>
>
>
>
>
>
> ------------------------------
>
> Message: 5
> Date: Wed, 13 Mar 2013 00:21:30 +1100
> From: Jesper S?rnesj? <[email protected]>
> Subject: Re: [Haskell-beginners] Performance problem with
> Haskell/OpenGL/GLFW
> To: The Haskell-Beginners Mailing List - Discussion of primarily
> beginner-level topics related to Haskell <[email protected]>
> Message-ID:
> <CALex+Wg_Q_azi3cqE_nRYDsdoutUoKS6=Zm=vb6h4nrc1-j...@mail.gmail.com>
> Content-Type: text/plain; charset=ISO-8859-1
>
> On Mon, Mar 11, 2013 at 8:23 AM, Jesper S?rnesj? <[email protected]> wrote:
>> I used gfxCardStatus to show which card was in use. When I ran
>> test2.c, the system briefly switched to the discrete card. However,
>> when I ran Test2.hs, the system kept using the integrated chip the
>> whole time. Presumably, the Intel chip lacks a hardware implementation
>> of OpenGL 3.2, which causes the system to fall back to a software
>> renderer. I then used gfxCardStatus to force the system to *always*
>> use the discrete card and - boom! - this time Test2.hs received a
>> hardware renderer!
>>
>> So it seems that the problem is a) Mac OS X-specific, or possibly
>> specific to systems with multiple graphics cards, b) related to
>> triggering the *switch* to the better graphics card. I don't yet
>> understand why the C program triggers a switch, while the Haskell
>> program does not, but I'll keep investigating.
>
> I haven't had much time to look at this, unfortunately, but I did
> notice one interesting thing: the Haskell program *does* in fact
> trigger a switch of graphics cards, just... not as quickly.
>
> To see this, you can check the system console (using Console.app).
> Here is what a switch from the integrated card to the discrete one
> looks like on my machine:
>
> 3/13/13 12:02:22.486 AM WindowServer[77]: Received display connect
> changed for display 0x4272dc0
> 3/13/13 12:02:22.548 AM WindowServer[77]: Received display connect
> changed for display 0x3f003d
> 3/13/13 12:02:22.549 AM WindowServer[77]: CGXMuxAcknowledge:
> Posting glitchless acknowledge
> 3/13/13 12:02:22.593 AM WindowServer[77]: Received display connect
> changed for display 0x4272dc0
>
> When I run the C program, this get logged immediately following the
> execution of glfwOpenWindow (I stepped through the program using GDB).
>
> For the Haskell program, well... If I run the program normally, the
> above gets logged with roughly a second's delay, and the program
> receives a software renderer. However, if I step through it using
> GHCi, it gets logged immediately following the execution of
> glfwOpenWindow - and the program receives a hardware renderer!
>
> Shot in the dark here, but could this be due to lazy I/O? I seem to
> recall reading something about GHCi forcing stricter I/O.
>
> --
> Jesper S?rnesj?
> http://jesper.sarnesjo.org/
>
>
>
> ------------------------------
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
>
> End of Beginners Digest, Vol 57, Issue 18
> *****************************************
------------------------------
Message: 2
Date: Tue, 12 Mar 2013 14:48:26 +0000 (GMT)
From: doaltan <[email protected]>
Subject: [Haskell-beginners] Checking if a Stack is Empty
To: "[email protected]" <[email protected]>
Message-ID:
<[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
Hello,
I have this stack data structure :
data Stack = Empty | Element Char Stack deriving Show
I want to check if it is equal to "Empty"
When I try something like this :
"a = Empty" or "a = (Empty)" in a haskell file and then write this on ghci : "a
= Empty"
I get this :
<interactive>:1:0:
??? No instance for (Eq Stack)
????? arising from a use of `==' at <interactive>:1:0-11
??? Possible fix: add an instance declaration for (Eq Stack)
??? In the expression: a == (Empty)
??? In the definition of `it': it = a == (Empty)
I don't know how to fix this. Can you help me so that I can check if a stack is
Empty without getting this error?
Thank you.
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Message: 3
Date: Tue, 12 Mar 2013 10:57:51 -0400
From: Brandon Allbery <[email protected]>
Subject: Re: [Haskell-beginners] Checking if a Stack is Empty
To: doaltan <[email protected]>, The Haskell-Beginners Mailing List
- Discussion of primarily beginner-level topics related to Haskell
<[email protected]>
Message-ID:
<CAKFCL4WUyz=quz5brtjjbmq01qdlrqams5jpqcywwvuxeum...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
On Tue, Mar 12, 2013 at 10:48 AM, doaltan <[email protected]> wrote:
> I have this stack data structure :
> data Stack = Empty | Element Char Stack deriving Show
> I want to check if it is equal to "Empty"
> When I try something like this :
> "a = Empty" or "a = (Empty)" in a haskell file and then write this on ghci
> : "a = Empty"
>
Have you studied any Haskell tutorials yet? Usually you want to use a
pattern match, not an equality test; if you must for some reason use an
equality test, you need an Eq instance (as the error message tells you).
--
brandon s allbery kf8nh sine nomine associates
[email protected] [email protected]
unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net
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