- Beginners - Haskell.org

Using scanl for Fibonacci sequence
by Lai Boon Hui 12 Oct '16

12 Oct '16

Hi All, i am not very sure how this can work fibs = 1 : scanl (+) 1 fibs Appreciate it if someone can guide me through by showing me a few steps of the function evaluation -- Best Regards, Boon Hui

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Regarding defining cases exhaustively
by Lai Boon Hui 10 Oct '16

10 Oct '16

Hi all, I have this code myMaximumBy :: (a -> a -> Ordering) -> [a] -> a myMaximumBy _ [] = undefined myMaximumBy _ (x:[]) = x myMaximumBy f (x:xs) = case f x maxRestOfList of GT -> x _ -> maxRestOfList where maxRestOfList = myMaximumBy f xs whereby the case myMaximumBy _ [] will never happen because case myMaximumBy _ (x:[]) = x prevents that from happening. However i will still like to define it to keep -Wall warning from complaining. Is using undefined acceptable or is there a better way? -- Best Regards, Boon Hui

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Re: [Haskell-beginners] Beginners Digest, Vol 100, Issue 7
by Lai Boon Hui 09 Oct '16

09 Oct '16

Hi Tom, this indeed led to more questions. I believe that was happening because x is not a list of concrete types. In determining the concrete type of the list GHCI evaluated all values hence λ> let foo = [1,2,3] :: [Int] Prelude| foo :: [Int] λ> :sprint foo foo = [1,2,3] I tried on other concrete types like [Char] and String...seems like [Char] is not entire the same as String after all λ> let x = ['a','b','c'] Prelude| x :: [Char] λ> let y = "abc" Prelude| y :: [Char] λ> :sprint x x = "abc" λ> :sprint y y = _ On Sun, Oct 9, 2016 at 8:00 PM, <beginners-request(a)haskell.org> wrote: > Send Beginners mailing list submissions to > beginners(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > or, via email, send a message with subject or body 'help' to > beginners-request(a)haskell.org > > You can reach the person managing the list at > beginners-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Beginners digest..." > > > Today's Topics: > > 1. Re: Basic sound playing on Windows? (Tilmann) > 2. Re: Basic sound playing on Windows? (Cleverson Casarin Uliana) > 3. Lazy evaluation, trying to find out when its done (Lai Boon Hui) > 4. Re: Lazy evaluation, trying to find out when its done > (Tom Murphy) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Sat, 8 Oct 2016 15:19:35 +0200 > From: Tilmann <t_gass(a)gmx.de> > To: The Haskell-Beginners Mailing List - Discussion of primarily > beginner-level topics related to Haskell <beginners(a)haskell.org> > Subject: Re: [Haskell-beginners] Basic sound playing on Windows? > Message-ID: <417e430b-14d5-1bda-217d-1df8e512831d(a)gmx.de> > Content-Type: text/plain; charset=utf-8; format=flowed > > I used ALUT on OSX and it worked perfectly. Not used it on windows yet, > but according to the documentation it's supported. > > > Have a look here for how to use it on windows: > > https://hackage.haskell.org/package/OpenAL > > > and here for some examples: > > https://github.com/haskell-openal/ALUT/tree/master/examples/Basic > > > Best, > > Tilmann > > > > > Am 06.10.16 um 17:17 schrieb Cleverson Casarin Uliana: > > Hello all, is it easy to play/stop sound wave files on Windows? For > > now I'd like just playing and stopping them assynchronously. Do I need > > to install any package besides Haskell Platform? > > > > Thanks, > > Cleverson > > _______________________________________________ > > Beginners mailing list > > Beginners(a)haskell.org > > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > > > > ------------------------------ > > Message: 2 > Date: Sat, 8 Oct 2016 10:54:54 -0300 > From: Cleverson Casarin Uliana <clever97(a)gmail.com> > To: The Haskell-Beginners Mailing List - Discussion of primarily > beginner-level topics related to Haskell <beginners(a)haskell.org> > Subject: Re: [Haskell-beginners] Basic sound playing on Windows? > Message-ID: <46d55c3c-0c5c-e1e1-c413-1406ec12a56c(a)gmail.com> > Content-Type: text/plain; charset=utf-8; format=flowed > > Thank you Tilmann, it's quite good. > > Greetings, > Cleverson > > > ------------------------------ > > Message: 3 > Date: Sun, 9 Oct 2016 10:27:22 +0800 > From: Lai Boon Hui <laiboonh(a)gmail.com> > To: beginners(a)haskell.org > Subject: [Haskell-beginners] Lazy evaluation, trying to find out when > its done > Message-ID: > <CAJdQggm7L--6Wb6kdqdDiSExEn10_Q8JwWHexVsUN4-LHmB2hw@mail. > gmail.com> > Content-Type: text/plain; charset="utf-8" > > Hi all, > > I understand that the take method will evaluate the value inside the cons > cell whereas length will just evaluate the spine or structure of the list > > λ> let y = "abc" > Prelude| > y :: [Char] > λ> :sprint y > y = _ > λ> take 1 y > "a" > it :: [Char] > λ> :sprint y > y = 'a' : _ > λ> > > Well and good but why doesn't the same work on a list of Nums?? > > λ> let x = [1,2,3] > Prelude| > x :: Num t => [t] > λ> :sprint x > x = _ > λ> take 1 x > [1] > it :: Num a => [a] > λ> :sprint x > x = _ > λ> > > I expected to see x = 1 : _ > > -- > Best Regards, > Boon Hui >

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Lazy evaluation, trying to find out when its done
by Lai Boon Hui 09 Oct '16

09 Oct '16

Hi all, I understand that the take method will evaluate the value inside the cons cell whereas length will just evaluate the spine or structure of the list λ> let y = "abc" Prelude| y :: [Char] λ> :sprint y y = _ λ> take 1 y "a" it :: [Char] λ> :sprint y y = 'a' : _ λ> Well and good but why doesn't the same work on a list of Nums?? λ> let x = [1,2,3] Prelude| x :: Num t => [t] λ> :sprint x x = _ λ> take 1 x [1] it :: Num a => [a] λ> :sprint x x = _ λ> I expected to see x = 1 : _ -- Best Regards, Boon Hui

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Basic sound playing on Windows?
by Cleverson Casarin Uliana 08 Oct '16

08 Oct '16

Hello all, is it easy to play/stop sound wave files on Windows? For now I'd like just playing and stopping them assynchronously. Do I need to install any package besides Haskell Platform? Thanks, Cleverson

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Re: [Haskell-beginners] how to enter EOF in emacs haskell mode
by Shrivats 06 Oct '16

06 Oct '16

Hi, If it's anything like other interactive modes, does C-c C-d do the job of signalling EOF? Shrivats On Sep 14, 2016 15:53, "Chul-Woong Yang" <cwyang(a)aranetworks.com> wrote: Hi, all. I use emacs haskell mode for editing and testing simple programs. I have no success in finding out how to enter EOF to haskell program in emacs "Interactive-Haskell" buffer. So I am unable to test haskell program which reads from stdin. :-( Any help would be appreciated deeply. Regards, Chul-Woong Yang _______________________________________________ Beginners mailing list Beginners(a)haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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Problems with lifting code
by PATRICK BROWNE 02 Oct '16

02 Oct '16

Hi, I am trying, without sucess, to run the code below from [1] (Section 4.3). Obviouly I am missing something. I would like to keep as close to the original code as possible. Any help would be apreciated. Regards, Pat [1] http://publik.tuwien.ac.at/files/pub-geo_2321.pdf data MyNumbers a = MyNum a deriving Show class (Floating a) => Numbers a where sqr :: a -> a -- Not sure if i need these -- instance Num (MyNumbers a) where -- instance Fractional (MyNumbers a) where class Lifts b a where lift0 :: a -> b a lift1 :: (a -> a) -> b a -> b a instance Lifts MyNumbers a where lift0 x = MyNum x lift1 o x = MyNum (o x) -- ** Couldn't match expected type `a' with actual type `MyNumbers a' ** instance (Floating (MyNumbers Float)) => Numbers (MyNumbers Float) where sqr x = lift1 sqr x -- ** could not deduce (Numbers Float) arising from a use of `sqr' ** -- This email originated from DIT. If you received this email in error, please delete it from your system. Please note that if you are not the named addressee, disclosing, copying, distributing or taking any action based on the contents of this email or attachments is prohibited. www.dit.ie Is ó ITBÁC a tháinig an ríomhphost seo. Má fuair tú an ríomhphost seo trí earráid, scrios de do chóras é le do thoil. Tabhair ar aird, mura tú an seolaí ainmnithe, go bhfuil dianchosc ar aon nochtadh, aon chóipeáil, aon dáileadh nó ar aon ghníomh a dhéanfar bunaithe ar an ábhar atá sa ríomhphost nó sna hiatáin seo. www.dit.ie Tá ITBÁC ag aistriú go Gráinseach Ghormáin – DIT is on the move to Grangegorman <http://www.dit.ie/grangegorman>

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Just wanted to share some GHCI macros with fellow beginners
by Lai Boon Hui 01 Oct '16

01 Oct '16

Hi fellow beginners, wanted to share some macros you can add to your ghci config file so that you don't have to keep switching between ghci and command line. In my case i use Git a lot :def pwd (\_-> System.Directory.getCurrentDirectory >>= print >> return "") :def gitA (\_ -> System.Process.rawSystem "git" ["add", "-A"] >>= print >> return "") :def gitC (\m -> System.Process.rawSystem "git" ["commit", "-am", m] >>= print >> return "") :def gitP (\_ -> System.Process.rawSystem "git" ["push"] >>= print >> return "") -- Best Regards, Boon Hui

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short-circuit like Maybe monad with a difference
by Dennis Raddle 30 Sep '16

30 Sep '16

I have a function 'evalRule' that applies rules to steps in a backtracking search optimization problem and evaluates the step's fitness or decides that it must be eliminated entirely. In the following, a result of Nothing means "eliminate the step," Just x means the step has fitness score x. evalRule :: Rule -> Step -> Maybe Double I would like to write a function that applies a bunch of rules but short-circuits the computation if it hits Nothing. However, unlike the way the Maybe monad works, I want to know the partial results. In the following, the rules are applied in order and the "Just" results are collected up the point where a rule returns Nothing. evalRules :: [Rule] -> Step -> [Double] What's a nice way to do this? D

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Could not deduce (Frame a r0) arising from a use of `len'
by PICCA Frederic-Emmanuel 30 Sep '16

30 Sep '16

Hello I try to write something like this {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE RankNTypes #-} module Hkl.Frame ( Frame(..) , frames , frames' ) where import Pipes import Control.Monad class (Frame a r) where len :: a -> IO (Maybe Int) row :: a -> Int -> IO r frames :: (Frame a r) => Pipe a r IO () frames = do d <- await (Just n) <- lift $ len d forM_ [0..n-1] (\i' -> do f <- lift $ row d i' yield f) But when I try to compile this I get this error message. Could not deduce (Frame a r0) arising from a use of `len' from the context (Frame a r) bound by the type signature for frames :: Frame a r => Pipe a r IO () at src/Hkl/Frame.hs:17:11-39 The type variable `r0' is ambiguous Possible fix: add a type signature that fixes these type variable(s) In the second argument of `($)', namely `len d' In a stmt of a 'do' block: (Just n) <- lift $ len d In the expression: do { d <- await; (Just n) <- lift $ len d; forM_ [0 .. n - 1] (\ i' -> do { f <- lift $ row d i'; .... }) } What should I do in order to solve this problem. Thanks Frederic

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