May 2017 - Beginners - Haskell.org

Multiline code in GHCi
by Shishir Srivastava 29 Aug '17

29 Aug '17

Hi, I am trying to run this basic multiline code in GHCi. I am not sure if this is even possible without writing it in a file and compiling it in a traditional way. ------- Prelude> :set +m Prelude> let main1 = do Prelude| a <- readLn Prelude| return a Prelude| Prelude> Prelude> main1 <interactive>:75:1: No instance for (Show (IO b0)) arising from a use of `print' In the first argument of `print', namely `it' In a stmt of an interactive GHCi command: print it ------- Could someone please confirm if the function can be defined and called like this in GHCi or point to where am going wrong. Thanks, Shishir Srivastava

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Got "parse error on input ‘=’” in Emacs Intero REPL when define any function
by 乃潇张 07 Jun '17

07 Jun '17

Hi, I am trying to define some functions in a REPL window of emacs intero. However, I always got "parse error on input ‘=’” as blow: Starting: stack ghci --with-ghc intero "--docker-run-args=--interactive=true --tty=false" --no-build --no-load --ghci-options -odir=/Users/spike/Code/haskell/learn-haskell/baby/.stack-work/intero/intero445zea --ghci-options -hidir=/Users/spike/Code/haskell/learn-haskell/baby/.stack-work/intero/intero445zea baby Intero 0.1.20 (GHC 8.0.2) Type :intro and press enter for an introduction of the standard commands. addTwo x y = x + y <interactive>:2:12: error: parse error on input ‘=’ Perhaps you need a 'let' in a 'do' block? e.g. 'let x = 5' instead of 'x = 5’ I did some research, seems like “let” is not a must here for ghci version above 8.0. And my ghc version is 8.0.2. So is it a issue of ghci or intero? Thanks, Sid

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Attoparsec parser question
by PICCA Frederic-Emmanuel 01 Jun '17

01 Jun '17

Hello I have this kind ot type data PoniEntry a = PoniEntry { poniEntryHeader :: [Text] , poniEntryDetector :: (Maybe (Detector a)) -- ^ Detector Name , poniEntryPixelSize1 :: (Length Double) -- ^ pixels size 1 , poniEntryPixelSize2 :: (Length Double) -- ^ pixels size 1 , poniEntryDistance :: (Length Double) -- ^ pixels size 2 , poniEntryPoni1 :: (Length Double) -- ^ poni1 , poniEntryPoni2 :: (Length Double) -- ^ poni2 , poniEntryRot1 :: (Angle Double) -- ^ rot1 , poniEntryRot2 :: (Angle Double) -- ^ rot2 , poniEntryRot3 :: (Angle Double) -- ^ rot3 , poniEntrySpline :: (Maybe Text) -- ^ spline file , poniEntryWavelength :: WaveLength -- ^ wavelength } deriving (Show) type Poni a = [PoniEntry a] And I try to create a Parser for the Poni type lengthP :: Text -> Parser (Length Double) lengthP key = do value <-doubleP key pure $ value *~ meter detectorP ∷ ToPyFAI a ⇒ a → Parser a detectorP a = do _ ← "Detector: " *> string (toPyFAI a) <* endOfLine pure a poniEntryP ∷ Detector a ⇒ a → Parser (PoniEntry a) poniEntryP a = PoniEntry <$> headerP <*> optional (detectorP a) <*> lengthP "PixelSize1: " <*> lengthP "PixelSize2: " <*> lengthP "Distance: " <*> lengthP "Poni1: " <*> lengthP "Poni2: " <*> angleP "Rot1: " <*> angleP "Rot2: " <*> angleP "Rot3: " <*> optional ("SplineFile: " *> takeTill isEndOfLine <* endOfLine) <*> lengthP "Wavelength: " <?> "poniEntryP" poniP :: Detector a ⇒ a → Parser (Poni a) poniP a = some (poniEntryP a) But when I compile this I get this error message. src/Hkl/PyFAI/Poni.hs:(118,16)-(131,24): Couldn't match type `attoparsec-0.10.4.0:Data.Attoparsec.Internal.Types.Parser Text (PoniEntry a)' with `a -> Parser (PoniEntry a)' Expected type: a -> Parser (PoniEntry a) Actual type: Parser (PoniEntry a) In the expression: PoniEntry <$> headerP <*> optional (detectorP a) <*> lengthP "PixelSize1: " <*> lengthP "PixelSize2: " <*> lengthP "Distance: " <*> lengthP "Poni1: " <*> lengthP "Poni2: " <*> angleP "Rot1: " <*> angleP "Rot2: " <*> angleP "Rot3: " <*> optional ("SplineFile: " *> takeTill isEndOfLine <* endOfLine) <*> lengthP "Wavelength: " <?> "poniEntryP" In an equation for `poniEntryP': poniEntryP a = PoniEntry <$> headerP <*> optional (detectorP a) <*> lengthP "PixelSize1: " <*> lengthP "PixelSize2: " <*> lengthP "Distance: " <*> lengthP "Poni1: " <*> lengthP "Poni2: " <*> angleP "Rot1: " <*> angleP "Rot2: " <*> angleP "Rot3: " <*> optional ("SplineFile: " *> takeTill isEndOfLine <* endOfLine) <*> lengthP "Wavelength: " <?> "poniEntryP" src/Hkl/PyFAI/Poni.hs:134:11-29: Couldn't match type `[Parser (PoniEntry a)]' with `Parser (Poni a)' Expected type: a -> Parser (Poni a) Actual type: a -> [Parser (PoniEntry a)] In the return type of a call of `some' In the expression: some (poniEntryP a) In an equation for `poniP': poniP a = some (poniEntryP a) I must say, that I do not understand what is wrong with my code. The lengthP code is ok, so why the detectorP is wrong ? thanks for your help Frederic

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ANNOUNCE: Haskell Communities and Activities Report (32nd ed., May 2017)
by Mihai Maruseac 24 May '17

24 May '17

On behalf of all the contributors, we are pleased to announce that the Haskell Communities and Activities Report (32nd edition, May 2017) is now available, in PDF and HTML formats: http://haskell.org/communities/05-2017/report.pdf http://haskell.org/communities/05-2017/html/report.html All previous editions of HCAR can be accessed on the wiki at https://wiki.haskell.org/Haskell_Communities_and_Activities_Report Many thanks go to all the people that contributed to this report, both directly, by sending in descriptions, and indirectly, by doing all the interesting things that are reported. We hope you will find it as interesting a read as we did. If you have not encountered the Haskell Communities and Activities Reports before, you may like to know that the first of these reports was published in November 2001. Their goal is to improve the communication between the increasingly diverse groups, projects, and individuals working on, with, or inspired by Haskell. The idea behind these reports is simple: Every six months, a call goes out to all of you enjoying Haskell to contribute brief summaries of your own area of work. Many of you respond (eagerly, unprompted, and sometimes in time for the actual deadline) to the call. The editors collect all the contributions into a single report and feed that back to the community. When we try for the next update, six months from now, you might want to report on your own work, project, research area or group as well. So, please put the following into your diaries now: ======================================== End of September 2016: target deadline for contributions to the November 2017 edition of the HCAR Report ======================================== Unfortunately, many Haskellers working on interesting projects are so busy with their work that they seem to have lost the time to follow the Haskell related mailing lists and newsgroups, and have trouble even finding time to report on their work. If you are a member, user or friend of a project so burdened, please find someone willing to make time to report and ask them to "register" with the editors for a simple e-mail reminder in November (you could point us to them as well, and we can then politely ask if they want to contribute, but it might work better if you do the initial asking). Of course, they will still have to find the ten to fifteen minutes to draw up their report, but maybe we can increase our coverage of all that is going on in the community. Feel free to circulate this announcement further in order to reach people who might otherwise not see it. Enjoy! -- Mihai Maruseac (MM) "If you can't solve a problem, then there's an easier problem you can solve: find it." -- George Polya

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Runge-Kutta and vectors
by mike h 12 May '17

12 May '17

Having looked at 'Learn Physics by Programming in Haskell’ I became quite wobbly with excitement! :) So I took and implemented some of their ideas. My University level maths is around 40 years old and so I struggled with some of the later things - but it's slowly coming back! (I have a copy at https://github.com/banditpig/vectors/blob/master/LearnPhysics.pdf) So I have type Scalar = Double type XYZ = (Scalar, Scalar, Scalar) newtype Vector = V { xyz :: XYZ} and set of vector operations (^+^) :: Vector -> Vector -> Vector (^-^) :: Vector -> Vector -> Vector (*^) :: Scalar -> Vector -> Vector (^*) :: Vector -> Scalar -> Vector (^/) :: Vector -> Scalar -> Vector (>.<) :: Vector -> Vector -> Scalar -- dot (><) :: Vector -> Vector -> Vector -- cross etc I then have type Time = Double type Displacement = Vector type Velocity = Vector type State = (Time, Displacement, Velocity) eulerStep :: (State -> Vector) -> Double -> State -> State eulerStep f dt st@(t, r, v) = (t', r', v') where t' = t + dt r' = r ^+^ v ^* dt v' = v ^+^ f st ^* dt and so for a given acceleration function, time step and start state a solution is given by solution :: (State -> Vector) -> Double -> State -> [State] solution a dt = iterate (eulerStep a dt) What I'm trying to do is replace the Euler method of solving with the Rung-Kutta method. I'm really struggling in seeing how the Rung-Kutta examples I've seen are implemented using vectors. How should I progress? Any advice would be welcome! (I really want to implement it using the types I have rather than using an external library ) Many Thanks Mike

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Quickcheck: pushing element onto stack increases stack size
by PATRICK BROWNE 11 May '17

11 May '17

Hi, I am trying to use Quickcheck to check that pushing an element increases the size of a stack. Something like: prop_size2 x s = (size (Push x s)) > (size s) Is this possible? Below is my effort using an empty stack. Thanks in advance, Pat module Stack (empty, push, pop, top, isEmpty) where import Test.QuickCheck data Stack a = Empty | Push a (Stack a) deriving Show empty :: Stack a empty = Empty push :: a -> Stack a -> Stack a push x ss = Push x ss pop :: Stack a -> Stack a pop Empty = error "pop emptyStack" pop (Push x ss) = ss top :: Stack a -> a top Empty = error "top emptyStack" top (Push x ss) = x isEmpty :: Stack a -> Bool isEmpty Empty = True isEmpty (Push x ss) = False size :: (Stack a) -> Int size Empty = 0 size (Push x ss) = succ (size ss) prop_size x = (size (Push x Empty)) > (size Empty) -- 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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hGetContents and modules architecture idea
by PY 09 May '17

09 May '17

Hello everyone! I'm trying to understand base idea of Haskell modules architecture. In other languages, "reading from file" is placed in something like "io", "istream", "file", etc modules. Also most languages have a concept of reading from abstract bytes streams. You read bytes from something and translate them into your high level object/type/etc. In Haskell I see that, for example, function *hGetContents* exists in (this is my local installation): * GHC.IO.Handle * System.IO * Data.ByteString * Data.ByteString.Char8 * Data.ByteString.Lazy * Data.ByteString.Lazy.Char8 * Data.Text.IO * Data.Text.Lazy.IO * System.IO.Strict * Text.Pandoc.UTF8 * Data.ListLike * Data.ListLike.IO * ClassyPrelude * Hledger.Utils.UTF8IOCompat * Data.IOData * Darcs.Util.Ratified * Sound.File.Sndfile * Sound.File.Sndfile.Buffer * Data.String.Class * Network.BufferType If I'll create module SuperMegaShapes with some Triangle, Rectangle, Square and other things, I'll create (to be consistent with Haskell-way)... hGetContents there??! So, I have 2 questions here: First one: let's imagine that we have Haskell compiler for embedded. And I want to read Text, ByteString, Sndfile and SuperMegaShapes from... SPI. There are many devices andprotocols, right? And I have not FILE HADNLER for most of them. So, this mean that Haskell (like simple script language) supports only concept of FILE HANDLER reading?! And no other ABSTRACTIONS? Second question is: must any new type which we plan to read/write to have hGetContents? What if it is packed in some tricky container? Matreshka? Something else, more tricky? :) And more: what other I/O functions must be injected in our model definitions modules? === Best regards

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Monads of functions
by Szymon Pajzert 07 May '17

07 May '17

Hello, Lately I'm working with Reader monad. Because of that, I have to use function of signature: Monad m => (a -> m b) -> m (a -> b). Is this even possible to do so? Thanks in advance for your help. Best Regards, Szymon Pajzert

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Foldable for (,)
by Jonathon Delgado 03 May '17

03 May '17

I've seen many threads, including the one going on now, about why we need to have: length (2,3) = 1 product (2,3) = 3 sum (2,3) = 3 or (True,False) = False but the justifications all go over my head. Is there a beginner-friendly explanation for why such seemingly unintuitive operations should be allowed by default?

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Struggling with integrating standalone function with existing vty based app
by Leon x 02 May '17

02 May '17

I have the following code working fine: https://github.com/leonstafford/pandoctest But struggling in bringing it into this vty based app. I admit to not quite understanding the control flow / IO here and could use some tips as to what I'm doing wrong with this one: https://github.com/leonstafford/cli-rss-reader/blob/feature/pandoc-render-a… As per the comments, I've tried to understand the existing code, but now just finding myself flailing keys and switching type signatures to no avail. Humbly seeking guidance from above! Cheers, Leon

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