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Question regarding error messages
by Choi Waikit 19 Jun '17

19 Jun '17

Hi guys, I have difficulty understanding how GHC determines what kind of error message to return. Could you help explain a bit ? Thanks ! The code can be accessed here <http://lpaste.net/356325> My question is Why does calling `settleDown 9` return different error message from `settleDown "sting"` ? Thanks, David

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Non type-variable argument
by Nikolay Kurtov 14 Jun '17

14 Jun '17

Hello everyone, Please help me compile the program without the -XFlexibleContexts extension. I don't understand the "Non type-variable argument" error message and googling does not shed any light on the problem. Any help is appreciated. % ghc --version The Glorious Glasgow Haskell Compilation System, version 7.10.3 % ghc orfest.hs orfest.hs:6:1: Non type-variable argument in the constraint: AM.MArray a (t, t, t, [i]) m (Use FlexibleContexts to permit this) When checking that ‘go’ has the inferred type go :: forall (m :: * -> *) t t1 t2 t3 (a :: * -> * -> *) i. (Read t, AM.Ix i, AM.MArray a (t, t1, t2, [i]) m) => a i (t, t1, t2, [i]) -> [[Char]] -> t3 -> i -> m (t3, i) This works: % ghc orfest.hs -XFlexibleContexts Regards, Orfest

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2d layout description?
by Mario Lang 12 Jun '17

12 Jun '17

Hi. I am looking for a good method to describe 2-dimensional panels of various technical, mostly music-production related, devices. Motivation: I am blind and am looking for a future-proof solution to write down labels of jacks/knobs/switches on front/back panels of all the devices I use on a daily basis. Complex devices have so many options that you tend to forget the not-so-often used ones, hence, the need to have a reference "card" somewhere. This need has recently surfaced for me rather clearly as I have entered the realm of eurorack modular synthesizers as a hobby. One module is typicall 3U in either and between 2HP and 50HP in vertical size. And there are literally hundreds of these available these days. So while it becomes surprisingly easy to remember the jacks/knobs of regularily used modules after a rather short while, the need for some reference to go back to is unavoidable. Why is this on haskell-beginners? I am a haskell fan, and think this is a perfect job for a functional programming language. What I *want* is some sort of data format/DSL that makes it easy for me to describe the positions, functions and labels of physical interface elements. From that, I'd like to generate easily readable 2d ascii-diagram alike descriptions which would be particularily well fit to be read with a Braille display. Also, knowing the position/function of interface elements (jacks, knobs, switches) it would be neat if I could implement convenience functions on top of the known data, like the ability to calculate cable lengths for a particular patch, or even figuring out a module layout to minimize patch cable lengths given a list of possible patches. While browsing hackage, I found asciidiagrams, which is * pretty cool! * and might be useful to specify the layout However, I would have to fork and rewrite it to fit my actual needs, and it isn't clear if specifying the machine readable description like that wouldn't be a little bit too verbose. As an example, I have written down the layout of http://www.doepfer.de/A114.html (which admittably, is a rather simple module): +-------------+ | | | /-------\ | | | A-114 | | | |RingMod| | | \-------/ | | | | +--------+ | | | /----\ | | | | | X | | | | | |{i} | | | | | \----/ | | | | /----\ | | | | | Y | | | | | |{i} | | | | | \----/ | | | | /----\ | | | | |X*Y | | | | | |{o} | | | | | \----/ | | | +========+ | | | /----\ | | | | | X | | | | | |{i} | | | | | \----/ | | | | /----\ | | | | | Y | | | | | |{i} | | | | | \----/ | | | | /----\ | | | | |X*Y | | | | | |{o} | | | | | \----/ | | | +--------+ | | | +-------------+ This looks cool, but once I add all the tags I probably need (jack(in/out/in|out), rotary knob, n-way rotary, n-way switch, ...) this will probably grow into something quite cumbersome. A DSL might be better there, and somehow recunstruct a simple ascii diagram from that? In essence, I think I am trying to solve a rather general problem here, even though my actual use-case is specific to eurorack modules. Once I have an easily useable DSL, I will likely want to describe everything I own in this format, to be able to remember individual functions at a later day. And once I start to see this as a database (which is the main reason for wanting to move it from text files to a programming language) I can see the need to search for exact descriptions. For instance, I might find something with 7 RJ45 jacks in groups of 4 and 3, and would like to find the description for it without knowing the product name... Or search for a label of a key, without knowing on which remote it was... There are quite some possibilities here. I am basically asking for everything that comes to your mind, if you have read so far without having been bored to hell. Is there any library on hackage that might help? Do you know of similar projects with different goals? Anything else? TL;DR: I am looking for a DSL to describe physical interfaces in a form that makes it easy to make this information available to the blind in a very accessible way. Description files should be easy enough to maintain, to maybe even start a trend of a central repository to avoid people having to re-create them over and over again. The metadata should be detailed enough to allow for search/query operations on the universe of described hardware. -- CYa, ⡍⠁⠗⠊⠕ | Blog: <https://blind.guru/> GitHub: <https://github.com/mlang/> .''`. | Twitter: @blindbird23 FaceBook: disyled : :' : | SoundCloud: <soundcloud.com/mario-lang> `. `' | YouTube: <youtube.com/user/mlang23> `-

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mempty and "No instance for (Monoid Int)"
by Baa 07 Jun '17

07 Jun '17

Maybe a is the Monoid: instance Monoid a => Monoid (Maybe a) -- Defined in ‘GHC.Base’ so I can compare its values with empty value: mempty == Nothing => True But if I try: mempty == Just 4 I get: <interactive>:1:1: error: • Ambiguous type variable ‘a0’ arising from a use of ‘mempty’ prevents the constraint ‘(Monoid a0)’ from being solved. Probable fix: use a type annotation to specify what ‘a0’ should be. These potential instances exist: instance Monoid a => Monoid (IO a) -- Defined in ‘GHC.Base’ instance Monoid Ordering -- Defined in ‘GHC.Base’ instance Monoid a => Monoid (Maybe a) -- Defined in ‘GHC.Base’ ...plus 7 others (use -fprint-potential-instances to see them all) • In the first argument of ‘(==)’, namely ‘mempty’ In the expression: mempty == Just 4 In an equation for ‘it’: it = mempty == Just 4 OK, I try: mempty::Maybe Int and get: <interactive>:1:1: error: • No instance for (Monoid Int) arising from a use of ‘mempty’ • In the expression: mempty :: Maybe Int In an equation for ‘it’: it = mempty :: Maybe Int so, how is related Int to Monoid, why does ghc expect from mempty::Maybe Int, Int to be Monoid?! As I understand, this means only that I mean "mempty" from (Maybe Int) type, which is Monoid and exists sure. Interesting is, that: mempty::Maybe [Int] => Nothing but how is related "monoidality" of "Maybe a" with "monoidality of "a" ??? Initial idea was to make comparison: mempty :: Maybe Int == Just 4 => False /Best regards, Paul

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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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help designing types for a gsl fit
by PICCA Frederic-Emmanuel 02 Jun '17

02 Jun '17

Hello I imagine that I have two types A and B I want to create a ADT data C = C A B 1) the toGsl class class ToGsl a where toGsl :: a -> [Double] this class allow to convert a type to a Vector of double in order to use gsl for a fit so I can create instance for A and B (I do not write the instance but the result of the toGsl function) A -> [a1, ... an] B -> [b1, ... bm] Now I want this to work also with C C -> [a1..., an] ++ [b1, .... bm] So the C instance is juste the (toGsl a) ++ (toGsl b) Is there a way to write this generically for other type construct like (data D = D C A) etc.... Functor, Applicative, Monoid ? 2) the fromGsl case now the inverse method is cladd FromGsl a where fromGsl :: a -> [Double] -> a so now the question is if I have a [Double], I need t osplit this list for each composant of the ADT. in the case of C fromGSL (C a b) v = ...split the vector... and apply fromGsl to a and b So my question is the symetic of the previous one. Is it possible to create a generic function inorder to do this. thanks for your help. Frederic

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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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