November 2016 - Beginners

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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Real World Haskell: Deprecated Libraries
by John Ky 27 Nov '16

27 Nov '16

Hello Haskellers, I have been working through the Real World Haskell chapter on Profiling and Optimisation <http://book.realworldhaskell.org/read/profiling-and-optimization.html>, and got to the section *Advanced techniques: fusion*, where I encountered some code using Data.Array.Vector from the deprecated library uvector. Does anyone know how to rewrite this to not use deprecated libraries? I have tried to use Data.Vector.Unboxed from vector, but my version consumes a lot of memory. The code I have is here: https://github.com/haskell-works/hw-tutorial-performance/blob/master/hw-tut… With that code, I get “1831 MB total memory in use”, which is terrible Build run steps I used were: stack build --executable-profiling --library-profiling --ghc-options="-fprof-auto -rtsopts -auto-all -caf-all -fforce-recomp" time /Users/jky/wrk/haskell-works/hw-tutorial-performance/.stack-work/install/x86_64-osx/lts-7.10/8.0.1/bin/hw-tutorial-performance-rwhe +RTS -sstderr -p -hy -RTS 1e7 I get very large numbers of allocations of Pair and Double. There is also a very large upfront allocation ARR_WORDS, which I feared would happen when using Data.Vector Any help appreciated. Cheers, -John

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Setting up WXHaskell on the 8.0.1 platform?
by Cleverson Casarin Uliana 27 Nov '16

27 Nov '16

Hello all, I've sent this to the WXHaskell list, but no answers. Please what is the recommended way to set up WXWidgets and WXHaskell for the 8.0.1 Haskell Platform on Windows? I tried using the Achelanne installer, but it gives errors apparently related to packages versions incompatibilities. Thanks, Cleverson

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pattern matching on a common element
by briand＠aracnet.com 25 Nov '16

25 Nov '16

Here's what I'm doing: data X = A1 String Double | A2 String Int | A3 String Double Int name c = case c of A1 name _ -> name A2 name _ -> name A3 name _ _ -> name I'm sure there's a better way...

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Equivalence (or not) of lists
by Lawrence Wickert 13 Nov '16

13 Nov '16

Hello all, I am a rank beginner to functional languages. Working through Lipovaca's book, up to Chapter 3. Ok, setup this function in editor and compiled: length' :: (Num b) => [a] -> b length' [] = 0 length' (_:xs) = 1 + length' xs skippy@skippy:~$ ghci GHCi, version 7.10.3: http://www.haskell.org/ghc/ :? for help Prelude> :l baby [1 of 1] Compiling Main ( baby.hs, interpreted ) Ok, modules loaded: Main. *Main> length' [1,2,3] 3 *Main> 1:2:3:[] [1,2,3] *Main> length' 1:2:3:[] <interactive>:5:9: Could not deduce (Num [a0]) arising from the literal '1' from the context (Num a) bound by the inferred type of it :: Num a => [a] at <interactive>:5:1-16 The type variable 'a0' is ambiguous In the first argument of 'length'', namely '1' In the first argument of '(:)', namely 'length' 1' In the expression: length' 1 : 2 : 3 : [] *Main> Obviously, there is something I don't understand about the apparent non-equivalence of the lists [1,2,3] and 1:2:3:[]I am guessing that the solution is contained in that error message but I can't quite decipher it. Thanks for any help.

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How to tell haskell that 'a' is 'a' ?
by Lai Boon Hui 13 Nov '16

13 Nov '16

Hi all, skips :: [a] -> [[a]] skips xs = go 1 [] where go :: Int -> [[a]] -> [[a]] *-- Compiles if i remove this line* go n acc | n > (length xs) = acc | n == 1 = xs : (go (n+1) acc) | n < 1 = [] | otherwise = (everyN n xs) : (go (n+1) acc) everyN :: Int -> [a] -> [a] everyN n xs = case (drop (n-1) xs) of y:ys -> y : (everyN n ys) [] -> [] How can i tell haskell compiler that a is the sub program if the same as a in the main program? -- Best Regards, Boon Hui

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Request for Code Review: Dice Game Distribution
by Jan Brusch 12 Nov '16

12 Nov '16

Hi, I'm currently learning Haskell and as a first project I wrote a little helper for a dice game. It's very short. Nonetheless I would really appreciate a little Code Review or any other input. The Game: Each player writes down the numbers from 1 to 42. Each turn you roll 3 dice (6-sided). The rolling player can cross of any one number that he can construct from the three dice using the math operators +, -, *, /. First player to cross off all numbers wins. The Helper: Calculates the reachable numbers for each dice roll. It the aggregates a count for each number, from how many dice rolls it can be reached. E.g.: There are 216 possible dice rolls (6^3), the number 2 can be reached form 171 of them. The helper gives out the data in JSON format that can be read by NVD3 to be displayed in a browser. The graph helps you to make a decision on which number you should prefer to cross off with your current roll. The Code: http://lpaste.net/338358 The Graph: http://imgur.com/a/NSDT8 Looking forward to any feedback and thanks in advance Jan

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Equivalence (or not) of lists
by Lawrence Wickert 12 Nov '16

12 Nov '16

Thanks, Sylvain. That works and I think I understand the difference now. Larry

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OI UTCTime to String
by sasa bogicevic 10 Nov '16

10 Nov '16

Hi All, This is a small program {-# LANGUAGE OverloadedStrings #-} module Main where import Control.Monad.IO.Class (liftIO) import qualified Data.ByteString.Lazy.Char8 as L import Data.Time import Network (withSocketsDo) import Network.HTTP.Conduit createRequestData today = [("index:brKursneListe",""), ("index:year","2016"), ("index:inputCalendar1", today), ("index:vrsta","3"), ("index:prikaz","0"), ("index:buttonShow","Prikazi")] timeFromString s = parseTimeOrError True defaultTimeLocale "%d %b %Y %l:%M %p" s formatDateString time = formatTime defaultTimeLocale "%m/%d/%Y" time getDateString = getCurrentTime getFormatedDate = formatDateString $ timeFromString getDateString main = do print $ getFormatedDate And here is my error main.hs:25:54: error: • Couldn't match type ‘IO UTCTime’ with ‘[Char]’ Expected type: String Actual type: IO UTCTime • In the first argument of ‘timeFromString’, namely ‘getDateString’ In the second argument of ‘($)’, namely ‘timeFromString getDateString’ In the expression: formatDateString $ timeFromString getDateString Thanks!

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Re: [Haskell-beginners] Constructor classes and type classes.
by PATRICK BROWNE 03 Nov '16

03 Nov '16

Imants Thanks for the sanity check on the instances. I am concerned about the use of the function immsucc' from the same paper <http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.36.9395>, which is defined in instances the Time class (but omitted from the class). The function immsucc' is first used without arguments, which to me implies that it 'might' a function passed to the higher-order closure function. It is then used as just an ordinary function with arguments. I have found a version of closure based on Craft of Function Programming. <https://github.com/buntine/Haskell--Craft-of-FP/blob/master/AbsTypes/Relati…> which does not require a function as an argument. So basically my question is: does the usage of immsucc' make sense in the code below?, do the types match up? Regards, Pat data TimeFacts t = Bef (t, t, TimeFacts t) | New class SetClass t => Time l t where new :: (t, t) -> l t bef :: (t, t, l t) -> l t succ , pred :: (t, l t) -> Set t immsucc , immpred :: (t, l t) -> Set t before, after :: (t, t, l t) -> Bool immedbefore, immedafter :: (t, t, l t) -> Bool immedafter (a, b, t) = immedbefore (b, a, t) after (a, b, t) = before (b, a, t) before (a, b, t) = memberInSet (b, succ (a, t)) succ (a, tl) = closure ([a], immsucc' , tl) pred (a, tl) = closure ([a], immpred' , tl) immsucc (a, tl) = immsucc' ([a], [], tl) immpred (a, tl) = immpred' ([a], [], tl) instance (SetClass t) => Time TimeFacts t where immsucc’ (a, s, New ) = s immsucc’ (a, s, Bef (e, f, tt)) = if memberInSet (e,a) then immsucc’ (a, insertInSet (f,s), tt) else immsucc’ (a, s,tt) On 3 November 2016 at 18:05, Imants Cekusins <imantc(a)gmail.com> wrote: > this builds. not sure if the instances are as intended > > > {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-} > module SetClass where > > data TimeFacts t = Bef (t, t, TimeFacts t) | New > > class SetClass t where > class SetClass t => Time l t where > class Time l t => TimeE l t where > class Time l t => TimeTO l t where > > > instance SetClass (TimeFacts t) where -- OK > instance (SetClass t) => Time (TimeFacts t) t where > instance (SetClass t) => TimeE (TimeFacts t) t where > instance (SetClass t) => TimeTO (TimeFacts t) t where > > > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > > -- 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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