- Beginners - Haskell.org

Equivalence (or not) of lists
by Lawrence Wickert 14 Nov '16

14 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 14 Nov '16

14 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 04 Nov '16

04 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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Java-Bridge, Ambiguous occurrence ‘<$’
by Desonte Jolivet 03 Nov '16

03 Nov '16

Hello All: Im interested interfacing with the JVM through Haskell and I found this neat library on Hackag name java-bridge although when I try and install with cabal I get the below error, "Ambiguous occurrence ‘<$’". Any tips on how to clear this up or how I should proceed would be very helpful, Thank you all in advance. src/Foreign/Java/IO.hs:5:7: Warning: -XOverlappingInstances is deprecated: instead use per-instance pragmas OVERLAPPING/OVERLAPPABLE/OVERLAPS[ 1 of 24] Compiling Foreign.Java.Tutorial ( src/Foreign/Java/Tutorial.hs, dist/build/Foreign/Java/Tutorial.o )[ 2 of 24] Compiling Foreign.Java.IO ( src/Foreign/Java/IO.hs, dist/build/Foreign/Java/IO.o )[ 3 of 24] Compiling Foreign.Java.Control ( src/Foreign/Java/Control.hs, dist/build/Foreign/Java/Control.o )[ 4 of 24] Compiling Foreign.Java.Bindings.ReflectHaskell ( src/Foreign/Java/Bindings/ReflectHaskell.hs, dist/build/Foreign/Java/Bindings/ReflectHaskell.o )[ 5 of 24] Compiling Foreign.Java.Bindings.Haskell2Java ( dist/build/Foreign/Java/Bindings/Haskell2Java.hs, dist/build/Foreign/Java/Bindings/Haskell2Java.o )[ 6 of 24] Compiling Foreign.Java.Bindings.HaskellTypes ( src/Foreign/Java/Bindings/HaskellTypes.hs, dist/build/Foreign/Java/Bindings/HaskellTypes.o )[ 7 of 24] Compiling Foreign.Java.Bindings.JavaSE6 ( src/Foreign/Java/Bindings/JavaSE6.hs, dist/build/Foreign/Java/Bindings/JavaSE6.o )[ 8 of 24] Compiling Foreign.Java.Utils ( src/Foreign/Java/Utils.hs, dist/build/Foreign/Java/Utils.o )[ 9 of 24] Compiling Foreign.Java.Bindings.JavaTypes ( src/Foreign/Java/Bindings/JavaTypes.hs, dist/build/Foreign/Java/Bindings/JavaTypes.o )[10 of 24] Compiling Foreign.Java.Util ( src/Foreign/Java/Util.hs, dist/build/Foreign/Java/Util.o ) src/Foreign/Java/Util.hs:5:19: Ambiguous occurrence ‘<$’ It could refer to either ‘Foreign.Java.Util.<$’, defined at src/Foreign/Java/Util.hs:24:1 or ‘Prelude.<$’, imported from ‘Prelude’ at src/Foreign/Java/Util.hs:4:8-24 (and originally defined in ‘GHC.Base’)cabal: Error: some packages failed to install:java-bridge-0.20130606.3 failed during the building phase. The exception was:ExitFailure 1 -Jo

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

03 Nov '16

Hi Patrick, I am not sure I understand what you are trying to do, but here are some thoughts. - your use of the names `typeVar` and `typeCons` make total sense to me. But your name `dataCons` in the definition of SetClass2 does not. You are only talking about types here. All you are doing is talking about *type* constructors. You are not constructing a specific value of that type. Maybe one way to see the difference between the type constructors and the data constructors is to change your first line to: data SetType typeVar = SetTypeDataConstructor [typeVar] deriving Show - I also don't understand why you named your type variable `dataVariable` instead of sticking to `typeVar` in SetClass3. It is a *type* variable in the type signature of the functions in the type class. - I don't understand what you mean by a "constructor class". My simple understanding of what a type constructors and data constructors are is here https://wiki.haskell.org/Constructor#Type_constructor. - I don't see the terms super class and sub class ever used in haskell land. Although I do understand they make sense, I think they are really OO terms. I've tried to map OO concepts into haskell and there are many different ways to do it. The best description of all those ways that I found was in Oleg Kiselyov's work https://arxiv.org/pdf/cs/0509027.pdf - I think parametric polymorphism as used in haskell forces us to compare types in terms of equality constraints (this type is "the same" as that), whereas subtype polymorphism as used in OO languages forces us to think in terms of inequalities (this type is "more specific" than that). I think those force fundamentally different ways to think about and model your problems. I would encourage you to use equality as much as possible, but if you want to model subtyping in haskell, I think Oleg's work above has a few approaches that I think are better than type classes. In particular, in OO there was always an ambiguity between modelling subtyping with inheritance or with a "has a" relationship when the subclass explicitly contains an object of the superclass. You can use the latter idea in haskell, Oleg's work has a good way to do it. Hope this helps. Cheers, Dimitri > From: PATRICK BROWNE <patrick.browne(a)dit.ie> > To: The Haskell-Beginners Mailing List - Discussion of primarily > beginner-level topics related to Haskell <beginners(a)haskell.org> > Subject: [Haskell-beginners] Constructor classes and type classes. > Message-ID: > <CAGFLrKfQVaQQJybMTn-pJksV1VKUXrmBks14Jkm==Zqt=bdy5A(a)mail.gmail.com> > Content-Type: text/plain; charset="utf-8" > > {- > I am trying to understand constructor classes and their relationship with > ordinary type classes. > I wrote the code below to help me understand the distinction. The code is > only for explanatory purposes, set operations use a tuple syntax. > > I use the naming convention of 'typeVar', 'typeCons', and 'dataCons' for > type variables, type constructors, and data constructors respectively. > Question1: > Is my naming convention correct? > I am particularly concerned about SetClass3 where the super class seems to > use a type constructor but the subclass seems to use the same term as a > data constructor. > > Q2: > In this case constructor classes and type classes seem to provide similar > functionality. > In general what situation are each best suited? > -} > > import Data.List > data SetType typeVar = SetType [typeVar] deriving Show > > > class SetClass1 typeCons where > member1 :: Eq typeVar => (typeVar, typeCons typeVar) -> Bool > intersect1 :: (Eq typeVar,Show typeVar) => (typeCons typeVar, typeCons > typeVar) -> typeCons typeVar > > > class SetClass2 dataCons typeVar where > member2 :: (typeVar, (dataCons typeVar)) -> Bool > intersect2 :: (dataCons typeVar, dataCons typeVar) -> dataCons typeVar > > > class SetClass1 typeCons => SetClass3 typeCons dataVariable where > union3 :: (Eq dataVariable,Show dataVariable) => (typeCons dataVariable, > typeCons dataVariable) -> typeCons dataVariable > > instance SetClass1 SetType where > member1 (x ,(SetType y)) = elem x y > intersect1 ((SetType x),(SetType y)) = SetType (intersect x y) > > instance SetClass2 SetType Int where > member2 (x ,SetType y) = elem x y > intersect2 (SetType x,SetType y) = SetType (intersect x y) > > instance SetClass3 SetType Int where > union3 (SetType x,SetType y) = SetType (union x y) > > > test1a = member1 (1, (SetType [1,2])) > test1b = intersect1 ((SetType [1,3,4]),(SetType [1,2])) > test2a = member2 (1, (SetType [1::Int])) > test2b = intersect2 ((SetType [1::Int]), (SetType [(1::Int)])) > test3a = union3 ((SetType [1::Int,2::Int,3::Int]),(SetType [4::Int,5::Int])) > -- 2E45 D376 A744 C671 5100 A261 210B 8461 0FB0 CA1F -- 2E45 D376 A744 C671 5100 A261 210B 8461 0FB0 CA1F

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Sum a tuple of Monoids
by Lai Boon Hui 01 Nov '16

01 Nov '16

Hi all, Is it possible to sum a tuple of monoids for example sum (Just 1, Nothing) expected result Just 1 sum (Just 1, Just 2) expected result Just 3 -- Best Regards, Boon Hui

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Re: [Haskell-beginners] Constructor classes and type classes.
by PATRICK BROWNE 31 Oct '16

31 Oct '16

My intended use of the term 'subclass' is as the Haskell report: <https://www.haskell.org/onlinereport/basic.html> "class Num of numeric types is a subclass of Eq, since all numbers may be compared for equality" Of course what I have written may not be consistent with the report. Regards, Pat On 31 October 2016 at 09:26, Imants Cekusins <imantc(a)gmail.com> wrote: > HelloPat, > > I am not sure if terms *superclass* and *subclass* are applicable to > Haskell classes. > > Haskell class is very different from Java class. It is more like an > interface. > > ;) > > -- 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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