- Beginners - Haskell.org

Stack implementation
by Uneeb Adeel Agha 10 Sep '16

10 Sep '16

Hi, I'm very, very new to Haskell. I am trying to understand the following error, but I'm having a hard time wrapping my head around the whole type system. Stack module Stack (Stack(..)) where import Prelude hiding (head, tail) class Stack s where empty :: s a isEmpty :: s a -> Bool cons :: s a -> a -> s a head :: s a -> a tail :: s a -> s a data ListStack a = LS [a] deriving (Show) instance Stack ListStack where empty = LS [] isEmpty (LS s) = null s cons (LS s) x = LS(x:s) head(LS(x:_)) = x tail(LS(x:xs)) = LS xs Not using Mixmax yet? The problem is that when I load this code in the interpreter and write>emptyI get the following error <interactive>:108:1: No instance for (Show (s0 a0)) arising from a use of ‘print’ The type variables ‘s0’, ‘a0’ are ambiguous I mean when I write "LS []" it works just fine. But something is wrong with empty. Thanks, Uneeb

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GADTs and method signature
by PICCA Frederic-Emmanuel 06 Sep '16

06 Sep '16

Hello I have these types data ImageXY data Image3d' data ScPipeParams a where ScPipeParamsImageXY :: ScBitSize -> Channel -> Modulo -> Sc3du -> Roi -> Accumulation -> ScPipeParams ImageXY ScPipeParamsImage3d' :: ScBitSize -> Channel -> Modulo -> Sc3du -> Roi -> Accumulation -> ScPipeParams Image3d' and now I write a method scPipeOpen2' (ScDevDesc dev) p = alloca $ \params -> do poke params p res <- c_sc_pipe_open2 dev (scPipeType p) params checkError res PipeId scPipeOpen2 :: ScDevDesc -> ScPipeParams a -> IO (PipeId) scPipeOpen2 d p@(ScPipeParamsImageXY _ _ _ _ _ _) = scPipeOpen2' d p scPipeOpen2 d p@(ScPipeParamsImage3d' _ _ _ _ _ _) = scPipeOpen2' d p it works fine In order to avoid these _ _ _ _ _ _, I tryed to removed the @(Constructor ____) since I just pass the p parameter to the ' method. But in that case , I get this error message C:\Users\TEMPO\Downloads\tdc\tdc\srcLib.hsc:247:19: No instance for (Storable (ScPipeParams a)) arising from a use of scPipeOpen2' In the expression: scPipeOpen2' d p In an equation for `scPipeOpen2': scPipeOpen2 d p = scPipeOpen2' d p Indeed I already defined two instance of Storable instance Storable (ScPipeParams ImageXY) where ... and instance Storable (ScPipeParams Image3d') where ... Should I use a special extension in order to be able to write only scPipeOpen2 d p = scPipeOpen2' d p thanks for your help Frederic

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Re: [Haskell-beginners] New edition of "Programming in Haskell" (Graham Hutton)
by Dimitri DeFigueiredo 01 Sep '16

01 Sep '16

This book is the best introduction to Haskell I know. The chapter on lazy evaluation was so compelling I am thoroughly convinced I have been thinking about evaluation the wrong way all my life (minus space complexity issues ;-). As much as I prefer short books (such as Hudak et al's "A Gentle Introduction to Haskell" -- which is also good, but *not* gentle once you get far enough into it) the main critique I had to the 1st edition is that it only scratched the surface and did not go far enough. It seems this has been thoroughly addressed in this edition. Thanks for making this a reality! :-) Dimitri On 01/09/16 9:00 AM, 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. New edition of "Programming in Haskell" (Graham Hutton) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Thu, 1 Sep 2016 08:59:09 +0000 > From: Graham Hutton <Graham.Hutton(a)nottingham.ac.uk> > To: "beginners(a)haskell.org" <beginners(a)haskell.org> > Subject: [Haskell-beginners] New edition of "Programming in Haskell" > Message-ID: > <A96EF479-E2D7-4C9D-A2EC-31CD1ECFC691(a)exmail.nottingham.ac.uk> > Content-Type: text/plain; charset="us-ascii" > > Dear all, > > I'm delighted to announce that the new edition of "Programming in > Haskell" is now available! Further details are provided below, > and are also available from: http://tinyurl.com/hnfjdgc. > > Best wishes, > > Graham > > ================================================================= > > *** BOOK ANNOUNCEMENT *** > > Programming in Haskell - 2nd Edition > > Graham Hutton, University of Nottingham > > Cambridge University Press, 1st September 2016 > > 320 pages, 120 exercises, ISBN 9781316626221 > > http://tinyurl.com/hnfjdgc > > ================================================================= > > DESCRIPTION: > > Haskell is a purely functional language that allows programmers > to rapidly develop clear, concise, and correct software. The > language has grown in popularity in recent years, both in teaching > and in industry. This book is based on the author's experience > of teaching Haskell for more than twenty years. All concepts > are explained from first principles and no programming experience > is required, making this book accessible to a broad spectrum > of readers. While Part I focuses on basic concepts, Part II > introduces the reader to more advanced topics. > > This new edition has been extensively updated and expanded to > include recent and more advanced features of Haskell, new examples > and exercises, selected solutions, and freely downloadable lecture > slides and example code. The presentation is clean and simple, > while also being fully compliant with the latest version of > the language, including recent changes concerning applicative, > monadic, foldable, and traversable types. > > ================================================================= > > CONTENTS: > > Foreword > Preface > Part I. Basic Concepts: > 1. Introduction > 2. First steps > 3. Types and classes > 4. Defining functions > 5. List comprehensions > 6. Recursive functions > 7. Higher-order functions > 8. Declaring types and classes > 9. The countdown problem > Part II. Going Further: > 10. Interactive programming > 11. Unbeatable tic-tac-toe > 12. Monads and more > 13. Monadic parsing > 14. Foldables and friends > 15. Lazy evaluation > 16. Reasoning about programs > 17. Calculating compilers > Appendix A. Selected solutions > Appendix B. Standard prelude > Bibliography > Index > > ================================================================= > > AUTHOR: > > Graham Hutton is Professor of Computer Science at the University > of Nottingham. He has taught Haskell to thousands of students > and received numerous best lecturer awards. Hutton has served as > an editor of the Journal of Functional Programming, Chair of the > Haskell Symposium and the International Conference on Functional > Programming, and Vice-Chair of the ACM Special Interest Group on > Programming Languages, and he is an ACM Distinguished Scientist. > > ================================================================= > > > > > This message and any attachment are intended solely for the addressee > and may contain confidential information. If you have received this > message in error, please send it back to me, and immediately delete it. > > Please do not use, copy or disclose the information contained in this > message or in any attachment. Any views or opinions expressed by the > author of this email do not necessarily reflect the views of the > University of Nottingham. > > This message has been checked for viruses but the contents of an > attachment may still contain software viruses which could damage your > computer system, you are advised to perform your own checks. Email > communications with the University of Nottingham may be monitored as > permitted by UK legislation. > > > > ------------------------------ > > Subject: Digest Footer > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > > > ------------------------------ > > End of Beginners Digest, Vol 99, Issue 1 > **************************************** -- 2E45 D376 A744 C671 5100 A261 210B 8461 0FB0 CA1F

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New edition of "Programming in Haskell"
by Graham Hutton 01 Sep '16

01 Sep '16

Dear all, I'm delighted to announce that the new edition of "Programming in Haskell" is now available! Further details are provided below, and are also available from: http://tinyurl.com/hnfjdgc. Best wishes, Graham ================================================================= *** BOOK ANNOUNCEMENT *** Programming in Haskell - 2nd Edition Graham Hutton, University of Nottingham Cambridge University Press, 1st September 2016 320 pages, 120 exercises, ISBN 9781316626221 http://tinyurl.com/hnfjdgc ================================================================= DESCRIPTION: Haskell is a purely functional language that allows programmers to rapidly develop clear, concise, and correct software. The language has grown in popularity in recent years, both in teaching and in industry. This book is based on the author's experience of teaching Haskell for more than twenty years. All concepts are explained from first principles and no programming experience is required, making this book accessible to a broad spectrum of readers. While Part I focuses on basic concepts, Part II introduces the reader to more advanced topics. This new edition has been extensively updated and expanded to include recent and more advanced features of Haskell, new examples and exercises, selected solutions, and freely downloadable lecture slides and example code. The presentation is clean and simple, while also being fully compliant with the latest version of the language, including recent changes concerning applicative, monadic, foldable, and traversable types. ================================================================= CONTENTS: Foreword Preface Part I. Basic Concepts: 1. Introduction 2. First steps 3. Types and classes 4. Defining functions 5. List comprehensions 6. Recursive functions 7. Higher-order functions 8. Declaring types and classes 9. The countdown problem Part II. Going Further: 10. Interactive programming 11. Unbeatable tic-tac-toe 12. Monads and more 13. Monadic parsing 14. Foldables and friends 15. Lazy evaluation 16. Reasoning about programs 17. Calculating compilers Appendix A. Selected solutions Appendix B. Standard prelude Bibliography Index ================================================================= AUTHOR: Graham Hutton is Professor of Computer Science at the University of Nottingham. He has taught Haskell to thousands of students and received numerous best lecturer awards. Hutton has served as an editor of the Journal of Functional Programming, Chair of the Haskell Symposium and the International Conference on Functional Programming, and Vice-Chair of the ACM Special Interest Group on Programming Languages, and he is an ACM Distinguished Scientist. ================================================================= This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system, you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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regex and Unicode
by Brian Sammon 30 Aug '16

30 Aug '16

I tried to write a program using Text.Regex.PCRE to search through a UTF8-encoded document. It appears that the presence of non-breaking-space characters (Charpoint 160) triggers some weird behavior in my program. This is using the Debian stable(Jessie) packages of ghc 7.6.3 and libraries. Now I find myself at a fork in the road, not sure which direction to head in. Do I: 1) Continue looking (or get help with looking) for bugs in my code? (I have this reduced to a pretty small test case) 2) Assemble a bug-report against debian? 3) Assemble a bug-report against Text.Regex.PCRE (or Text.Regex.Base) for "upstream" 4) Uninstall Text.Regex.PCRE (and/or some other packages) and switch to something that works with Unicode/UTF8? Any ideas?

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Learning Monads with 'setjmp' and 'longjmp' like actions
by Michael Roth 30 Aug '16

30 Aug '16

Hi, I'm really trying hard to understand monads but I guess my brain is trapped in the imperative world. Below what I have done so far. But now I would like to implement something like this: do foo (label, jumped) <- setjmp bar longjmp label But I'm stuck. How can I implement 'setjmp' and 'longjmp' with my monad? I'm not able to grasp how can I design 'setjmp' and somehow create a label to use in 'longjmp'. Maybe my monad structure is not appropriate? Code follows: --------------------------------------------------------------- data World = SomeWorld deriving (Eq, Show) data ProgramResult a = Result { world :: World, result :: a } | Continue { world :: World, continue :: Program a } | Stopped { world :: World } newtype Program a = Program { runProgram :: World -> ProgramResult a } runComplete :: Program a -> World -> (World, a) runComplete prg world0 = case runProgram prg world0 of (Result world1 x1) -> (world1, x1) (Continue world1 prg1) -> runComplete prg1 world1 nop :: Program () nop = return () yield :: Program () yield = Program $ \world -> Continue world nop stop :: Program () stop = Program $ \world -> Stopped world instance Functor Program where fmap = liftM instance Applicative Program where pure = return (<*>) = ap instance Monad Program where return x = Program $ \world0 -> Result world0 x ma >>= fb = Program $ \world0 -> case runProgram ma world0 of (Result world1 x1) -> runProgram (fb x1) world1 (Continue world1 prg1) -> Continue world1 (prg1 >>= fb) (Stopped world1) -> Stopped world1

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Could anyone explain what this means? (Haskell wiki entry on monads)
by umptious 30 Aug '16

30 Aug '16

>>Each monad, or computation type<< That seems to be saying that monad and "computation type" are synonyms. But surely this is just bad writing? A monad is a CT, but a CT doesn't have to be a monad? And what is the point of adding the verbiage of "computation type" in what will be a massive sentence anyway? >>provides means, subject to *Monad Laws*,<< I think it's expected that monads will follow monad laws and provide means to do something.. >>> to *(a)* *create* a description of computation action that will produce (a.k.a. "return") a given Haskell value<<< This is just bizarre. The monad can create a description of a computation action? What does that mean - a comment is a description - does it mean that? And does it mean the description will do the returning or that the computation action will? And why say "means" instead of something more specific and meaningful (for example, I'm guessing the means might be a function..)

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Why is the type constraint different from signature?
by Lai Boon Hui 29 Aug '16

29 Aug '16

Hi All, can some one explain to me why ghci> let f:: (Ord a, Num b) => a -> b -> a ; f=undefined ghci> :t f 1 2 ghci> f 1 2 :: (Num a, Ord a) => a The initial type signature just required *a* to be a type that is an instance of Ord but after it had the additional constraint of Ord as well??? -- Best Regards, Boon Hui

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How do you show a Data.ByteArray?
by haskell＠verge.info.tm 28 Aug '16

28 Aug '16

Prelude Data.ByteArray> c = Data.ByteArray.zero 20 Prelude Data.ByteArray> :type c c :: ByteArray ba => ba Prelude Data.ByteArray> c <interactive>:38:1: error: • Ambiguous type variable ‘a0’ arising from a use of ‘print’ Uhm... Prelude Data.ByteArray> index c 3 <interactive>:39:1: error: • Ambiguous type variable ‘a0’ arising from a use of ‘index’ I can't print a byte?

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Basic Trie Implementation: Request for Feedback and QuickCheck Question
by Dominik Bollmann 28 Aug '16

28 Aug '16

Hello Haskellers, I just finished implementing a very simple Trie module, consisting of a Trie data type as well as operations `insert` and `lookup`. Function `insert` inserts a new element into the Trie; function lookup searches for all words in the Trie that match a specific pattern. Besides regular words, a pattern may consist of a single dot '.', indicating that at this position of the search term any letter may occur. For example a pattern "hello" would just search for the word "hello" in the Trie, while ".ello" would search words starting in *any* letter and followed by the sequence "ello" (i.e., it searches for "hello", "Hello", "Aello", etc.) The code for the Trie module can be found here: http://lpaste.net/180768. Regarding the code I have two questions: (1) Since I'm new to Haskell, I'd very much welcome feedback on the general implementation of the Trie module and its two methods what I could improve. (2) While testing the Trie module with QuickCheck, I wrote the property `prop_insert_then_lookup_succeeds` stating that after inserting a word into a Trie it can always successfully be looked up again. While quickchecking this property seems to succeed, I cannot verbose check it. In particular, when I run `verboseCheck prop_insert_then_lookup_succeeds` this property consumes all my main memory and results in the program hanging. Does anyone know what I messed up in devising the quickcheck property? I guess I left open some memory leak... Maybe it has to to with the Arbitrary instance which I might have defined incorrectly? Responses to any of the two questions above are very welcome :-) Thanks! Dominik.

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