- Beginners - Haskell.org

Re: [Haskell-beginners] Newbie question about function type constraints
by Tushar Tyagi 22 Sep '16

22 Sep '16

What happens if you change the signature of meanList to meanList :: ( Fractional b) => [b] ->b The integrals in [1,2,3] would be converted to [1.0, 2.0, 3.0] before you act upon them. On 22 Sep 2016 6:49 p.m., "Lai Boon Hui" <laiboonh(a)gmail.com> wrote: Hi, can someone explain to me why i cannot define meanList as: meanList :: (Integral a, Fractional b) => [a] -> a meanList xs = (sumList xs) / (lengthList xs) I want to restrict the function to only accept lists like [1,2,3] and return answer 2.0 sumList :: (Num a) => [a] -> a sumList [] = 0 sumList (x:xs) = x + (sumList xs) lengthList :: (Num a) => [t] -> a lengthList [] = 0 lengthList (_:xs) = 1 + (lengthList xs) _______________________________________________ Beginners mailing list Beginners(a)haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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is this right?
by Dennis Raddle 22 Sep '16

22 Sep '16

I want to evaluate a function on a series of inputs: f :: a -> Maybe b then collect the results [b] if they are all Just, or terminate the computation immediately upon hitting Nothing. This is exactly what mapM does in the Maybe monad, correct? In particular I want to make sure that it will not try to evaluate anything past the first 'Nothing' result as the efficiency of my design is based on that. D

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why isnt this optimized?
by Gunnar Quehl 16 Sep '16

16 Sep '16

Hi, I wrote a recursive fibonacci function in ghci: :set +m let f 0 = 0 f 1 = 1 f n = f (n-1) + f (n-2) As expected calculation f 30 takes a while. About 5s on my machine. What I don't understand is that let x = f 30 in (x,x) takes twice as long. Why is ghci reevaluating the x twice? Isn't in always said, that it can optimize because we can replace same by same and so on. Actually if compiled the behaviour is as expected. main = print $ let x = f 34 in (x) and main = print $ let x = f 34 in (x,x) both take about 3s. Why is this not the case in the interactive enviroment? Thanks Gunnar

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how to enter EOF in emacs haskell mode
by Chul-Woong Yang 14 Sep '16

14 Sep '16

Hi, all. I use emacs haskell mode for editing and testing simple programs. I have no success in finding out how to enter EOF to haskell program in emacs "Interactive-Haskell" buffer. So I am unable to test haskell program which reads from stdin. :-( Any help would be appreciated deeply. Regards, Chul-Woong Yang

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help with optimizing memory usage of thunk size of Monte Carlo backtracking search algorithm
by Dennis Raddle 12 Sep '16

12 Sep '16

I'm a Haskell semi-beginner, and I've implemented a crude Monte Carlo search optimization algorithm, and I want to know how to use strict evaluation to prevent it from consuming too much memory. I am willing to learn new things to do this---if you want to point me in the right direction and maybe give some references, I'll take it from there. However, if there is something simple that can take care of the whole problem, please let me know. I have not run this with the profiler so I don't actually know precisely what it is doing now. I have not run out of memory using it in small cases, but I hope to use it on much larger cases. Here is the code so far: import qualified Data.List as L import Control.Monad import Data.Function -- Suppose that we do backtracking search for an optimal arrangement of -- elements in some kind of "structure". The "structure" is built one step at -- a time. -- -- An example would be searching for an optimal arrangement of furniture. The -- structure is a represenation of the room and all items that have been -- placed in it so far. We could list the options (or "steps") available to us -- at any point in building the structure, that is a list of furniture items -- and locations. -- -- We have the notion that the structure, at some point in adding steps, -- becomes complete. We have an evaluation function providing a "goodness -- score" on either a partially built or complete structure. -- -- The search will optimize the goodness score over all possible complete -- structures. (Or perhaps with Monte Carlo search, an approximate optimal -- value.) -- -- Class Opt (for "optimization") defines a structure type "struct" and a step -- type "step". class Opt struct step | struct -> step where -- number of steps chosen so far in current state of 'struct': oSize :: struct -> Int -- list all the available steps to choose next. If this list is null, then -- the structure is complete. oList :: struct -> [step] -- apply a step to the structure to create a new structure: oApply :: struct -> step -> struct -- evaluate the "goodness" of the current state of a structure. Higher is -- better. oEval :: struct -> Double -- Implement a kind of Monte Carlo search. (I have a vague idea of the -- literature on Monte Carlo; this algorithm is my guess at something that -- does the job). We work in a state monad of class -- "RandMonad" which holds the StdGen data and provides several methods for -- accessing it. The only method we need from RandMonad is -- -- rChooseItem :: RandMonad m => [a] -> m a -- -- which chooses a random item from a non-null list. -- -- The basic algorithm is this: at each point in building the structure, we -- have a structure S and a list of next steps step_list. We apply each step -- in step_list to S, in turn. After applying a step x to S, call the result -- S_x. We evaluate the "monte carlo fitness" of S_x by making random -- completions of it---that is, choosing a bunch of additional steps -- randomly---doing 'nExper' completions (nExper might be 1000 to 10000). The -- fitness as measured by oEval of the very best random completion becomes -- the "monte carlo fitness" of S_x. We then choose the step from step_list, -- x, that maximizes the "monte carlo fitness" of S_x. -- -- We then repeat this process until S is complete. -- Function monteCarlo will take a partially complete structure, and optimize -- it over an investigation of 'nExper' possible "completions". monteCarlo :: (RandMonad r, Opt a b) => Int -> a -> r a monteCarlo nExper struct = case oList struct of -- If structure is complete, then it is its own optimization. [] -> return struct -- Otherwise find optimal step according to "monte carlo fitness" and -- recursively call 'monteCarlo' steps -> do let doStep step = do let newStruct = oApply struct step score <- monteCarloEval nExper newStruct return (score,newStruct) (_,winner) <- L.maximumBy (compare `on` fst) `liftM` mapM doStep steps monteCarlo nExper winner -- monteCarloEval -- -- Evaluate the "monte carlo fitness" of a structure 'struct' by completing it -- in nExper random ways (that is, make all remaining choices in purely random -- way) and finding the maximum value of the evaluated final state among all -- nExper ways. -- monteCarloEval :: (RandMonad r, Opt a b) => Int -> a -> r Double monteCarloEval nExper struct = case oList struct of [] -> return $ oEval struct _ -> do scores <- replicateM nExper (randomComplete struct) return . maximum . map oEval $ scores -- Make random choices of steps until a structure 's' is complete. randomComplete :: (RandMonad r, Opt a b) => a -> r a randomComplete s = case oList s of [] -> return s steps -> rChooseItem steps >>= (randomComplete . oApply s)

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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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