- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] adding the elements of two lists
by Doug McIlroy 29 Mar '12

29 Mar '12

> Date: Tue, 27 Mar 2012 11:03:54 +1300 > From: "Richard O'Keefe" <ok(a)cs.otago.ac.nz> > Subject: Re: [Haskell-cafe] adding the elements of two lists > To: <jerzy.karczmarczuk(a)unicaen.fr> > Cc: haskell-cafe(a)haskell.org > > And *that* is why I stopped trying to define instance Num t => Num [t]. > If "I KNEW that the length of the lists is ... fixed ..." then the type > wasn't *really* [t], but some abstract type that happened to be implemented > as [t], and that abstract type deserved a newtype name of its own. > > Naming the type > - makes the author's intent clearer to readers > - lets the compiler check it's used consistently > - lets you have instances that don't match instances for > other abstract types that happen to have the same implementation > - provides a natural place to document the purpose of the type > - gives you a way to enforce the intended restrictions > all for zero run-time overhead. Quite taken by this manifesto for high style and morality, I resolved to do right by some old code, of which I had been quite proud: www.cs.dartmouth.edu/~doug/powser.html. Sadly, the exercise took some bloom off the rose. What the code gained in honesty and safety, it lost in beauty and readability. Here's the contrast, seen in overloading arithmetic to handle addition and multiplication of power series. --------- without newtype toSeries f = f : repeat 0 -- coerce scalar to series instance Num a => Num [a] where (f:fs) + (g:gs) = f+g : fs+gs (f:fs') * gs@(g:gs') = f*g : fs'*gs + (toSeries f)*gs' --------- with newtype newtype Num a => PS a = PS [a] deriving (Show, Eq) fromPS (PS fs) = fs -- extract list toPS f = PS (f : repeat 0) -- coerce scalar to series instance Num a => Num (PS a) where (PS (f:fs)) + (PS (g:gs)) = PS (f+g : fs+gs) (PS (f:fs)) * gPS@(PS (g:gs)) = PS $ f*g : fromPS ((PS fs)*gPS + (toPS f)*(PS gs)) The code suffers a pox of PS. When one goes on to introduce efficiencies and generalize to handle finite series (polynomials), it only gets worse. One unpleasant technical problem: the deriving clause is almost useless--one can't print or detect equality of infinite objects. For output one must apply something like (take 10 . fromPS). Yet this modest package would likely pose problems if it were combined with others in a grander suite for automated mathematics. What to do? I think I might write the package without newtype, and then wrap it in PS for export in hopes of exploiting the advantages of both styles.

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Mathematics and Statistics libraries
by Ben Jones 29 Mar '12

29 Mar '12

I am a student currently interested in participating in Google Summer of Code. I have a strong interest in Haskell, and a semester's worth of coding experience in the language. I am a mathematics and cs double major with only a semester left and I am looking for information regarding what the community is lacking as far as mathematics and statistics libraries are concerned. If there is enough interest I would like to put together a project with this. I understand that such libraries are probably low priority, but if anyone has anything I would love to hear it. Thanks for reading, -Benjamin

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a code that cannot compile with or without NoMonomorphismRestriction
by Ting Lei 29 Mar '12

29 Mar '12

Hi I have met a piece of code that cannot be compiled whether I add or remove the NoMonomorphismRestriction flag (as of GHC 7.0.4, Haskell platform 2011.4.0.0).I have extracted a minimal example below: {-# LANGUAGE NoMonomorphismRestriction #-} (f1, f2) = let commond_definitions = undefined in let f1 = id.show f2 x = (< x) in (f1, f2) I needed this format because there are many shared definitions in common_definitions for f1 and f2, and I want to keep them local. If I compile them with NoMonomorphismRestriction, I get: D:\work\test.hs:7:8: Ambiguous type variable `a0' in the constraint: (Show a0) arising from a use of `f1' Possible cause: the monomorphism restriction applied to the following: f1 :: a0 -> String (bound at D:\work\hsOcaml\test.hs:2:2) Probable fix: give these definition(s) an explicit type signature In the expression: f1 In the expression: (f1, f2) In the expression: let f1 = id . show f2 x = (< x) in (f1, f2)D:\work\test.hs:7:12: Ambiguous type variable `a1' in the constraint: (Ord a1) arising from a use of `f2' Possible cause: the monomorphism restriction applied to the following: f2 :: a1 -> a1 -> Bool (bound at D:\work\hsOcaml\test.hs:2:6) Probable fix: give these definition(s) an explicit type signature In the expression: f2 In the expression: (f1, f2) In the expression: let f1 = id . show f2 x = (< x) in (f1, f2) Failed, modules loaded: none. If I comment out -- {-# LANGUAGE NoMonomorphismRestriction #-} I get: D:\work\hsOcaml\test.hs:4:17: Ambiguous type variable `a0' in the constraint: (Show a0) arising from a use of `show' Possible cause: the monomorphism restriction applied to the following: f1 :: a0 -> String (bound at D:\work\hsOcaml\test.hs:2:2) Probable fix: give these definition(s) an explicit type signature or use -XNoMonomorphismRestriction In the second argument of `(.)', namely `show' In the expression: id . show In an equation for `f1': f1 = id . showD:\work\hsOcaml\test.hs:7:12: Ambiguous type variable `a1' in the constraint: (Ord a1) arising from a use of `f2' Possible cause: the monomorphism restriction applied to the following: f2 :: a1 -> a1 -> Bool (bound at D:\work\hsOcaml\test.hs:2:6) Probable fix: give these definition(s) an explicit type signature or use -XNoMonomorphismRestriction In the expression: f2 In the expression: (f1, f2) In the expression: let f1 = id . show f2 x = (< x) in (f1, f2) Failed, modules loaded: none. Can anyone show me why this does not work and how to fix it (e.g. by adding type signature as the error message suggested)?I tried to add type signature by couldn't figure out the right way of doing it. Thanks in advance! Ting

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Re: [Haskell-cafe] adding the elements of two lists
by Doug McIlroy 29 Mar '12

29 Mar '12

> From: "Richard O'Keefe" <ok(a)cs.otago.ac.nz> > Date: Thu, 29 Mar 2012 16:34:46 +1300 > > On 29/03/2012, at 3:08 PM, Doug McIlroy wrote: > > --------- without newtype > > > > toSeries f = f : repeat 0 -- coerce scalar to series > > > > instance Num a => Num [a] where > > (f:fs) + (g:gs) = f+g : fs+gs > > (f:fs') * gs@(g:gs') = f*g : fs'*gs + (toSeries f)*gs' > > > > --------- with newtype > > > > newtype Num a => PS a = PS [a] deriving (Show, Eq) > > > > fromPS (PS fs) = fs -- extract list > > toPS f = PS (f : repeat 0) -- coerce scalar to series > > > > instance Num a => Num (PS a) where > > (PS (f:fs)) + (PS (g:gs)) = PS (f+g : fs+gs) > > (PS (f:fs)) * gPS@(PS (g:gs)) = > > PS $ f*g : fromPS ((PS fs)*gPS + (toPS f)*(PS gs)) > > Try it again. > > newtype PS a = PS [a] deriving (Eq, Show) > > u f (PS x) = PS $ map f x > b f (PS x) (PS y) = PS $ zipWith f x y > to_ps x = PS (x : repeat 0) > > ps_product (f:fs) (g:gs) = whatever > > instance Num a => Num (PS a) > where > (+) = b (+) > (-) = b (-) > (*) = b ps_product > negate = u negate > abs = u abs > signum = u signum > fromInteger = to_ps . fromInteger > > I've avoided defining ps_product because I'm not sure what > it is supposed to do: the definition doesn't look commutative. You have given the Hadamard product--a construction with somewhat esoteric properties. The product I have in mind is the ordinary mathematical product that one meets in freshman calculus. The distributive law yields this symmetric formulation (f:fs) * (g:gs) = f*g : (toSeries f)*gs + fs*(toSeries g) + (0 : fs*gs) The version I gave is an optimization (which I learned from Jerzy). For more explanation see www.cs.dartmouth.edu/~doug/powser.html. I like the lifting functions b and u, but they don't get one very far. The product is where the PS pox begins to bite badly. I would welcome a perspicuous formulation of that using newtype. Incidentally, a more efficient way to write the symmetric product is (f:fs) * (g:gs) = f*g : zipWith (f*) gs + zipWith fs (*g) + (0 : fs*gs) toSeries and zipWith appear in the formulas because Haskell overloading won't let one use the multiplication symbol for both series*series and scalar*series. One might also invent a distinct operator for the purpose. Coercion with toSeries strikes me as the least jarring of these approaches. zipWith suffers from mixing levels of abstraction--it signifies not the idea of multiplication, but the algorithm. A new operator would suffer both from unfamiliarity and lack of commutativity. > > > The code suffers a pox of PS. > > But it doesn't *need* to. > >

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adding the elements of two lists
by TP 29 Mar '12

29 Mar '12

Hello, My primary problem may be reduced to adding elements of two lists: [1,2,3] + [4,5,6] = [5,7,9] My first idea was to declare a list of Int as an instance of Num, and define (+) in the correct way. However, it seems it is not possible to do that: ------------------- instance Num [Int] where l1 + l2 = .... ------------------- Why? It seems it is necessary to do: ------------------ newtype ListOfInt = ListOfInt { getList :: [Int] } deriving (Show, Eq) instance Num ListOfInt where l1 + l2 = ... ------------------- Am I correct? Is it the best way to do that? Now, what is the most usual way to implement l1+l2? I have just read about applicative functors, with which I can do: ------------------- import Control.Applicative let l1 = [1,2,3] let l2 = [4,5,6] print $ getZipList $ (+) <$> ZipList l1 <*> ZipList l2 [5,7,9] ------------------- Is it the correct way to do that? I have tried: ------------------- instance Num ListOfInt where l1 + l2 = ListOfInt $ getZipList $ (+) <$> ZipList (getList l1) <*> ZipList (getList l2) ------------------- Isn't it too much complicated? Thanks TP

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Haskell Weekly News: Issue 220
by Daniel Santa Cruz 29 Mar '12

29 Mar '12

Welcome to issue 220 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of March 18 to 24, 2012. Quotes of the Week * shachaf: Type classes are a scary story that you tell new Haskellers so they can go to sleep at night. * acowley: Parametric polymorphism is the inspiring story you tell them to get them riled up and raring to go. It's like Braveheart for programmers. * monochrom: the purpose of types is to ban more programs * rostayob: I was a type constructor, then I took an arrow in the kind * shachaf: @let otherfoolish = not otherwise * flebron: Monoidal objects in the monoidal category of endofunctors on a category. I took a small detour through tensor products to understand monoidal categories better, with one example of such a product. mauke: I have no idea what you just said * weexplat: starts a proposal to replace launchMissiles with atomicDestroyBiosphere * monochrom: rumour: SPJ will write a thick book "A New Science of Kind" * JonathanShapiro: [on abandoning BitC] "And I *still* don't believe in monads. :-)" Top Reddit Stories * Starting the Mezzo Haskell Book Domain: yesodweb.com, Score: 58, Comments: 28 On Reddit: [1] http://goo.gl/pOSlR Original: [2] http://goo.gl/ecy4O * First patches for iPhone cross compiler against GHC head Domain: haskell.org, Score: 55, Comments: 27 On Reddit: [3] http://goo.gl/OOAai Original: [4] http://goo.gl/p7C3L * Retrospective thoughts on BitC by its author, and why type classes didn't work for him Domain: coyotos.org, Score: 44, Comments: 22 On Reddit: [5] http://goo.gl/0sL80 Original: [6] http://goo.gl/5pCp2 * Google Summer of Code 2012 - Haskell.org is participating Domain: haskell.org, Score: 42, Comments: 11 On Reddit: [7] http://goo.gl/9EwzD Original: [8] http://goo.gl/XqcUJ * Conduits vs. Pipes. Using Void as an Input or Output type can reduce the number of concepts (source, sink, conduit) from 3 to 1 (pipe). Domain: twanvl.nl, Score: 34, Comments: 50 On Reddit: [9] http://goo.gl/bc8RB Original: [10] http://goo.gl/eb7oz * Announcing conduit 0.3 Domain: yesodweb.com, Score: 33, Comments: 9 On Reddit: [11] http://goo.gl/JrgV0 Original: [12] http://goo.gl/OsDl7 * Haskell for embedded domain-specific languages (Functional Programming eXchange 2012) Domain: skillsmatter.com, Score: 31, Comments: 4 On Reddit: [13] http://goo.gl/pSREX Original: [14] http://goo.gl/1azVh * Domain theory course Domain: cs.nott.ac.uk, Score: 31, Comments: 5 On Reddit: [15] http://goo.gl/5VHyG Original: [16] http://goo.gl/o2pRC * ANNOUNCE: options, an easy-to-use command-line option parser Domain: self.haskell, Score: 29, Comments: 21 On Reddit: [17] http://goo.gl/s6ktw Original: [18] http://goo.gl/s6ktw * A Case Study in Creating Rich Internet Apps in Haskell (Silk at FP eXchange 2012) Domain: skillsmatter.com, Score: 22, Comments: 18 On Reddit: [19] http://goo.gl/ZdtDq Original: [20] http://goo.gl/Ta2HQ * It just occurred to me that (lift .) defines a functor... Domain: self.haskell, Score: 22, Comments: 15 On Reddit: [21] http://goo.gl/GMzPt Original: [22] http://goo.gl/GMzPt Top StackOverflow Questions * Haskell platform: nested functions and optimization votes: 15, answers: 2 Read on SO: [23] http://goo.gl/nreIr * newtype with RankNTypes votes: 14, answers: 2 Read on SO: [24] http://goo.gl/UT9DV * Pattern Matching - Prolog Vs Haskell votes: 13, answers: 4 Read on SO: [25] http://goo.gl/3hZV1 * How does this definition of ArrowLoop.loop work? votes: 13, answers: 2 Read on SO: [26] http://goo.gl/ocSCM * Data.Set : does it always know best? votes: 12, answers: 2 Read on SO: [27] http://goo.gl/NUjkB * Building a histogram with haskell, many times slower than with python votes: 11, answers: 3 Read on SO: [28] http://goo.gl/kbh5i * What is the scope of let when used without in? votes: 11, answers: 2 Read on SO: [29] http://goo.gl/bNwna * Strange "half to even" rounding in different languages [closed] votes: 11, answers: 3 Read on SO: [30] http://goo.gl/lnPch * Practical reasons for Сhurch Encoding votes: 10, answers: 1 Read on SO: [31] http://goo.gl/xYoef * Can the FFI deal with arrays? If so, how? votes: 9, answers: 3 Read on SO: [32] http://goo.gl/d4R3H Until next time, Daniel Santa Cruz References 1. http://www.yesodweb.com/blog/2012/03/start-mezzo-haskell 2. http://www.reddit.com/r/haskell/comments/r9xdq/starting_the_mezzo_haskell_b… 3. http://www.haskell.org/pipermail/iphone/2012-March/000120.html 4. http://www.reddit.com/r/haskell/comments/r3ge7/first_patches_for_iphone_cro… 5. http://www.coyotos.org/pipermail/bitc-dev/2012-March/003300.html 6. http://www.reddit.com/r/haskell/comments/rbk7z/retrospective_thoughts_on_bi… 7. http://www.haskell.org/pipermail/haskell-cafe/2012-March/100247.html 8. http://www.reddit.com/r/haskell/comments/r5h81/google_summer_of_code_2012_h… 9. http://twanvl.nl/blog/haskell/conduits-vs-pipes 10. http://www.reddit.com/r/haskell/comments/rbgvz/conduits_vs_pipes_using_void… 11. http://www.yesodweb.com/blog/2012/03/announcing-conduit-0-3 12. http://www.reddit.com/r/haskell/comments/r6wdh/announcing_conduit_03/ 13. http://skillsmatter.com/podcast/scala/haskell-for-embedded-domain-specific-… 14. http://www.reddit.com/r/haskell/comments/r22d6/haskell_for_embedded_domains… 15. http://www.cs.nott.ac.uk/~gmh/domains.html 16. http://www.reddit.com/r/haskell/comments/r250h/domain_theory_course/ 17. http://www.reddit.com/r/haskell/comments/rbpmb/announce_options_an_easytous… 18. http://www.reddit.com/r/haskell/comments/rbpmb/announce_options_an_easytous… 19. http://skillsmatter.com/podcast/home/silkapp-a-case-study-in-creating-rich-… 20. http://www.reddit.com/r/haskell/comments/r1xey/a_case_study_in_creating_ric… 21. http://www.reddit.com/r/haskell/comments/raxsk/it_just_occurred_to_me_that_… 22. http://www.reddit.com/r/haskell/comments/raxsk/it_just_occurred_to_me_that_… 23. http://stackoverflow.com/questions/9757515/haskell-platform-nested-function… 24. http://stackoverflow.com/questions/9779301/newtype-with-rankntypes 25. http://stackoverflow.com/questions/9780779/pattern-matching-prolog-vs-haske… 26. http://stackoverflow.com/questions/9856342/how-does-this-definition-of-arro… 27. http://stackoverflow.com/questions/9755851/data-set-does-it-always-know-best 28. http://stackoverflow.com/questions/9772098/building-a-histogram-with-haskel… 29. http://stackoverflow.com/questions/9777399/what-is-the-scope-of-let-when-us… 30. http://stackoverflow.com/questions/9829769/strange-half-to-even-rounding-in… 31. http://stackoverflow.com/questions/9806729/practical-reasons-for-hurch-enco… 32. http://stackoverflow.com/questions/9854782/can-the-ffi-deal-with-arrays-if-…

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Can't install snap (problem with syb)
by Michael Iles 29 Mar '12

29 Mar '12

I uninstalled ghc, removed my ~/.cabal directory, reinstalled ghc, then tried to install snap. I get: $ cabal install snap Resolving dependencies... Downloading syb-0.3.6... <command line>: cannot satisfy -package Cabal-1.14.0: Cabal-1.14.0-0338b6f52e6e6a56054371a110e1d79e is unusable due to missing or recursive dependencies: directory-1.1.0.2-8957520ced1fb19160c3a6126dcccfaa process-1.1.0.1-91185c964ab744c1f3cbca1863d2ba45 (use -v for more information) cabal: Error: some packages failed to install: aeson-0.6.0.0 depends on syb-0.3.6 which failed to install. snap-0.8.0.2 depends on syb-0.3.6 which failed to install. syb-0.3.6 failed during the configure step. The exception was: ExitFailure 1 Any ideas? Mike.

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Re: [Haskell-cafe] [Haskell] ANNOUNCE: haskell-src-exts-1.13.0
by dag.odenhall＠gmail.com 28 Mar '12

28 Mar '12

On 28 March 2012 21:05, Jurriaan Hage <J.Hage(a)uu.nl> wrote: > > Our first year students will be very unhappy to hear this. Wait, what?

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ANNOUNCE: haskell-src-exts-1.13.0
by Niklas Broberg 28 Mar '12

28 Mar '12

Fellow Haskelleers, I'm pleased to announce the release of haskell-src-exts-1.13.0! * On hackage: http://hackage.haskell.org/package/haskell-src-exts * Via cabal: cabal install haskell-src-exts * Darcs repo: http://code.haskell.org/haskell-src-exts It's been a *very short* while since the last update before this, and another (smallish) major release it is. Sorry about that (especially to Neil who was quick to update). The main update of this release is to add support for DoAndIfThenElse, which means haskell-src-exts at long last is compliant with Haskell2010. This is also the cause for the backwards-incompatible change: default parse mode is now to use Haskell2010 mode. In particular this means that NPlusKPatterns is no longer recognized by default. No old code using haskell-src-exts should fail to compile (unless really unlucky with clashing imports from L.H.E.Extension), but it may now fail to work on some Haskell98-compliant source files. Changelog: 1.12.0 --> 1.13.0 =============== * Add extensions DoAndIfThenElse and NPlusKPatterns to Language.Haskell.Exts.Extensions. * DoAndIfThenElse is now supported, at long last, making HSE compatible with Haskell2010 * Introduce haskell98 and haskell2010 extension groups, exported from Language.Haskell.Exts.Extensions. * Backwards-incompatible change: default parse mode is now to use haskell2010, which means the following features are recognized by default: DoAndIfThenElse, PatternGuards, ForeignFunctionInterface, EmptyDataDecls. NPlusKPatterns is no longer recognized by default. Cheers, /Niklas

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ghci can't load file whose name contains non-ASCII chars?
by Johannes Waldmann 28 Mar '12

28 Mar '12

I must be making some obvious mistake here, but I'm not seeing it. The file name contains O-umlaut, and the OS handles it fine, but ghci does not like it (although it accepts umlauts in the contents of the file (UTF-8) e.g., as a module name) $ cat fÖÖbar.hs main = print $ product [1..100] $ ghci fÖÖbar.hs GHCi, version 7.4.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. <no location info>: can't find file: f??bar.hs Failed, modules loaded: none. this is on a standard ubuntu install (11.10 and 11.04) with $ locale LANG=en_US.UTF-8 LANGUAGE= LC_CTYPE="en_US.UTF-8" ... $ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.4.1 $ uname -a Linux octopus 3.0.0-16-generic #29-Ubuntu SMP Tue Feb 14 12:48:51 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux

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