January 2016 - Beginners

Custom type classes
by Daniel Hinojosa 25 Jan '16

25 Jan '16

I am pretty sure I have a good handle on type classes, but this one is perplexing me in Haskell. I created custom Tuples called Tuple2 and Tuple3 (I know Haskell already has Tuples, just thought I would create my own for this exercise). Then I wanted to create a type class that would have a method called first that would get the first element of the tuple regardless of what kind of Tuple it is. Tuple2, Tuple3, etc. Here is what I have: data Tuple3 a b c = Tuple3 a b c deriving (Show) data Tuple2 a b = Tuple2 a b deriving (Show) class Indexable idx where first :: idx c -> a instance Indexable (Tuple2 a) where first (Tuple2 a b) = a In my main, I try to get call putStrLn $ show $ first $ Tuple2 1 "One" I was greeted with the following trace: Couldn't match expected type ‘a1’ with actual type ‘a’ ‘a’ is a rigid type variable bound by the instance declaration at TypeClasses.hs:35:10 ‘a1’ is a rigid type variable bound by the type signature for first :: Tuple2 a c -> a1 at TypeClasses.hs:36:4 Relevant bindings include a :: a (bound at TypeClasses.hs:36:18) first :: Tuple2 a c -> a1 (bound at TypeClasses.hs:36:4) In the expression: a In an equation for ‘first’: first (Tuple2 a b) = a Help is appreciated.

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Functional Programming for the Object Oriented
by Øystein Kolsrud 24 Jan '16

24 Jan '16

I gave a presentation on functional programming a couple of weeks ago that I thought might be of interest to this community as well: https://www.youtube.com/watch?v=I2tMmsZC1ZU It's target audience is programmers familiar with object oriented programming, and it presents how concepts from the functional paradigm are relevant also in most modern OO languages. For more information about the presentation, please refer to the following site: http://www.foocafe.org/previous_event/functional-programming-for-the-object… -- Mvh Øystein Kolsrud

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FFI and opac struct
by PICCA Frederic-Emmanuel 24 Jan '16

24 Jan '16

Hello Here the code I try to use I get an array of (HklFactory *) via the GetAll method and I want to read for each of these HklFactory the name of the factory with the NameGet method. -- data HklFactory newtype HklFactory = HklFactory (Ptr HklFactory) deriving (Show, Storable) hklFactoryGetAll :: IO [HklFactory] hklFactoryGetAll = alloca $ \ptr -> do factories <- c_hkl_factory_get_all ptr n <- peek ptr peekArray n factories foreign import ccall unsafe "hkl.h hkl_factory_get_all" c_hkl_factory_get_all :: Ptr Int -> IO (Ptr HklFactory) hklFactoryNameGet :: Ptr HklFactory -> IO String hklFactoryNameGet factory = do name <- c_hkl_factory_name_get factory peekCString name foreign import ccall unsafe "hkl.h hkl_factory_name_get" c_hkl_factory_name_get :: Ptr HklFactory -> IO CString main :: IO () main = do initGUI factories <- hklFactoryGetAll names <- mapM hklFactoryNameGet factories print factories print names the signature of the C method are: typedef struct _HklFactory HklFactory; HKLAPI HklFactory **hkl_factory_get_all(size_t *n) HKL_ARG_NONNULL(1); HKLAPI const char *hkl_factory_name_get(const HklFactory *self) HKL_ARG_NONNULL(1); when I run the haskell code, I just get garbage when I try to extract the name of all HklFActoies here the output [HklFactory 0xb777b920,HklFactory 0xb777ba6c,HklFactory 0xb777ba98,HklFactory 0xb777bac4,HklFactory 0xb777bbf8,HklFactory 0xb777bef8,HklFactory 0xb777c4f0,HklFactory 0xb777c524,HklFactory 0xb777c6c0,HklFactory 0xb777c784,HklFactory 0xb777c8a8,HklFactory 0xb777c8dc,HklFactory 0xb777c908,HklFactory 0xb777c9cc] [",v+v<w\STX","2v,vw\EOT","2v|.vw\EOT","2v\DC40vw\EOT","5v\\3v\DC4w\EOT","_8v86v\DC4w\ACK","z@v9v\fw\ACK","@v\EOT<v@w\ACK","EvBvw\ACK","GvEvw\ENQ","yNvGvw\ACK","NvHJvw\EOT","Nv,Lv$w\ENQ","PvNvw\EOT"] Instead of human readable names I get this garbage. So I do not understand what is going on and what is wrong in my haskell code. thanks for your help Frederic

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FW: Beginners Digest, Vol 91, Issue 26
by pmcilroy＠gmail.com 22 Jan '16

22 Jan '16

Don’t worry about the time to append rather than pretend, it is not “exponential” nor even cubic. You are making a list of length in an N^2 algorithm. Appending to a list adds another tiny O(N^2) step, which is dominated by the main algorithm (also O(N^2), which both takes longer and evaluates vastly more values (failures as well as successes.) Message: 3 Date: Thu, 21 Jan 2016 19:05:17 +0000 From: Rein Henrichs <rein.henrichs(a)gmail.com> To: The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell <beginners(a)haskell.org>, masonmlai(a)gmail.com Subject: Re: [Haskell-beginners] program not running lazily Message-ID: <CAJp6G8zemeHpYwZ36ShMJ-m_BishVnUJpoqEmK=DosKHRVaqeg(a)mail.gmail.com> Content-Type: text/plain; charset="utf-8" s/not/note, sorry On Thu, Jan 21, 2016 at 10:42 AM Rein Henrichs <rein.henrichs(a)gmail.com> wrote: > But not that doing so will cause the program to have an exponential > runtime as each new ys must be repeatedly traversed to append a [y].. The > alternative is to *unfold* your list by recursing on the right hand side of > the (:) to add new elements. > > On Thu, Jan 21, 2016 at 5:43 AM Doug McIlroy <doug(a)cs.dartmouth.edu> > wrote: > >> Each time you find another good 9-mer, you add it to >> the head of the list. This means that the ultimate >> list will be in reverse order of discovery: the first element >> to be printed is the last one to be found. To get >> output in the order it was discovered, build the >> output by ys++[y] rather than y:ys. >> _______________________________________________ >> Beginners mailing list >> Beginners(a)haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners >> >

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program not running lazily
by Doug McIlroy 21 Jan '16

21 Jan '16

Each time you find another good 9-mer, you add it to the head of the list. This means that the ultimate list will be in reverse order of discovery: the first element to be printed is the last one to be found. To get output in the order it was discovered, build the output by ys++[y] rather than y:ys.

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program not running lazily
by Mason Lai 20 Jan '16

20 Jan '16

Hi, I'm teaching myself Haskell and attempting to use it for day-to-day tasks at work. I signed up for this list a few weeks ago, and this is my first post. Here's my problem: I'm given a list of 45 Strings, each nine Chars long. The only characters are 'A', 'C', 'G' and 'T'. (This is for a bioinformatics application.) I'll refer to any nine-length String composed of these Chars as a "9-mer". I need to generate a larger list of 9-mers such that no two 9-mers have a Levenshtein distance of less than 3. (The list I start with satisfies this requirement.) I can generate as many 9-mers as I possibly can, but this process is very slow. It's also not being computed lazily; calling head on the output list forces the entire list of 9-mers to be computed. *I'd like to understand why this list isn't being computed lazily, and how I can change my code to make it so. *My knowledge of monads is pretty poor as well, so a digression or a series of questions about why the line [9] >>= (`replicateM` "ACGT") works would also be helpful, as would general tips about writing clean code. This is an O(n^2) operation, so I'm not expecting it to be slow. However, I figured that I'd just be able to take the first N elements from the output list. Here's what I have: import Control.Monad main :: IO () main = interact processData processData :: String -> String processData = unlines . (`merge` ([9] >>= (`replicateM` "ACGT"))) . lines -- Merges two lists of things into a single list merge :: Eq a => [[a]] -> [[a]] -> [[a]] merge xs [] = xs merge xs ys = merge ((head ys) `addInto` xs) $ tail ys -- Adds a thing into a list if it is "different" enough addInto :: Eq a => [a] -> [[a]] -> [[a]] y `addInto` ys = if ((minimum $ map (dist y) ys) < 3) then ys else y:ys -- Calculate the Levenshtein distance -- Lloyd Allison algorithm. See Haskell wiki -- code omitted dist :: Eq a => [a] -> [a] -> Int My workaround to getting a smaller subset of the whole list is to replace [9] >>= (`replicateM` "ACGT") with take 10000 $ [9] >>= (`replicateM` "ACGT") Thanks! Mason Lai

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Problem using stack
by Robert Weisser 20 Jan '16

20 Jan '16

I recently installed stack via Haskell for Mac OS X (https://ghcformacosx.github.io/) which gave me stack 0.1.8.0 x86_64 and ghc 7.10.2. Following the User Guide (http://docs.haskellstack.org/en/stable/GUIDE.html) I tried to create the first example (hello world, naturally). Everything went according to plan until I entered 'stack test', which failed. The output was supposed to be, 'Test suite not yet implemented.' I tried to find information online, but was unsuccessful. I play with Haskell for fun and know nothing about cabal, so I don't know if it is a cabal issue. The error message includes 'helloworld-test: executable not found'. helloworld-test is indeed missing. I searched for it using find: find . -name '*test' Here is the terminal session, followed by the contents of the generated helloworld.cabal file (with some user info removed): $ stack new helloworld new-template Downloading template "new-template" to create project "helloworld" in helloworld/ ... Writing default config file to: /Users/rpw/self/haskell/stack/user_guide/helloworld/stack.yaml Basing on cabal files: - /Users/rpw/self/haskell/stack/user_guide/helloworld/helloworld.cabal Checking against build plan lts-3.13 Selected resolver: lts-3.13 Wrote project config to: /Users/rpw/self/haskell/stack/user_guide/helloworld/stack.yaml $ cd helloworld/ $ stack setup stack will use the GHC on your PATH For more information on paths, see 'stack path' and 'stack exec env' To use this GHC and packages outside of a project, consider using: stack ghc, stack ghci, stack runghc, or stack exec $ stack build helloworld-0.1.0.0: configure Configuring helloworld-0.1.0.0... helloworld-0.1.0.0: build Preprocessing library helloworld-0.1.0.0... [1 of 1] Compiling Lib ( src/Lib.hs, .stack-work/dist/x86_64-osx/Cabal-1.22.4.0/build/Lib.o ) In-place registering helloworld-0.1.0.0... Preprocessing executable 'helloworld-exe' for helloworld-0.1.0.0... [1 of 1] Compiling Main ( app/Main.hs, .stack-work/dist/x86_64-osx/Cabal-1.22.4.0/build/helloworld-exe/helloworld-exe-tmp/Main.o ) Linking .stack-work/dist/x86_64-osx/Cabal-1.22.4.0/build/helloworld-exe/helloworld-exe ... helloworld-0.1.0.0: copy/register Installing library in /Users/rpw/self/haskell/stack/user_guide/helloworld/.stack-work/install/x86_64-osx/lts-3.13/7.10.2/lib/x86_64-osx-ghc-7.10.2/helloworld-0.1.0.0-6urpPe0MO7OHasGCFSyIAT Installing executable(s) in /Users/rpw/self/haskell/stack/user_guide/helloworld/.stack-work/install/x86_64-osx/lts-3.13/7.10.2/bin Registering helloworld-0.1.0.0... $ stack exec helloworld-exe someFunc $ stack test Test suite helloworld-test executable not found for helloworld Test suite failure for package helloworld-0.1.0.0 helloworld-test: executable not found Logs printed to console $ cat helloworld.cabal name: helloworld version: 0.1.0.0 synopsis: Initial project template from stack description: Please see README.md homepage: ... license: BSD3 license-file: LICENSE author: ... maintainer: ... copyright: None category: Example build-type: Simple -- extra-source-files: cabal-version: >=1.10 library hs-source-dirs: src exposed-modules: Lib build-depends: base >= 4.7 && < 5 default-language: Haskell2010 executable helloworld-exe hs-source-dirs: app main-is: Main.hs ghc-options: -threaded -rtsopts -with-rtsopts=-N build-depends: base , helloworld default-language: Haskell2010 test-suite helloworld-test type: exitcode-stdio-1.0 hs-source-dirs: test main-is: Spec.hs build-depends: base , helloworld ghc-options: -threaded -rtsopts -with-rtsopts=-N default-language: Haskell2010 source-repository head type: git location: ... $ Robert Weisser

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proving fmap law for recursive data types
by 鲍凯文 16 Jan '16

16 Jan '16

Hi, Suppose you have something like: data Tree a = Leaf a | Branch (Tree a) (Tree a) instance Functor Tree where fmap f (Leaf a) = Leaf $ f a fmap f (Branch l r) = Branch (fmap f l) (fmap f r) To check that fmap id = id, I can see that the case for Leaf is ok: fmap id (Leaf a) = Leaf $ id a = Leaf a How would you prove it for the second case? Is some sort of inductive proof necessary? I found this: http://ssomayyajula.github.io/posts/2015-11-07-proofs-of-functor-laws-with-…, which goes over an inductive (on the length of the list) proof for the list type, but I'm not sure how to do it for a Tree. Thanks, toz

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Ambiguous module name `Prelude': it was found in multiple packages (trying to install HXQ)
by Stanislaw Findeisen 16 Jan '16

16 Jan '16

Hi! I am trying to install HXQ: https://hackage.haskell.org/package/HXQ . But somehow it doesn't work: $ cabal install hxq Resolving dependencies... Configuring HXQ-0.19.0... Building HXQ-0.19.0... Failed to install HXQ-0.19.0 Build log ( /home/ct/.cabal/logs/HXQ-0.19.0.log ): Configuring HXQ-0.19.0... Building HXQ-0.19.0... Preprocessing library HXQ-0.19.0... src/Text/XML/HXQ/XQuery.hs:1:1: Ambiguous module name `Prelude': it was found in multiple packages: base haskell98-2.0.0.2 cabal: Error: some packages failed to install: HXQ-0.19.0 failed during the building phase. The exception was: ExitFailure 1 Why? Thanks! -- http://people.eisenbits.com/~stf/ http://www.eisenbits.com/ OpenPGP: 9EC2 5620 2355 B1DC 4A8F 8C79 0EC7 C214 E5AE 3B4E

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Implementing instance of '^' operator
by pmcilroy＠gmail.com 16 Jan '16

16 Jan '16

As a ‘hello world’ example for type definitions, I like to define a numeric type that can handle the mod p multiplicative group, where p is prime. This requires: • Implementing interface functions • Defining non-trivial implementations, where constructor must be private, etc. • Invoking an abstract superclass concrete instance method from within the subclass method definition The latter appears not to be possible in Haskell. Is this true? Here’s the basic code, but I punted on x^n. It looks like I’d have to paste in the entire original definition of ‘^’. data Modp a = Modp a a deriving (Eq, Show) mkModp p n | isPrime p = Modp p (n `mod` p) | otherwise = error $ show p ++ " is not a prime" instance Integral a => Num (Modp a) where (Modp q n) + (Modp p m) | p==q = Modp p $ (n+m) `mod` p | otherwise = error $ "unequal moduli" (Modp p n) * (Modp q m) | p==q = Modp p $ (n*m) `mod` p | otherwise = error $ "unequal moduli" negate (Modp p n) = Modp p (p-n) -- can't reuse base because ^ is impl. directly in prelude {- (Modp p x) ^ n | n <= p = (Modp p x) `baseExp` n | n1 == 0 = (Modp p x) | n > p = x ^ n1 where baseExp = ^ in Num n1 = n `mod` p -} instance Integral a => Fractional (Modp a) where recip (Modp p n) = (Modp p n)^(p-2) isPrime p = True -- stub

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