July 2015 - Beginners

Config data
by Mike Houghton 19 Sep '15

19 Sep '15

Hi, I’ve haskell files parser.hs and email.hs email.hs is a module imported into parser and parser has the main. email has various passwords and server names needed to connect and send email. I want to have those details in a config file and read it in at start - using say config module. I’m ok with this part but the practicality of doing it eludes me... It seems to me that email module can either get the config details itself or be told them by parser. If email wants to get them how would it do this? It does not have a main that gets run so can’t load a config file.(can it ??) However parser can load the config in its main and then tell mail what the values are but how would mail save them? Of course I could chanage the signatures of the email send/receive functions in mail to take the connection details but that seems wrong. Thanks Mike

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Haskell equivalent to Clojure's partition fn
by Timothy Washington 05 Aug '15

05 Aug '15

Is there a Haskell equivalent to Clojure's partition <http://clojuredocs.org/clojure.core/partition> function? I'm particularly interested in the stepwise feature, where we can select a certain `n` size of the list, then step `m` and take again. What's returned is a list of lists. Each list being a sublist incremented by `m`. I didn't see anything in the below packages. And I wanted to check before I go about writing my own. - Prelude <https://hackage.haskell.org/package/base-4.8.0.0/docs/Prelude.html#g:13> - Data.List <https://hackage.haskell.org/package/base-4.8.1.0/docs/Data-List.html> - Data.Array <http://hackage.haskell.org/package/array-0.5.0.0/docs/Data-Array.html> - Data.Vector <http://hackage.haskell.org/package/vector-0.11.0.0/docs/Data-Vector.html> Thanks Tim

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An unsigned int class.
by derek riemer 31 Jul '15

31 Jul '15

Is there anything in haskell that represents an unsigned Int class? namely, something bounded below at 0? If not, is it possible to define an Int in this way to add the bounded constraint (I can't immagine trying to define Int like this, there has got to be a better way that I haven't quite understood in my study of haskell). data UnsignedInt = 0 | 1 | 2 | (...) for a long time doing that ... deriving (Show, Eq, Ord, Bounded, Read) Thanks, Derek

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How to define ord manually.
by derek riemer 30 Jul '15

30 Jul '15

Hi, I was playing around with haskell, and decided to implement a RockPaperScissors type. data RPS= Rock | Paper | Scissors Then I manually derived show for fun. instance Show RPS where show Rock = "rock" show Paper = "paper" show Scissors = "scissors" But, I have a problem when defining ord. How do I look in the sourcecode for ord? In ghci I did prelude> :i Ord <snip> Ord (a, b, c, d, e, f, g, h, i, j) -- Defined in `GHC.Classes' instance (Ord a, Ord b, Ord c, Ord d, Ord e, Ord f, Ord g, Ord h, Ord i) => Ord (a, b, c, d, e, f, g, h, i) -- Defined in `GHC.Classes' instance (Ord a, Ord b, Ord c, Ord d, Ord e, Ord f, Ord g, Ord h) => Ord (a, b, c, d, e, f, g, h) -- Defined in `GHC.Classes' instance (Ord a, Ord b, Ord c, Ord d, Ord e, Ord f, Ord g) => Ord (a, b, c, d, e, f, g) -- Defined in `GHC.Classes' instance (Ord a, Ord b, Ord c, Ord d, Ord e, Ord f) => Ord (a, b, c, d, e, f) -- Defined in `GHC.Classes' instance (Ord a, Ord b, Ord c, Ord d, Ord e) => Ord (a, b, c, d, e) -- Defined in `GHC.Classes' instance (Ord a, Ord b, Ord c, Ord d) => Ord (a, b, c, d) -- Defined in `GHC.Classes' instance (Ord a, Ord b, Ord c) => Ord (a, b, c) -- Defined in `GHC.Classes' instance (Ord a, Ord b) => Ord (a, b) -- Defined in `GHC.Classes' instance Ord () -- Defined in `GHC.Classes' Prelude> Where is ghc.classes? I looked under the ghc directory and didn't find a classes.hs file. I tried defining it as such. instance Ord RPS where ord Scissors `compare` Rock = LT ord Paper `compare` Scissors = LT ord Rock `compare` Paper = LT But this fails with verry little errors, and I really don't know how I define an instance of Ord for this. I want rock to be less than paper, but greater than scissors, scissors to be less than rock but greater than paper, and paper to be greater than rock but less than scissors. Kind of geeky, but it's one way of learning. Also, What is the best way to write useful programs, One example I tried is to make a simple guess what I am guessing game, but I couldn't make heads or tails of the IO typeclasses, or the System.Random module. How can I possibly get random numbers and do things on them, and does this mean main must be imperative with a do statement? Thanks, Derek

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Traversing generic trees
by Ali Baharev 24 Jul '15

24 Jul '15

Dear All, I have started learning Haskell recently, and I try to reimplement interesting algorithms that are challenging to do in other languages. I would like to ask you for your kind help with a problem that I am stuck with. A generic tree is given in the form of nested sequences (nested lists or nested tuples) and I would like to traverse it in (1) depth-first order, (2) lazily and (3) without copying the data into some other datastructure first. I test the algorithm by pretty printing the tree. My goal is learning, so Data.Tree is not an option (although I did look into its implementation). Here is my first attempt: data Tree a = Leaf a | Node [Tree a] tree = Node [Leaf 'a', Node [Leaf 'b', Leaf 'c', Node []], Leaf 'd'] toString :: (Show a) => Tree a -> String toString (Leaf leaf) = " " ++ show leaf ++ " " toString (Node nodes) = " [ " ++ concatMap toString nodes ++ " ] " main = print $ toString tree It (more or less) satisfies the above requirements. The pretty printing is not very pretty but the part that I am really not happy with is the lot of code duplication in giving the tree. I would like to enter it as: tree = ['a', ['b', 'c', []], 'd'] which of course won't work. Is there a (not too obscure) way to help the compiler so that it understands that 'a' means Leaf 'a', etc? I have also tried tree = ('a', ('b', 'c', ()), 'd') but then I do not know how to destructure the nested tuples. Any help is greatly appreciated. Ali

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(->) reader/environment
by Martin Vlk 23 Jul '15

23 Jul '15

Hi, this one is a bit of a mystery to me. I am working on solutions to cis194/NICTA Haskell exercises and among others I am asked to work with (->). I understand it refers to the reader/environment design patterns, but I am stumped as for what exactly it is. It doesn't seem to be an ordinary type, I know it is used as type constructor in type declarations, so that suggests it's a function, but somehow special. Looking in Hoogle it seems to be Haskell keyword - e.g. not an ordinary function. So what is it - type constructor, a keyword, how is it related to the reader/environment pattern? >From GHCI's :info (->) I get: data (->) a b -- Defined in ‘ghc-prim-0.4.0.0:GHC.Prim’ instance P.Monad ((->) r) -- Defined in ‘GHC.Base’ instance P.Functor ((->) r) -- Defined in ‘GHC.Base’ instance P.Applicative ((->) a) -- Defined in ‘GHC.Base’ instance P.Monoid b => P.Monoid (a -> b) -- Defined in ‘GHC.Base’ Cheers Martin

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Re: [Haskell-beginners] Traversing generic trees
by Ali Baharev 22 Jul '15

22 Jul '15

Dear Elliot, The question is whether there is a (not too obscure) way to eliminate the code duplication, or how the code duplication can be avoided using nested tuples instead. Please try to answer the question. Ali On Wed, Jul 22, 2015 at 11:51 PM, elliot <elliot(a)citrusbytes.net> wrote: > In general you don't produce large trees (or any data structure) by hand. It's slightly frustrating to test of course (as you've discovered) but for the most part you'll only need to produce small structures. > > -- > e: elliot(a)citrusbytes.net > p: +44 751 070 5158 > ---- Original Message ---- > From: "Ali Baharev" > To: beginners(a)haskell.org > Sent: Wed, Jul 22, 2015, 10:24 PM > Subject: [Haskell-beginners] Traversing generic trees > Dear All, > > I have started learning Haskell recently, and I try to reimplement > interesting algorithms that are challenging to do in other languages. > I would like to ask you for your kind help with a problem that I am > stuck with. > > A generic tree is given in the form of nested sequences (nested lists > or nested tuples) and I would like to traverse it in (1) depth-first > order, (2) lazily and (3) without copying the data into some other > datastructure first. I test the algorithm by pretty printing the tree. > My goal is learning, so Data.Tree is not an option (although I did > look into its implementation). > > Here is my first attempt: > > data Tree a = Leaf a | Node [Tree a] > > tree = Node [Leaf 'a', Node [Leaf 'b', Leaf 'c', Node []], Leaf 'd'] > > toString :: (Show a) => Tree a -> String > toString (Leaf leaf) = " " ++ show leaf ++ " " > toString (Node nodes) = " [ " ++ concatMap toString nodes ++ " ] " > > main = print $ toString tree > > It (more or less) satisfies the above requirements. The pretty > printing is not very pretty but the part that I am really not happy > with is the lot of code duplication in giving the tree. I would like > to enter it as: > > tree = ['a', ['b', 'c', []], 'd'] > > which of course won't work. > > Is there a (not too obscure) way to help the compiler so that it > understands that 'a' means Leaf 'a', etc? > > I have also tried > > tree = ('a', ('b', 'c', ()), 'd') > > but then I do not know how to destructure the nested tuples. > > Any help is greatly appreciated. > > Ali > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org (mailto:Beginners@haskell.org) > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners (http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners)

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seq vs deepseq
by Ryan Warner 22 Jul '15

22 Jul '15

I'm trying to grok Haskell's laziness. In this example, why does the seq not force youTyped to wait for s to be fully evaluated, before returning a result to putStrLn? -------------------- import System.IO (hSetBuffering,stdin,BufferMode(..)) youTyped :: String -> String youTyped s = s `seq` "you typed: " ++ s main = do hSetBuffering stdin NoBuffering s <- getContents let l = map youTyped $ lines $ takeWhile ( /= '\04' ) s mapM_ putStrLn ("Start Typing (Ctrl-D to exit):":l) -------------------- The output looks like: -------------------- $ runhaskell seqLazy.hs Start Typing (Ctrl-D to exit): Hyou typed: Heelllloo wwoorrlldd fyou typed: faaiill ^D -------------------- Changing seq, to deepseq does the trick though. -------------------- import System.IO (hSetBuffering,stdin,BufferMode(..)) import Control.DeepSeq youTyped :: String -> String youTyped s = s `deepseq` "you typed: " ++ s main = do hSetBuffering stdin NoBuffering s <- getContents let l = map youTyped $ lines $ takeWhile ( /= '\04' ) s mapM_ putStrLn ("Start Typing (Ctrl-D to exit):":l) -------------------- Output: $ runhaskell deepSeqLazy.hs Start Typing (Ctrl-D to exit): Hello world you typed: Hello world success you typed: success ^D When does it make sense to use seq then?

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Re: [Haskell-beginners] Cabal dependencies are per target or global?
by Dimitri DeFigueiredo 19 Jul '15

19 Jul '15

I guess my example was incomplete. I can make GHC run. Here's a modified version where I do have the ghc-options line. I can build an executable for the second target (myPersonalMain) by commenting the "build-depends: section" for the first target (agent). My .cabal file with (commented lines) is: ------------------------------------------------ name: agent version: 0.1.0.0 build-type: Simple cabal-version: >=1.20 ------------------------------------------------ executable agent main-is: Main.hs hs-source-dirs: ./src -- build-depends: base >=4.6 && <4.7 -- , unordered-containers >= 0.2.3.0 -- , unix >= 2.6.0.1 -- , process >= 1.1.0.2 -- , stm >= 2.4.2 -- Base language which the package is written in. default-language: Haskell2010 ---------------------------------------------- executable myPersonalMain main-is: Mpm.hs hs-source-dirs: ./src ghc-options: -main-is Mpm build-depends: base >=4.4 default-language: Haskell2010 ---------------------------------------------- The contents of Mpm.hs are: module Mpm where main = putStrLn "Hi!" But if I remove the comments in the .cabal file above, I get this: Dis-machine:cabal-tests dimitri$ cabal build myPersonalMain ./agent.cabal has been changed. Re-configuring with most recently used options. If this fails, please run configure manually. Resolving dependencies... Configuring agent-0.1.0.0... cabal: At least the following dependencies are missing: process >=1.1.0.2, stm >=2.4.2, unix >=2.6.0.1, unordered-containers >=0.2.3.0 Is this a bug? Or am I missing something? Cheers, Dimitri On 17/07/15 15:10, Imants Cekusins wrote: > this could shed some light: > > http://stackoverflow.com/questions/14238729/producing-multiple-executables-… > > answer #2 was marked as answered > > ghc-options: -O2 -threaded -with-rtsopts=-N -main-is FirstExecutable

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Re: [Haskell-beginners] basic Functor, Applicative and Monad instances
by amindfv＠gmail.com 17 Jul '15

17 Jul '15

Right, you'll want to say eg fmap :: (a -> b) -> Val a -> Val b Tom El Jul 17, 2015, a las 12:23, Imants Cekusins <imantc(a)gmail.com> escribió: >> InstanceSigs > > Thank you Tom. > > this sig does not work though: > fmap ::(a -> b) -> f a -> f b

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