March 2013 - Beginners

hiding members of a data, separate accessors instead
by Emmanuel Touzery 24 Mar '13

24 Mar '13

hi, i was looking at the response type in http-streams: http://hackage.haskell.org/packages/archive/http-streams/0.4.0.0/doc/html/N… I'm used that simply the data type and all its "members" are visible -- the functions to access its contents. But in this case on the HTML documentation the response type looks like it has no members. And the author has defined like "public accessors" later in the code: getStatusCode :: Response -> StatusCode getStatusCode = pStatusCode So I'm not even sure how he achieved that the members are not visible, the data are exported with (..) as is usually done... And the other thing is why would you do that.. You could name the member getStatusCode in the first place, but then it might increase encapsulation to hide it (depending on how he managed to hide the members).. But did you then make it impossible to deconstruct a Response through pattern matching? That sounds like a minus... Although pattern matching on a data with 6 fields is always going to be a pain and decreasing the chances for modifying the data type without breaking compatibility. These "members" are also causing me problems in other situations, for instance I have some cases when I use a data type only a few times and with -Wall the compiler tells me I don't use the accessor; in fact I read that value from the data, but through pattern matching/deconstruction only, not through that particular function. I'm thinking to try to hide the warning as I think my code is correct. Anyway I'm curious on the mechanism used by that library... I've already noticed a few nice tricks in this library, like a small state monad to take optional parameters, much more elegant than any other mechanism i've seen so far to achieve the same effect. Thank you! Emmanuel

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binary parsing problem
by Alexander Polakov 23 Mar '13

23 Mar '13

I'm trying to parse a binary format. One of the components is defined like this: "BITDOUBLE: 1st 2 bits : what it is 00 : A double follows 01 : 1.0 10 : 0.0 11 : not used Doubles are eight byte IEEE standard floating point values." So I wrote this code which looks almost like verbatim translation of the spec. import Data.Binary import Data.Binary.Get import Control.Applicative import qualified Data.Binary.Bits.Get as Bits newtype DWG_BD = DWG_BD Double deriving (Show) instance Binary DWG_BD where put = undefined get = do d <- Bits.runBitGet $ Bits.getWord8 2 >>= \i -> case i of 0 -> decode <$> Bits.getLazyByteString 8 1 -> return 1.0 2 -> return 0.0 _ -> fail "bad DWG_BD" return (DWG_BD d) But then when I try to parse anything starting with 00 bits, I get this: *Main> decodeFile "/tmp/foo" :: IO DWG_BD DWG_BD *** Exception: Data.Binary.Get.runGet at position 2: demandInput: not enough bytes I'm 100% sure there're enough bytes in the file. I recall seeing this problem when my record's fields were not strict, but this is not the case here. I suppose the problem lies in getLazyByteString, but have no idea how to solve it. -- Alexander Polakov | plhk.ru

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Programming with Arrows
by Michael Baker 23 Mar '13

23 Mar '13

I'm reading through http://www.cse.chalmers.se/~rjmh/afp-arrows.pdf I was following it up until this example in the section about flip-flops: class Arrow arr => ArrowLoop arr where loop :: arr (a,c) (b,c) -> arr a b instance ArrowLoop (->) where loop f a = b where (b,c) = f (a,c) Although I've never seen a function declaration like this, but I think I get it. Because f :: (a, c) -> (b, c) then then loop f :: ((a, c) -> (b, c)) -> (a -> b) which is the same as ((a, c) -> (b, c)) -> a -> b. However, I don't see where the c comes from in f (a,c). Is this a mistake or am I missing something? A friend of mine realized that this is just a recursive definition so f (a, c) == f (a, snd $ f (a, snd $ f (a, ...))). I don't really understand this definition. I can see how it compiles, but I don't see how it would ever produce a legitimate value. Do I have to assume that f never evaluates the second element in the pair and just passes it through?

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type vs data, again...
by Emmanuel Touzery 21 Mar '13

21 Mar '13

Hello, I'm again a bit stuck on a simple problem, and I think it's the same problem that I faced some time ago: http://comments.gmane.org/gmane.comp.lang.haskell.beginners/10843 There is obviously something that I didn't get yet about "data" versus "type". In the program in attachment, I want to parse a JSON file to a Value, then if that value is in fact a JSON object, I want to explore its hash member name -> member value. Now we have: data Value = Object Object <--- the case I hope for | Array Array | String Text | Number Number | Bool !Bool | Null deriving (Eq, Show, Typeable, Data) And then: -- | A JSON \"object\" (key\/value map).type Object = Map Text Value So my clearly flawed plan is to get the value, pattern mach it against (Object hash) and then work on the hash. However for the program in attachment, which I would expect to compile, I get this compile error: question.hs:12:62: Couldn't match expected type `Map.Map T.Text Value' with actual type `Object' In the first argument of `parseConfigMap', namely `map' In the second argument of `($)', namely `parseConfigMap map' In the expression: return $ parseConfigMap map That confuses me, because if the error means by Object the data, then I understood nothing. However if the error means by Object the type, then I would expect it to work, since the type exactly means "Map Text Value"... Any hint? And that really doesn't look like a good idea to me to define a type and a data by the same name, but it's apparently the right way to do it... Thank you! Emmanuel

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Beginners Code - Comments on Style
by Heinrich Ody 21 Mar '13

21 Mar '13

Hi, I'm trying to learn Haskell by writing a library for automata functions. So far I finished some functions that I use to calculate the union and intersection of 2 automata for the case of finite words. I wonder if somebody is willing to give comments on the code? For example how I could write a function to be nicer, better to understand etc. Please note that I don't know monads yet. Thanks for your time! Below is my code (I left out some functions to make it shorter) Greetings, Heinrich ------------------ Code import Text.Show.Functions import qualified Data.List as List import Data.Maybe import Prelude hiding (init) -- b is the type for the alphabet. -- Meaning of the parameters are (States, Alphabet, InitStates, Trans.Function, FinalStates) data NFA l = NFA [State] [l] [State] [(State, l, State)] [State] instance Show l => Show (NFA l) where show (NFA states alphabet init delta final) = "States: " ++ show states ++ "\n" ++ "Alphabet: " ++ show alphabet ++ "\n" ++ "Init: " ++ show init ++ "\n" ++ "Delta: " ++ show delta ++ "\n" ++ "Final: " ++ show final data State = I Integer | S [State] deriving Eq instance Show State where show (I i) = show i show (S xs) = show xs -- Advance all states in the set by one step, for the given input letter setTransition :: Eq l => [(State, l, State)] -> [State] -> l -> [State] setTransition delta xs l = [s' | (s, l', s') <- delta, s `List.elem` xs, l' == l] -- naivly test whether a given word is accepted -- for this we forward propagate the current state sets on our input word -- we assume the automaton is complete isAccepted :: Eq l => NFA l -> [l] -> Maybe Bool isAccepted (NFA states alphabet init delta final) word = if (List.nub word) `subset` alphabet then let f xs sigma = setTransition delta xs sigma in Just (((/= []) . (List.intersect final) . (List.foldl f init)) word) else Nothing -- makes an automaton complete s.t. for each pair in (States x Alphabet) a the transition function returns a state. -- For this a sink state is added to States which is the result of all previously unassigned pairs in (States x Alphabet). -- This function keeps DFA deterministc. It adds the sinkstate, but it will be unreachable. makeComplete :: Eq l => NFA l -> NFA l makeComplete (NFA states alphabet init delta final) = NFA (e:states) alphabet init (unassigned `List.union` delta) final where -- e is a new state, whose integer value does not occur in states e = I ((minState states) -1) r = ([e] `List.union` states) `times` alphabet unassigned = [(s,l,e) | (s,l) <- r, (s,l) `List.notElem` (map proj3' delta)] -- checks if 1st parameter is (non-strict) subset of 2nd parameter -- Assumes that there are no duplicates subset :: Eq l => [l] -> [l] -> Bool subset xs ys = (xs List.\\ ys) == [] -- comparision of lists where index of elements is ignored -- Assumes that there are no duplicates seteq :: Eq l => [l] -> [l] -> Bool seteq xs ys = (xs List.\\ ys) == (ys List.\\ xs) -- cartesian product going from states to states stateTimes :: [State] -> [State] -> [State] stateTimes xs ys = [ S [x,y] | x <- xs, y <- ys] -- Normal cartesian product times :: [a] -> [b] -> [(a,b)] times xs ys = [(x,y) | x <- xs, y <- ys] -- Normal cartesian product for three lists times3 :: [a] -> [b] -> [c] -> [(a,b,c)] times3 xs ys zs = [(x,y,z) | x <- xs, y <- ys, z <- zs] -- removes the last element from the tuple proj3' :: (a,b,c) -> (a,b) proj3' (x,y,z) = (x,y) -- adds the 2nd parameter as last element of the input tupple addToTuple :: (a,b) -> c -> (a,b,c) addToTuple (x,y) z = (x,y,z)

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virtualenv vs
by luc taesch 20 Mar '13

20 Mar '13

I run ( again) in some dependencies issues , and considering using virtualenv , or cabal-dev any comment fro users on these tow ? recommendations ? what to do ? what not to do ? -------------- [virtualenv](http://paczesiowa.blogspot.be/2011/11/virtual-haskell-environment.html)

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A good data structure for representing a tic-tac-toe board?
by Costello, Roger L. 19 Mar '13

19 Mar '13

Hi Folks, Currently I am representing a tic-tac-toe board as a string, with 'X' denoting player 1 and 'O' denoting player 2. For example, I represent this 2x2 game board: 'X' | ----------------------- | 'O' with this string: "X O" The nice thing about that representation is that it is each to identify which cells are filled or empty, and it is easy to mark a cell with an 'X' or 'O'. The problem with the representation is that it is difficult to determine when a player has won. Can you recommend a representation that makes it easy to: 1. determine when a player has won 2. identify cells that are filled or empty 3. mark an empty cell /Roger

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compiling a dynamic executable
by Markus Läll 19 Mar '13

19 Mar '13

Hi! I'm trying to compile a dynamic executable in Debian on a x86_64 architecture with ghc 7.4.1-4 and ghc-dynamic installed. To compile the program I started from a clean cabal user database, set the 'shared' and 'executable-dynamic' settings to 'True' in cabal conf and fired away with 'cabal install' in my package's directory. It compiled all the libraries fine-- both the static and dynamic --until it reached compiling my own program, when in stopped with the following error: xxx.hs:1:1: Dynamic linking required, but this is a non-standard build (eg. prof). You need to build the program twice: once the normal way, and then in the desired way using -osuf to set the object file suffix. /usr/bin/ghc returned ExitFailure 1 I don't have profiling enabled (or is it an example of non-standard build?), but I also don't understand the proposed solution of building the program twice. Compiling with "ghc --make -dynamic {-X...}* Main.hs" ends with the same error.. (I also know there is a multi-step manual way to do it, so maybe it could pinpoint the problem -- but I haven't tried it yet) What could be the problem? -- Markus Läll

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haddock parse error
by Christopher Howard 19 Mar '13

19 Mar '13

Hi. I tried to run haddock on a file with this content: code: -------- {-| The 'summation' function provides the same function as summation or sigma notation. The first argument the starting index number. The second argument is the ending index number. The third argument is a function representing the formula for each term. -} summation :: (Eq a, Num a1, Num a) => a -> a -> (a -> a1) -> a1 summation i n f | i == n = f i | otherwise = f i + summation (i + 1) n f ...snip... -------- And I get quote: -------- $ haddock summation.hs summation.hs:7:1: parse error on input `summation' -------- What is wrong with it? (The code runs fine with GHC.) $ haddock --version Haddock version 2.10.0, (c) Simon Marlow 2006 Ported to use the GHC API by David Waern 2006-2008 $ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.4.1 -- frigidcode.com

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Simpler than lifting
by Adrian May 18 Mar '13

18 Mar '13

Hi All, I have this in my code: infixl 6 ~+, ~- f ~+ g = \p -> f p + g p f ~- g = \p -> f p - g p but I feel that the prelude must have already taken care of such things. What's the proper way to do it? Thanks in advance, Adrian.

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