September 2010 - Beginners

removing duplicate tuples (including symmetrical ones)
by Martin Tomko 28 Sep '10

28 Sep '10

Hi all, I apologize for spamming this forum so frequently, but there is noone I can turn to around here... I have a list of (a,a) tuples, and am trying something like nub, but also matching for symmetrical tuples. I implemented it using the template from delete from Prelude. Seems like my typse signature has some troubles (Paarse error in pattern) but I am not sure where the problem is. removeDuplTuples :: [(a,a)] -> [(a,a)] removeDuplTuples [] = [] removeDuplTuples [b] = [b] -- using the syntactic sugar for single element in list removeDuplTuples x:xs = nub (if elem (snd x,fst x) xs then removeDuplTuples xs else [x] ++ removeDuplTuples xs) I assume the problem lies in elem (snd x,fst x) xs but I am not sure how to rewrite it. Thanks for all help, Martin

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Re: Beginners Digest, Vol 27, Issue 57
by Martin Tomko 28 Sep '10

28 Sep '10

Ahaaa, the heureka moment. Stupid me, I should have tried :t null Thanks Brent! On 9/27/2010 6:00 PM, 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://www.haskell.org/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. Re: matching the output of a standard function to my > function definition (non-exhaustive pattern problem) (Brent Yorgey) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Mon, 27 Sep 2010 12:21:08 -0400 > From: Brent Yorgey<byorgey(a)seas.upenn.edu> > Subject: Re: [Haskell-beginners] matching the output of a standard > function to my function definition (non-exhaustive pattern problem) > To: beginners(a)haskell.org > Message-ID:<20100927162108.GA5408(a)seas.upenn.edu> > Content-Type: text/plain; charset=us-ascii > > On Mon, Sep 27, 2010 at 06:15:22PM +0200, Martin Tomko wrote: > >> Dear Daniel >> that worked perfectly, thanks for the suggestion! >> >> One more question: how do you use null to check for empty lists? >> something like [] == null did not work ... just chekcing, I got my >> orignal code to work, thanks again! >> Martin >> > When in doubt, consult the types! > > null :: [a] -> Bool > > That is, null is a function for testing lists to see whether they are > empty. So rather than 'list == null' you write 'null list'. > > -Brent > > > ------------------------------ > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners > > > End of Beginners Digest, Vol 27, Issue 57 > ***************************************** > > -- Martin Tomko Postdoctoral Research Assistant Geographic Information Systems Division Department of Geography University of Zurich - Irchel Winterthurerstr. 190 CH-8057 Zurich, Switzerland email: martin.tomko(a)geo.uzh.ch site: http://www.geo.uzh.ch/~mtomko mob: +41-788 629 558 tel: +41-44-6355256 fax: +41-44-6356848

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matching the output of a standard function to my function definition (non-exhaustive pattern problem)
by Martin Tomko 27 Sep '10

27 Sep '10

Dear all I am sure this is a very beginner problem, but I am not sure how to solve it: I have a myFunct function: myFunct :: Int -> [a] -> a defined as: myFunct _ [] = error "empty list provided as arg" myFunct a [b] | length [x|x<-[b],fid x == a] == 0 = error "no match" | otherwise = head [x|x<-[b],fid x == a] Which works fine (may not be elegant, but works). Now, I am chaining it in a larger function, where at some stage I use a graph object from Data.Graph and check for its edges, resulting in [(Int,Int)] trying to run this fails: myFunct (fst (head (edges graph))) myList *** Exception: FunctStruct.hs:(136,1)-(138,48): Non-exhaustive patterns in function myFunct contrived example, but myList is a list of [a]). The problem arrises from the definition of the type of Edge as (Vertex,Vertex), but type Vertex = Int (http://hackage.haskell.org/packages/archive/containers/0.2.0.1/doc/html/Dat…) so that should be ok, or am I making a mistake somewhere and I should go from vertex to Integer explicitly? Thanks a lot! Martin

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Happy generated parser fails to compile.
by Rohit Garg 27 Sep '10

27 Sep '10

Hi, Following is a simple parser definition I am trying to write for simple numerical expressions. A sequence of numerical expressions actually. Happy reports that there are 3 un-used terminals. AFAIK, they should be let, in and '='. Is that correct? Although, happy generates a .hs file without any other message. GHC gives a type mismatch error. I don't suppose it could be a lexer error, could it? a) Could this be a bug in Happy (very unlikely, IMO) it is generating code that has type mismatches. b) If there is a bug in my parser definition, shouldn't Happy beep on it? Any suggestions to fix it will be highly appreciated. Thanks, =================== { module Main where import Tokens_posn } %name calc %tokentype { Token } %token let { Let _ } in { In _ } ';' { Newstatement _ } int { Int _ $$ } var { Var _ $$ } '=' { Sym _ '=' } '+' { Sym _ '+' } '-' { Sym _ '-' } '*' { Sym _ '*' } '/' { Sym _ '/' } '(' { Sym _ '(' } ')' { Sym _ ')' } %% Exprs :: { [ Expr ] } Exprs : Exprs Expr ';' { $2 : $1 } | Expr ';' { [$1] } Expr : Expr '+' Term { PlusE $1 $3 } | Expr '-' Term { MinusE $1 $3 } | Term { $1 } Term : Term '*' Factor { TimesE $1 $3 } | Term '/' Factor { DivE $1 $3 } | Factor { $1 } Factor : '-' Atom { NegE $2 } | Atom { $1 } Atom : int { IntE $1 } | var { VarE $1 } | '(' Expr ')' { $2 } { data Expr = LetE String Expr Expr | PlusE Expr Expr | MinusE Expr Expr | TimesE Expr Expr | DivE Expr Expr | NegE Expr | IntE Int | VarE String deriving Show flist :: [Int] -> Int -> [Int] flist x a = a : x main:: IO () main = interact (show.runCalc) runCalc :: String -> Expr runCalc = calc . alexScanTokens happyError :: [Token] -> a happyError tks = error ("Parse error at " ++ lcn ++ "\n") where lcn = case tks of [] -> "end of file" tk:_ -> "line " ++ show l ++ ", column " ++ show c where AlexPn _ l c = token_posn tk } -- Rohit Garg http://rpg-314.blogspot.com/

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Empty 'do' constructor
by Sok H. Chang 27 Sep '10

27 Sep '10

I use Haskell Platform 2010.2.0.0 on WinXP, and jEdit. I write my first code, and got error... main = do cs <- readFile "C:\\SPR.txt" putStrLn $ myManupulation cs myManupulation cs = (unlines cs) ! 3 The error message is "Empty 'do' construct" What's this mean? and how can I fix it? Thank you a lot! -- Sok Ha, CHANG Open Mind Clinic/Academy, 1551-1, Sa-Dong, SangRok-Gu, AnSan-City, KyongGi-Do Tel: 031-407-6114 HP: openmind.ac / www.openmind.ac

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Types and the difference between GHCi interactive and a file
by Russ Abbott 26 Sep '10

26 Sep '10

The following runs without a problem. Prelude> let null' xs = xs == [] null' :: (Eq a) => [a] -> Bool Prelude> let main' = do print (null' []) main' :: IO () Prelude> main' True it :: () But if I put essentially the same code in a file and try to load the file I get an error. File: Null null' xs = xs == [] main = do print (null' []) Prelude> :load "Null.hs" [1 of 1] Compiling Main ( Null.hs, interpreted ) Null.hs:3:17: Ambiguous type variable `a' in the constraint: `Eq a' arising from a use of `null'' at Null.hs:3:17-24 Probable fix: add a type signature that fixes these type variable(s) Failed, modules loaded: none. Why is that? I had thought that the ambiguity was referring to the type of [] in the print statement, i.e., that GHC can't figure out the type of []. If I modify the print statement to be print ( null' ([] :: [Int]) ) everything is ok. But if that's the case, why is this not a problem at the interactive level? Here is a related question. In the following what does it mean to say that x is of type [a] where "a" is a type variable? Prelude> let x = [] x :: [a] For example, Prelude> x == (tail [1 :: Int]) True it :: Bool Prelude> x == (tail [1 :: Float]) True it :: Bool Prelude> (tail [1 :: Int]) == (tail [1 :: Float]) <interactive>:1:28: Couldn't match expected type `Int' against inferred type `Float' In the expression: 1 :: Float In the first argument of `tail', namely `[1 :: Float]' In the second argument of `(==)', namely `(tail [1 :: Float])' Can a value like the value of x really be of an incompletely specified type? I couldn't do that in the file. When I tried the following in the file, print (null' ([] :: (Eq a) => a)) I got this error message on loading the file. Null.hs:3:24: Couldn't match expected type `a1' against inferred type `[a]' `a1' is a rigid type variable bound by an expression type signature at Null.hs:3:34 In the first argument of `null'', namely `([] :: (Eq a) => a)' In the first argument of `print', namely `(null' ([] :: (Eq a) => a))' In the expression: print (null' ([] :: (Eq a) => a)) Thanks. -- Russ

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E-mail
by Angela Sheard 24 Sep '10

24 Sep '10

I'd like to post to the mailing list. My e-mail is: angelasheard(a)gmail.com Thanks! :D

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generating arbitrary parameter lists
by Amy de Buitléir 24 Sep '10

24 Sep '10

Whenever I want to use quickCheck to test some function with multiple input parameters, I end up writing boilerplate code that looks like the example below. I have a feeling I'm missing out on some good Haskell tricks. In particular, writing a function like arbXYZ for every combination of parameter types that I need to test is a bit repetitive. Is there a better way? Thank you in advance, Amy ----- code example ----- functionIWantToTest :: Double -> Int -> Bool -> Double functionIWantToTest x y z = 27 -- pretend we're doing something useful arbXYZ :: Gen (Double, Int, Bool) arbXYZ = do x <- choose (0.0, 1.0) y <- choose (1, 10) z <- arbitrary return (x, y, z) conditionIWantToCheck :: (Double, Int, Bool) -> Bool conditionIWantToCheck (x, y, z) = True -- pretend we're checking something prop_i_want_to_check :: Property prop_i_want_to_check = forAll arbXYZ conditionIWantToCheck

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Implementing a toy network proxy
by Patrick LeBoutillier 23 Sep '10

23 Sep '10

Hi , I'm trying to write a toy generic network proxy that will accept connections on a port, and for each connection connect to a remote server and forward the traffic. Here's what I have do far: import Network import System.IO import Control.Exception import Control.Concurrent copy :: Handle -> Handle -> IO () copy a b = undefined redir :: Handle -> Handle -> IO () redir h1 h2 = forkIO (copy h1 h2) >> forkIO (copy h2 h1) >> return () acceptLoop :: Socket -> HostName -> PortID -> IO () acceptLoop sock rhost rport = loop where loop = do (local, host, port) <- accept sock remote <- connectTo rhost rport redir local remote loop >> return () main = do let local_port = 8000 let remote_host = "whatever" let remote_port = 80 server <- listenOn (PortNumber local_port) acceptLoop server remote_host (PortNumber remote_port) Basically I'm stuck at how to implement the copy function. I want it to block until some data is available on a, read it and then write it to b. I think I need to use hGetBufNonBlocking, but I don't know how to get a "Ptr a". I am on the right track with this? Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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Current status of GUM
by Monica Sakuray Pais 23 Sep '10

23 Sep '10

Hello, I would like to know about the current status of GUM: - What is the latest version available? - Does it run on distributed memory multiprocessors - clusters? What type of architectures? thanks. Monica

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