November 2008 - Beginners

Ensuring consistency in typeclass instances?
by Colin Paul Adams 26 Nov '08

26 Nov '08

Hello, I'm just reading through Real World Haskell - chapter 6 (typeclasses). The definition of Eq is shown, and it mentions that you can define either one or both of the two classes. What would happen if I were two define both == and /= for an instance, in such a way that they were not opposites? If I were doing this in Eiffel, the function definitions would have postconditions to state the relationships, and the first execution would trigger a violation, telling me what was wrong. Is there any similar facility in Haskell? -- Colin Adams Preston Lancashire

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Not quite another Haskell tutorial, but ...
by Janis Voigtlaender 26 Nov '08

26 Nov '08

... I submitted my Habilitation thesis last week. The first few chapters of it try to give an introduction to Haskell with emphasis on types and reasoning principles. That might be an interesting read for some, so I made it accessible at http://wwwtcs.inf.tu-dresden.de/~voigt/habil.pdf. And yes, the November HCAR will also be coming. Rather soon now that this cause for delay is out of the way. Ciao, Janis. -- Dr. Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de

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GTK + Reactive
by Levi Stephen 25 Nov '08

25 Nov '08

Hi, I'm trying to set up a simple program combining Reactive and GTK. I have the program below so far, but mainQuit is never called. After looking at a couple of adapters I tried adding the commented out line, but this didn't work for me. I'm sure I've probably missed something, but I'm not sure what. Thanks, Levi main = do initGUI clock <- makeClock (windowDestroy, sink) <- makeEvent clock w <- windowNew w `onDestroy` sink () widgetShowAll w -- forkE (tSync clock) $ quitOnDestroy windowDestroy mainGUI quitOnDestroy :: Event () -> Event (IO ()) quitOnDestroy e = fmap (const mainQuit) e tSync :: Clock TimeT-> ITime -> IO () tSync clock t = sleepPast (cGetTime clock) (exact t)

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Trouble with formatting, Real World Haskell example
by Robert Kosara 24 Nov '08

24 Nov '08

I'm having some trouble getting the InteractWith example from Real World Haskell's Chapter 4 to compile. When I use spaces (like below), it says "Interact.hs:10:30: Empty 'do' construct." When I use tabs (4 spaces per tab), I get "Interact.hs:16:13: parse error on input `='." In either case, I don't see the problem. Any help would be greatly appreciated. I'm also confused about the formatting of the example in the book (page 72). The last line ("myFunction = id") seems to be indented between the where and the next line, why is that? Is that simply a layout problem? To me, it seems it should be on the same level as the where. Below is my program: -- Interact.hs, simple filter in Haskell import System.Environment (getArgs) interactWith function inputFile outputFile = do input <- readFile inputFile writeFile outputFile (function input) main = mainWith myFunction where mainWith function = do args <- getArgs case args of [input, output] -> interactWith function input output _ -> putStrLn "Usage: Interact inputFile outputFile" myFunction = id Regards, Robert

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Efficient sieve of erastothenes, for solving project euler problem #10?
by Malcolm Reynolds 24 Nov '08

24 Nov '08

Hello all, I'm attempting to learn Haskell by going through the project euler problems. Number 10, http://projecteuler.net/index.php?section=problems&id=10 , involves summing prime numbers. It's easy in terms of coding something up that works, but I'm having a lot of trouble getting decent performance. I've learned a reasonable amount of ML at uni but Haskell is the first lazy language I've used.. I think the inefficiency is possibly due to the laziness but I'm not positive. I'd love if someone could show me how to do this in Haskell somewhere near as fast as C - at the moment I have a C version which runs in about a tenth of a second ( http://github.com/malcster/project-euler-solutions--c-/tree/master/10.c ). My haskell attempts are http://github.com/malcster/project-euler-solutions/tree/master/10better.hs (using the sieve) and http://github.com/malcster/project-euler-solutions/tree/master/10.hs (using possibly an even worse method, but seems to be a bit faster). If anyone could point out any neat strictness annotations or anything else I could put in, that would be cool. Cheers, Malcolm

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Type polymorphism with size
by Michael Snoyman 19 Nov '08

19 Nov '08

I am trying to write some code to read flat files from a mainframe system. This includes some character fields. This is a fixed width file, so each field will have a consistent length between records, but there are fields of different length within a record. For example, I might have a "name" field length 20 and an eye color field length 5. I am trying to use the binary library to read in this file. I've written a binary type, MFChar2, for reading in a 2-length character field. It is defined as such (you can safely ignore the ebcdicToAscii piece, it is just doing character conversion): data MFChar2 = MFChar2 [Word8] instance Binary MFChar2 where put = undefined get = do ebcdic <- replicateM 2 getWord8 return $ MFChar2 $ map ebcdicToAscii ebcdic What I would like to do is have some kind of generic "MFChar" data type which could take any character length, but I can't figure out how to do it. Any help would be appreciated. Thanks, Michael

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help
by haskell-beginners＠iorange.fastmail.fm 18 Nov '08

18 Nov '08

On Tue, 18 Nov 2008 17:12:57 -0500 (EST), beginners-request(a)haskell.org said: > 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: Profiling haskell code (Brent Yorgey) > 2. Parsing arithmentic expressions (Glurk) > 3. Re: Parsing arithmentic expressions (Bernie Pope) > 4. RE: Profiling haskell code (Sayali Kulkarni) > 5. Re: Profiling haskell code (Brent Yorgey) > 6. Type polymorphism with size (Michael Snoyman) > 7. Re: Type polymorphism with size (Brent Yorgey) > 8. Re: Type polymorphism with size (Michael Snoyman) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Fri, 14 Nov 2008 15:53:34 -0500 > From: Brent Yorgey <byorgey(a)seas.upenn.edu> > Subject: Re: [Haskell-beginners] Profiling haskell code > To: beginners(a)haskell.org > Message-ID: <20081114205334.GA30261(a)seas.upenn.edu> > Content-Type: text/plain; charset=us-ascii > > > > > quicksort [ ] = [ ] > > > > quicksort (x : xs) = quicksort larger ++ [x ] ++ quicksort smaller > > > > > > where > > > > > > smaller = [a | a <- xs, a <= x ] > > > > > > larger = [b | b <- xs, b > x ] > > > > > > > > > > > > When I compile the code with the following command : > > > > > > > > $ ghc --make Project.hs -prof -auto-all > > > > Then I tested it with the following command : > > > > $ Project +RTS -p > > > > It generates the .hi and the .o file but I cannot get the .prof file. > > > > Please let me know if any of the steps is missing or where could I check > > my profiling info. > > > > Hi Sayali, > > Is the code shown above *everything* in your Project.hs file? You > will also need a main function for it to actually do anything. If > there is more to your Project.hs file that you have not shown, could > you send the complete version? > > Do you get any errors? Does Project produce the output that you expect? > > -Brent > > > ------------------------------ > > Message: 2 > Date: Sun, 16 Nov 2008 00:15:29 +0000 (UTC) > From: Glurk <streborg(a)hotmail.com> > Subject: [Haskell-beginners] Parsing arithmentic expressions > To: beginners(a)haskell.org > Message-ID: <loom.20081115T235851-940(a)post.gmane.org> > Content-Type: text/plain; charset=us-ascii > > Hi, > > I'm just trying to learn how to use Parsec and am experimenting with > parsing > arithmetic expressions. > > This article gives a good example -> > http://www.haskell.org/haskellwiki/Parsing_expressions_and_statements > > However, like most other examples I could find, the grammar for the > expression > doesn't take operator precedence into account, and allows for expressions > of > any size by defining expr recursively, eg :- > > expr ::= var | const | ( expr ) | unop expr | expr duop expr > > So, you can keep extending the expression by adding another operator and > expression. > > The data to hold the expression is then very easily derived :- > > data Expr = Var String | Con Bool | Uno Unop Expr | Duo Duop Expr Expr > > The grammar I want to parse is slightly different in that it allows for > operator precendence. Part of the grammar is something like :- > > expression = SimpleExpression {relation SimpleExpression}. > SimpleExpression = ["+"|"-"] term {AddOperator term}. > > So, instead of recursively defining expression, it is made up of > multiples > occurrences of SimpleExpression joined together with Relation operators. > > Where I am confused is how I should best represent this stucture in my > data. > Should I have something like :- > > data Expr = Expr SimpleExpr [(RelOp, SimpleExpression)] > > ie, an initial SimpleExpr, followed by a list of operator and > SimpleExpression > pairs. > > I haven't seen any example similar to this, so I was wondering if I'm > going > down the wrong track ? > > Perhaps another alternative is to modify the grammar somehow ? > > I guess, the question is, in general how do you handle such repeated > elements > as definied in an EBNF grammar, in structuring your data ? > > Any advice appreciated ! > > Thanks :) > > > > ------------------------------ > > Message: 3 > Date: Mon, 17 Nov 2008 16:35:02 +1100 > From: Bernie Pope <bjpop(a)csse.unimelb.edu.au> > Subject: Re: [Haskell-beginners] Parsing arithmentic expressions > To: Glurk <streborg(a)hotmail.com> > Cc: beginners(a)haskell.org > Message-ID: <57383C16-16DD-4032-9CBC-5D6CC27A6E6E(a)csse.unimelb.edu.au> > Content-Type: text/plain; charset=US-ASCII; format=flowed; delsp=yes > > Hi, > > Have you seen the buildExpressionParser combinator in Parsec? > > http://legacy.cs.uu.nl/daan/download/parsec/parsec.html#buildExpressionPars… > > It allows you to specify precedence and associativity for operator > parsers declaratively, and it generally saves you from lots of > refactoring in the grammar. > > You could probably stick with the straightforward data representation > of expressions. > > Cheers, > Bernie. > > On 16/11/2008, at 11:15 AM, Glurk wrote: > > > Hi, > > > > I'm just trying to learn how to use Parsec and am experimenting with > > parsing > > arithmetic expressions. > > > > This article gives a good example -> > > http://www.haskell.org/haskellwiki/Parsing_expressions_and_statements > > > > However, like most other examples I could find, the grammar for the > > expression > > doesn't take operator precedence into account, and allows for > > expressions of > > any size by defining expr recursively, eg :- > > > > expr ::= var | const | ( expr ) | unop expr | expr duop expr > > > > So, you can keep extending the expression by adding another operator > > and > > expression. > > > > The data to hold the expression is then very easily derived :- > > > > data Expr = Var String | Con Bool | Uno Unop Expr | Duo Duop Expr Expr > > > > The grammar I want to parse is slightly different in that it allows > > for > > operator precendence. Part of the grammar is something like :- > > > > expression = SimpleExpression {relation SimpleExpression}. > > SimpleExpression = ["+"|"-"] term {AddOperator term}. > > > > So, instead of recursively defining expression, it is made up of > > multiples > > occurrences of SimpleExpression joined together with Relation > > operators. > > > > Where I am confused is how I should best represent this stucture in > > my data. > > Should I have something like :- > > > > data Expr = Expr SimpleExpr [(RelOp, SimpleExpression)] > > > > ie, an initial SimpleExpr, followed by a list of operator and > > SimpleExpression > > pairs. > > > > I haven't seen any example similar to this, so I was wondering if > > I'm going > > down the wrong track ? > > > > Perhaps another alternative is to modify the grammar somehow ? > > > > I guess, the question is, in general how do you handle such repeated > > elements > > as definied in an EBNF grammar, in structuring your data ? > > > > Any advice appreciated ! > > > > Thanks :) > > > > _______________________________________________ > > Beginners mailing list > > Beginners(a)haskell.org > > http://www.haskell.org/mailman/listinfo/beginners > > > > ------------------------------ > > Message: 4 > Date: Mon, 17 Nov 2008 09:35:26 +0530 > From: "Sayali Kulkarni" <Sayali.Kulkarni(a)kpitcummins.com> > Subject: RE: [Haskell-beginners] Profiling haskell code > To: "Brent Yorgey" <byorgey(a)seas.upenn.edu>, <beginners(a)haskell.org> > Message-ID: <82C3BC9106BCE149B63464D79D0A22FD0761A60E(a)sohm.kpit.com> > Content-Type: text/plain; charset="us-ascii" > > Hello Brent, > > I just have written a quick sort program. > There is nothing more in the code than that I have shown. > > What is it about the main function? > What do I need to do in the main function? > > I do not get any errors. > And I get the expected output. The only thing that I am stuck at is that > I do not get the ".prof" file which will give me the profile details of > the code. > > Also it would be great if you could through a light on whether there is > any other method to profile a code in Haskell? > > Regards, > Sayali. > > -----Original Message----- > From: beginners-bounces(a)haskell.org > [mailto:beginners-bounces@haskell.org] On Behalf Of Brent Yorgey > Sent: Saturday, November 15, 2008 2:24 AM > To: beginners(a)haskell.org > Subject: Re: [Haskell-beginners] Profiling haskell code > > > > > quicksort [ ] = [ ] > > > > quicksort (x : xs) = quicksort larger ++ [x ] ++ quicksort smaller > > > > > > where > > > > > > smaller = [a | a <- xs, a <= x ] > > > > > > larger = [b | b <- xs, b > x ] > > > > > > > > > > > > When I compile the code with the following command : > > > > > > > > $ ghc --make Project.hs -prof -auto-all > > > > Then I tested it with the following command : > > > > $ Project +RTS -p > > > > It generates the .hi and the .o file but I cannot get the .prof file. > > > > Please let me know if any of the steps is missing or where could I > check > > my profiling info. > > > > Hi Sayali, > > Is the code shown above *everything* in your Project.hs file? You > will also need a main function for it to actually do anything. If > there is more to your Project.hs file that you have not shown, could > you send the complete version? > > Do you get any errors? Does Project produce the output that you expect? > > -Brent > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners > > > ------------------------------ > > Message: 5 > Date: Mon, 17 Nov 2008 09:07:57 -0500 > From: Brent Yorgey <byorgey(a)seas.upenn.edu> > Subject: Re: [Haskell-beginners] Profiling haskell code > To: beginners(a)haskell.org > Message-ID: <20081117140757.GA13235(a)seas.upenn.edu> > Content-Type: text/plain; charset=us-ascii > > On Mon, Nov 17, 2008 at 09:35:26AM +0530, Sayali Kulkarni wrote: > > Hello Brent, > > > > I just have written a quick sort program. > > There is nothing more in the code than that I have shown. > > > > What is it about the main function? > > What do I need to do in the main function? > > > > I do not get any errors. > > And I get the expected output. The only thing that I am stuck at is that > > I do not get the ".prof" file which will give me the profile details of > > the code. > > > > Also it would be great if you could through a light on whether there is > > any other method to profile a code in Haskell? > > > > Regards, > > Sayali. > > Hi Sayali, > > Just writing a quicksort function by itself is fine if you want to > test it interactively in ghci. But if you want to profile it you will > have to make an executable, which means you will need a 'main' > function which says what to do when the program is run. Your main > function might look something like this: > > main = do print "Sorting..." > print (length (quicksort (reverse [1..1000000]))) > print "Done!" > > Of course, sorting a list in reverse order might not be a very > representative task; you might also want to look into the > System.Random module to generate a list of a million random elements > and sort that. > > -Brent > > > > > -----Original Message----- > > From: beginners-bounces(a)haskell.org > > [mailto:beginners-bounces@haskell.org] On Behalf Of Brent Yorgey > > Sent: Saturday, November 15, 2008 2:24 AM > > To: beginners(a)haskell.org > > Subject: Re: [Haskell-beginners] Profiling haskell code > > > > > > > > quicksort [ ] = [ ] > > > > > > quicksort (x : xs) = quicksort larger ++ [x ] ++ quicksort smaller > > > > > > > > > where > > > > > > > > > smaller = [a | a <- xs, a <= x ] > > > > > > > > > larger = [b | b <- xs, b > x ] > > > > > > > > > > > > > > > > > > When I compile the code with the following command : > > > > > > > > > > > > $ ghc --make Project.hs -prof -auto-all > > > > > > Then I tested it with the following command : > > > > > > $ Project +RTS -p > > > > > > It generates the .hi and the .o file but I cannot get the .prof file. > > > > > > Please let me know if any of the steps is missing or where could I > > check > > > my profiling info. > > > > > > > Hi Sayali, > > > > Is the code shown above *everything* in your Project.hs file? You > > will also need a main function for it to actually do anything. If > > there is more to your Project.hs file that you have not shown, could > > you send the complete version? > > > > Do you get any errors? Does Project produce the output that you expect? > > > > -Brent > > _______________________________________________ > > Beginners mailing list > > Beginners(a)haskell.org > > http://www.haskell.org/mailman/listinfo/beginners > > > > > ------------------------------ > > Message: 6 > Date: Tue, 18 Nov 2008 10:02:20 -0800 > From: "Michael Snoyman" <michael(a)snoyman.com> > Subject: [Haskell-beginners] Type polymorphism with size > To: beginners(a)haskell.org > Message-ID: > <29bf512f0811181002k7ea715f3o8d797aae6a4b3855(a)mail.gmail.com> > Content-Type: text/plain; charset="utf-8" > > I am trying to write some code to read flat files from a mainframe > system. > This includes some character fields. This is a fixed width file, so each > field will have a consistent length between records, but there are fields > of > different length within a record. For example, I might have a "name" > field > length 20 and an eye color field length 5. > > I am trying to use the binary library to read in this file. I've written > a > binary type, MFChar2, for reading in a 2-length character field. It is > defined as such (you can safely ignore the ebcdicToAscii piece, it is > just > doing character conversion): > > data MFChar2 = MFChar2 [Word8] > instance Binary MFChar2 where > put = undefined > get = do ebcdic <- replicateM 2 getWord8 > return $ MFChar2 $ map ebcdicToAscii ebcdic > > What I would like to do is have some kind of generic "MFChar" data type > which could take any character length, but I can't figure out how to do > it. > Any help would be appreciated. > > Thanks, > Michael >

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Profiling haskell code
by Sayali Kulkarni 17 Nov '08

17 Nov '08

Hello, I am new to Haskell. I have written a small quicksort function in Haskell. I want to profile it for time. I have GHC 6.10.1 version and I have also installed the extra-libs that it has on the Haskell home page. I followed the following steps for installing the extra-libs: $ cd /to/the/library/source $ runghc Setup.hs configure --enable-library-profiling $ runghc Setup.hs build $ runghc Setup.hs install Following is the quicksort code that I'm using: quicksort [ ] = [ ] quicksort (x : xs) = quicksort larger ++ [x ] ++ quicksort smaller where smaller = [a | a <- xs, a <= x ] larger = [b | b <- xs, b > x ] When I compile the code with the following command : $ ghc --make Project.hs -prof -auto-all Then I tested it with the following command : $ Project +RTS -p It generates the .hi and the .o file but I cannot get the .prof file. Please let me know if any of the steps is missing or where could I check my profiling info. Regards, Sayali.

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Parsing arithmentic expressions
by Glurk 17 Nov '08

17 Nov '08

Hi, I'm just trying to learn how to use Parsec and am experimenting with parsing arithmetic expressions. This article gives a good example -> http://www.haskell.org/haskellwiki/Parsing_expressions_and_statements However, like most other examples I could find, the grammar for the expression doesn't take operator precedence into account, and allows for expressions of any size by defining expr recursively, eg :- expr ::= var | const | ( expr ) | unop expr | expr duop expr So, you can keep extending the expression by adding another operator and expression. The data to hold the expression is then very easily derived :- data Expr = Var String | Con Bool | Uno Unop Expr | Duo Duop Expr Expr The grammar I want to parse is slightly different in that it allows for operator precendence. Part of the grammar is something like :- expression = SimpleExpression {relation SimpleExpression}. SimpleExpression = ["+"|"-"] term {AddOperator term}. So, instead of recursively defining expression, it is made up of multiples occurrences of SimpleExpression joined together with Relation operators. Where I am confused is how I should best represent this stucture in my data. Should I have something like :- data Expr = Expr SimpleExpr [(RelOp, SimpleExpression)] ie, an initial SimpleExpr, followed by a list of operator and SimpleExpression pairs. I haven't seen any example similar to this, so I was wondering if I'm going down the wrong track ? Perhaps another alternative is to modify the grammar somehow ? I guess, the question is, in general how do you handle such repeated elements as definied in an EBNF grammar, in structuring your data ? Any advice appreciated ! Thanks :)

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Unicode in GHCi
by Christos Chryssochoidis 13 Nov '08

13 Nov '08

Hello, I'd like to type in GHCi some non-latin characters, but when I try it, no characters are inserted at all. Latin characters though are typed in fine, and in any other shell except GHCi I try non-Latin characters get typed without problem. I mean I'm not able to type in GHCi an expression like 'ᾶ' == 'ᾶ' (can't type in the ᾶ) Is this a known GHCi restriction on the character set it accepts, or something is wrong with my setup of GHCi/terminal? I'm using GHC 6.10.1 in Mac OS X Terminal. Thank you very much, Christos

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