- Beginners - Haskell.org

exercise 3.10 in YAHT
by George Huber 09 Jun '09

09 Jun '09

I'm currently working through "Yet Another Haskell Tutorial" by H. Daume and have a question on how to proceed with his exercise 3.10. The exercise question is: Write a program that will repeatedly ask the user for numbers until he types in zero, at which point it will tell him the sum of all the numbers, the product of all the numbers, and, for each number, its factorial. The hint that is given is to write an IO action that reads a number and either returns an empty list (if the number is zero) or recurses itself making a list from the number and the result of the recursive call. Daume presents and example of such a function: askForWords = do putStrLn "Please enter a word:" word <- getLine if word == "" then return [] else do rest <- askForWords return (word : rest) so, based on his example I created the function: getNums = do putStrLn "Enter a number (zero to stop):"; strNum <- getLine let num = read strNum if num == 0 then return [] else do rest <- getNums return (num:rest) loading the file that contains the above function into GHCi (version 6.8.3) gives the following error "The last statement in a 'do' construct must be an expression" being a beginner, I'm at a loss at the cause of the error and how to fix it -- any suggestions? thanks, george

4 3

type classes and multiple implementations
by Michael Hendricks 06 Jun '09

06 Jun '09

I'm writing a tool for which there will be multiple storage formats. The user can choose his favorite storage mechanism through a configuration file. Possible storage mechanisms might include in-memory storage or on-disk storage. To represent this abstraction, I created a Storage type class and two simple instances: class Storage a where open :: String -> IO a data Disk = Disk instance Storage Disk where open file = do putStrLn $ "opening disk: " ++ file return Disk data Memory = Memory instance Storage Memory where open _ = do putStrLn "opening memory" return Memory I can load this code into ghci (version 6.10.1, if that matters) and it behaves as I expected it to: *Main> open "foo" :: IO Disk opening disk: foo *Main> open "foo" :: IO Memory opening memory Eventually a configuration facility will provide a string indicating which storage mechanism the user prefers. This will return a string like "memory" or "disk". Based on that string, I want to return a value whose type belongs to the Storage type class. I thought this would do the job: by_type :: Storage a => String -> String -> IO a by_type "disk" x = open x :: IO Disk by_type "memory" x = open x :: IO Memory by_type _ t = error $ "no such storage type " ++ t but I get the following compile errors: Couldn't match expected type `Disk' against inferred type `Memory' Expected type: IO a Inferred type: IO Memory In the expression: open x :: IO Memory In the definition of `by_type': by_type "memory" x = open x :: IO Memory It seems as though the type variable `a' is binding to Disk which prevents it from later binding to Memory. That makes sense but I don't see a way forward from here. How can I accomplish multiple implementations of the same interface? Are type classes the right way? Thank you for any help. -- Michael

3 2

Clearing Parsec error messages
by Giuliano Vilela 06 Jun '09

06 Jun '09

Hi all, In a Parsec project I used the *fail* parser, wanting to show a message to the user and halt the parsing process. That's okay, but the error message showed included some other "unexpected" and "expecting" messages that did not seem related to the fail. My guess is that Parsec keeps these messages in an internal state, to use them whenever needed. My question is, how can I clear those error messages and only show the string I pass to fail? -- []'s Giuliano Vilela.

2 3

HXT runtime error
by aditya siram 03 Jun '09

03 Jun '09

Hi all, I am trying to use HXT to parse an XML file. However whenever I try and run 'xread', GHCI crashes. The code and output are below. The following code queries isbndb.com ( a book ISBN database ) which responds with XML describing the book. I am using 'xread' from HXT to parse the response into an XMLTree. respHTML'' isbn = do rsp <- simpleHTTP (getRequest $ " http://isbndb.com/api/books.xml?results=details&access_key=" ++ key ++ "&index1=isbn&value1=" ++ isbn :: Request_String) case rsp of Left x -> putStrLn $ "Error " ++ (show x) Right r -> putStrLn $ show $ xread $ rspBody r When I try this respHTML'' in GHCI usin the isbn for "Real World Haskell" , this is what I get: *Main> respHTML'' rwhis Loading package syb ... linking ... done. Loading package base-3.0.3.1 ... linking ... done. Loading package array-0.2.0.0 ... linking ... done. Loading package containers-0.2.0.1 ... linking ... done. Loading package bytestring-0.9.1.4 ... linking ... done. Loading package old-locale-1.0.0.1 ... linking ... done. Loading package old-time-1.0.0.2 ... linking ... done. Loading package unix-2.3.2.0 ... linking ... done. Loading package filepath-1.1.0.2 ... linking ... done. Loading package directory-1.0.0.3 ... linking ... done. Loading package process-1.0.1.1 ... linking ... done. Loading package pretty-1.0.1.0 ... linking ... done. Loading package random-1.0.0.1 ... linking ... done. Loading package haskell98 ... linking ... done. Loading package parsec-2.1.0.1 ... linking ... done. Loading package network-2.2.1 ... linking ... done. Loading package mtl-1.1.0.2 ... linking ... done. Loading package parallel-1.1.0.1 ... linking ... done. Loading package HTTP-4000.0.6 ... linking ... done. Loading package HaXml-1.13.3 ... linking ... done. Loading package MaybeT-0.1.2 ... linking ... done. Loading package HUnit-1.2.0.3 ... linking ... done. Loading package curl-1.3.5 ... linking ... done. Loading package network-2.2.1.2 ... GHCi runtime linker: fatal error: I found a duplicate definition for symbol my_inet_ntoa whilst processing object file /home/deech/.cabal/lib/network-2.2.1.2/ghc-6.10.3/HSnetwork-2.2.1.2.o This could be caused by: * Loading two different object files which export the same symbol * Specifying the same object file twice on the GHCi command line * An incorrect `package.conf' entry, causing some object to be loaded twice. GHCi cannot safely continue in this situation. Exiting now. Sorry. It seems to want to load the 'network' package twice. Is this causing the trouble? thanks , deech

1 1

HaXML tutorial
by aditya siram 03 Jun '09

03 Jun '09

Hi all, Are there any current HaXml 1.13.2 tutorials/examples out there? I found the one from their webpage ( http://www.krowland.net/tutorials/haxml_tutorial.html) to be quite confusing. thanks .. -deech

1 0

Type Class question
by Malik H 02 Jun '09

02 Jun '09

I am reading article "Fun with type functions" and I have a few questions regarding examples presented in it. Here is the code class Add a b where type SumTy a b add :: a -> b -> SumTy a b instance (Add Integer a) => Add Integer [a] where type SumTy Integer [a] = [SumTy Integer a] add x y = map (add x) y Here are my questions: 1. Is type "SumTy Integer [a]" visible outside 'Add' instance? 2. If yes, how would you create and instance of that type without using 'add' function? 3. Will function 'add x y' create the following structure [SumTy x y1, SumTy x y2, etc...]? If not, what is 'add x y' creates? 4. How to print the result of 'add x y'? Also, I understand the following "instance (Ord a) => Monad a where ...." as type of the 'a' must be an extension of type Ord. How shall I understand code below? "instance (Add Integer a) => Add Integer [a]" Thanks a lot.

2 1

HaL4: Haskell-Meeting in Germany, 12th June 2009
by Janis Voigtlaender 02 Jun '09

02 Jun '09

Hi all, If you are anyway near Halle/Saale in June, be sure not to miss out on: http://iba-cg.de/hal4.html We have already close to 50 registered participants, so expect a very lively meeting. See you there? (Late registration still possible.) Ciao, Janis. -- Dr. Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de

1 1

A try on a bank machine algorithm...
by Bernhard Lehnert 01 Jun '09

01 Jun '09

Hi, in a Python Forum someone came up with his homework. Just beginning to learn Haskell I thougt I give it a try. I came up with a working function that I present here to ask you, if I am doing this right or how I could do better. The job was: A cash machine has to compute the division of bank notes for any given amount of money. According to German wikipedia a common algorithm is to start giving one 5$ bill, one 10$ and so on as long as possible (I call this uphill). After that the maximum possible amount of large bank notes is given... Thus every customer gets some small bills and still the number of bills is limited. My answer is a function called 'paymixture' which first calls a function 'uphill' and afterwards a function 'downhill', each of which realizes one part of the above mentioned algorithm. The downhill function might also be usefull apart from the rest which is why indentation ist different... Thanks a lot, Bernhard ------------------------------------------------------------- paymixture :: (Ord t, Num t)=> t->[t]->[t] -- usage: paymixture (sum to pay) (list of possible bills) -- list of bills has to be sorted, like in -- 'paymixture 175 [5,10,20,50,100]' -- last element of the returned list is what could not be paid. paymixture n list = init(up) ++ down where up = uphill n list down = downhill (last(up)) (reverse list) uphill :: (Ord t, Num t)=> t->[t]->[t] uphill n [] = [n] uphill 0 _ = [0] uphill n (x:xs) = if n>=x then x : uphill (n-x) xs else uphill n xs downhill :: (Ord t, Num t)=> t ->[t]->[t] -- usage: downhill (sum to pay) (list of possible bills) -- list if bills has to be reverse sorted, like e. g. -- 'downhill 175 [200, 100, 50, 20, 10,5]' -- last elemt of return value is the rest that could -- not be paid. downhill n supply | (n<last supply) = [n] | otherwise = max : downhill (n-max) supply where max = head([x|x<-supply, x<=n])

4 5

Maybe a and Maybe t
by Ivan Uemlianin 01 Jun '09

01 Jun '09

Dear All I have just started learning Haskell, using the O'Reilly book Real World Haskell [1]. I'm finding it a very exciting language, and I'm working my way painstakingly through the book (I'm up to the exercises at the end of Chapter 3). I have a query based on some code from the book. This question has been asked on the book web site [2], but no answer. I have also Googled about and scanned the documentation. So, here is some code (quoted from [3]): two implementations of a function to return the second element of a list. I have commented out the type signatures. -- file: rwh_examples2/ch03/MySecond.hs -- safeSecond :: [a] -> Maybe a safeSecond [] = Nothing safeSecond xs = if null (tail xs) then Nothing else Just (head (tail xs)) -- tidySecond :: [a] -> Maybe a tidySecond (_:x:_) = Just x tidySecond _ = Nothing My query concerns the inferred types of these functions --- the types inferred when the type signatures are commented out. Here's a ghci session: Prelude> :load seconds.hs [1 of 1] Compiling Main ( seconds.hs, interpreted ) Ok, modules loaded: Main. *Main> *Main> :type safeSecond safeSecond :: [a] -> Maybe a *Main> *Main> :type tidySecond tidySecond :: [t] -> Maybe t *Main> Why is safeSecond type [a] -> Maybe a, and tidySecond type [t] -> Maybe t? I mean why does one use "a" and the other "t"? What does it mean? If I change the order of the two functions in the source file, safeSecond still uses "a" and tidySecond still uses "t". Can anyone help, or point me to the right place in the documentation? In the meantime, if I find out from elsewhere I'll report back here. Thanks and best wishes Ivan [1] http://book.realworldhaskell.org/ [2] http://book.realworldhaskell.org/read/defining-types-streamlining-functions… [3] http://book.realworldhaskell.org/read/defining-types-streamlining-functions… -- ============================================================ Ivan A. Uemlianin Speech Technology Research and Development ivan(a)llaisdy.com www.llaisdy.com llaisdy.wordpress.com www.linkedin.com/in/ivanuemlianin "Froh, froh! Wie seine Sonnen, seine Sonnen fliegen" (Schiller, Beethoven) ============================================================

5 6

Re: [Haskell-cafe] can someone point me to more help about Database.HDBC.ODBC?
by Michael Mossey 30 May '09

30 May '09

This is on Windows, by the way. Do you know the equivalent file names and locations? What is DSN? Thanks, Mike Евгений Тарасов wrote: > Usually you should pass only DSN name in the string, if you used InnoDB > database in MySQL. > > For example, /etc/odbc.ini file: > ; > ; odbc.ini configuration for MyODBC and MyODBC 3.51 Drivers > ; > [.....] > > [Default] > Driver = MySQL > Description = MySQL ODBC 3.51 Driver DSN > Server = localhost > Port = > User = root > Password = > Database = test > Option = 3 > Socket = > > [......] > > Next step, you can test a connection with command: > $ isql Default > +---------------------------------------+ > | Connected! | > | | > | sql-statement | > | help [tablename] | > | quit | > | | > +---------------------------------------+ > SQL> > > And finally, the haskell code: > > conn <- connectODBC "DSN=Default;" > >

1 0