- Beginners - Haskell.org

haskell IO help
by Ashley Banks 06 May '09

06 May '09

From: applebiz89(a)hotmail.com To: beginners(a)haskell.org Subject: Date: Wed, 6 May 2009 13:28:18 +0000 I havent done much IO at all in haskell, only within the function itself. However I want to get the input from the interface for the function and havent done this before. // Say I have this as my data type and list of films data Film = Film String String Int [String] -- List of films testDatabase :: [Film] testDatabase = [(Film "Casino Royale" "Martin Campbell" 2006 ["Garry", "Dave", "Zoe"]) ] // with functions such as: becomeFan :: String -> String -> [Film] -> [Film] becomeFan _ _ [] = [] becomeFan filmName fanName ((Film title director year fans):xs) | filmName == title = (Film title director year fanName:fans) : xs | otherwise = (Film title director year fans) : becomeFan filmName fanName xs filmsInGivenYear :: Int -> [Film] -> [String] filmsInGivenYear filmYear films = [ title | (Film title director year fans) <- films, year == filmYear] // I want to ask the user what function they want to use, I have this so far main :: IO() do putStr "Hi there! what is your name: " fanName = getLine do putStr "1 = Insert film, 2 = Become a Fan, 3 = The number of fans of a film, 4 = Film released in a year: " input = getLine read input :: Int (if input == 1 then main x = insertFilm [] else if input == 2 then main x = becomeFan [] else if input == 3 then main x = numberOfFans []) // I thought using the if statement would work, but now I cant think how to gather the needed input for the function they have chosen? thanks apple Get the New Internet Explore 8 Optimised for MSN. Download Now _________________________________________________________________ Share your photos with Windows Live Photos – Free. http://clk.atdmt.com/UKM/go/134665338/direct/01/

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(no subject)
by Ashley Banks 06 May '09

06 May '09

I havent done much IO at all in haskell, only within the function itself. However I want to get the input from the interface for the function and havent done this before. // Say I have this as my data type and list of films data Film = Film String String Int [String] -- List of films testDatabase :: [Film] testDatabase = [(Film "Casino Royale" "Martin Campbell" 2006 ["Garry", "Dave", "Zoe"]) ] // with functions such as: becomeFan :: String -> String -> [Film] -> [Film] becomeFan _ _ [] = [] becomeFan filmName fanName ((Film title director year fans):xs) | filmName == title = (Film title director year fanName:fans) : xs | otherwise = (Film title director year fans) : becomeFan filmName fanName xs filmsInGivenYear :: Int -> [Film] -> [String] filmsInGivenYear filmYear films = [ title | (Film title director year fans) <- films, year == filmYear] // I want to ask the user what function they want to use, I have this so far main :: IO() do putStr "Hi there! what is your name: " fanName = getLine do putStr "1 = Insert film, 2 = Become a Fan, 3 = The number of fans of a film, 4 = Film released in a year: " input = getLine read input :: Int (if input == 1 then main x = insertFilm [] else if input == 2 then main x = becomeFan [] else if input == 3 then main x = numberOfFans []) // I thought using the if statement would work, but now I cant think how to gather the needed input for the function they have chosen? thanks apple _________________________________________________________________ Beyond Hotmail — see what else you can do with Windows Live. http://clk.atdmt.com/UKM/go/134665375/direct/01/

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Re: Programação Funcional - TP1
by Marco Túlio Gontijo e Silva 05 May '09

05 May '09

Olá Henrique, seu problema Polepos.hs excedeu o tempo limite de execução no SPOJ. Algumas dicas: você pode escrever: cars = map fst startGrid positions = map snd startGrid (cars, positions) = unzip take (length positions) [0..] [0 .. pred $ length positions] do scoreBoard <- parseScoreboard n let startGrid = getStartGrid scoreBoard putStrLn $ showGrid startGrid main parseScoreboard n >>= putStrLn . showGrid . getStartGrid >> main showGrid Nothing = "-1" showGrid (Just xs) = foldl1 (++) $ intersperse " " $ map show xs showGrid = maybe "-1" (foldl1 (++) . intersperse " " . map show) Talvez fosse mais legal criar um tipo de dados e definir a classe Ord para eles do que usar tupla e sortBy. Essas dicas não estão relacionadas com a demora no tempo de execução. Isso se deve, acredito eu, ao seu algoritmo. -- marcot http://marcot.iaaeee.org/

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GHCI: Main function seems to mask other functions
by aditya siram 05 May '09

05 May '09

Hi all, I'm having a weird problem where when I add the 'main' function to my program and load it into GHCI, whenever I check the type or try to run my other functions I get a 'Not in scope: <functionname>' error. When I remove the 'main' function and reload the code, the problem goes away. I can't seem to reproduce this with a trivial example, but this has happened to me several times in my larger programs. thanks ... deech

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Polymorphism question from an OO-speaking newbie
by Joel Neely 04 May '09

04 May '09

Short version: Is it possible/reasonable to define a single function that accepts a single string or a list of strings (in which case it maps the single-string "flavor" over the list)? Longer version: Thus far I know that Haskell allows me to define a function on a single string, then define another function that maps the first one across a list of strings, as in: *Main> let quote1 s = "\"" ++ s ++ "\"" *Main> "Quux said " ++ quote1 "foo" ++ " loudly" "Quux said \"foo\" loudly" *Main> let quote = map quote1 *Main> quote ["foo", "baz", "bletch"] ["\"foo\"","\"baz\"","\"bletch\""] (BTW, is it standard terminology to say that quote lifts quote1 to lists?) In the above I have different names for the two functions. OO languages such as Java allow me to overload quote to accept either String or List<String> (and return the same type). AFAICT, Haskell's parametric polymorphism allows me to define functions (e.g. length) which deal with "listness" concepts indifferent to the type contained by the list. Am I missing something, or should I admit to OO wrongheadedness and accept that my inability to write a single declaration unifying: quote :: String -> String quote :: [String] -> [String] is an emphatic clue that I should change my expectations? Thanks in advance for any enlightenment shed my way! -jn- -- Beauty of style and harmony and grace and good rhythm depend on simplicity. - Plato

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Re: [Haskell-beginners] Parsec tutorial
by Keith Sheppard 02 May '09

02 May '09

Hi Patrick, This isn't what you asked for but it may be useful to you. I've created a parsec based SQL parser for my "TxtSushi" command-line program. It's still pretty early in development but it does parse a subset of the SELECT grammar. The project's page is http://keithsheppard.name/txt-sushi/index.html Keith

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Selecting a GUI toolkit
by Daniel Carrera 02 May '09

02 May '09

Hello, This question is just idle curiosity. I don't actually plan to write a desktop GUI program. With that said, what library would you recommend to someone who wants to write a cross-platform desktop app with Haskell? Consider the following requirements: 1) Cross-platform. Windows, Linux and preferably OS X too. 2) Maintained, stable, mature, documented, etc. 3) Easy to use. Notice that this has much to do with the Haskell bindings. For example, Qt might be a fabulous toolkit, but qtHaskell might be immature (it looks like it started in Dec 2007). wxHaskell looks good. I like the idea behind wxWidgets, but I have never used it (or any GUI toolkit for that matter). Thank you for your input. Cheers, Daniel.

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Re: [Haskell-beginners] gcd
by Steve 01 May '09

01 May '09

On Fri, 2009-05-01 at 16:19 +0200, Peter Verswyvelen wrote: > I suggest you forward this to the Haskell Cafe, since it's not really > a beginners question, it's a really good question :) Thanks for the advice, I've reposted the question to the Haskell Cafe. Steve > On Fri, May 1, 2009 at 4:19 PM, Steve <stevech1097(a)yahoo.com.au> > wrote: > I had a look at the gcd definition in GHC 6.10.1 > ghc-6.10.1/libraries/base/GHC/Real.lhs > > -- | @'gcd' x y@ is the greatest (positive) integer that > divides both > @x@ > -- and @y@; for example @'gcd' (-3) 6@ = @3@, @'gcd' (-3) > (-6)@ = @3@, > -- @'gcd' 0 4@ = @4@. @'gcd' 0 0@ raises a runtime error. > gcd :: (Integral a) => a -> a -> a > gcd 0 0 = error "Prelude.gcd: gcd 0 0 is undefined" > gcd x y = gcd' (abs x) (abs y) > where gcd' a 0 = a > gcd' a b = gcd' b (a `rem` b) > > Why is gcd 0 0 undefined? > > http://en.wikipedia.org/wiki/Greatest_common_divisor says: > "It is useful to define gcd(0, 0) = 0 and lcm(0, 0) = 0 > because then the > natural numbers become a complete distributive lattice with > gcd as meet > and lcm as join operation. This extension of the definition is > also > compatible with the generalization for commutative rings given > below." > > An added advantage, for haskell, of defining gcd 0 0 = 0 is > that gcd > would change from being a partial function to a total > function. > > Regards, > Steve > > > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners > >

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gcd
by Steve 01 May '09

01 May '09

I had a look at the gcd definition in GHC 6.10.1 ghc-6.10.1/libraries/base/GHC/Real.lhs -- | @'gcd' x y@ is the greatest (positive) integer that divides both @x@ -- and @y@; for example @'gcd' (-3) 6@ = @3@, @'gcd' (-3) (-6)@ = @3@, -- @'gcd' 0 4@ = @4@. @'gcd' 0 0@ raises a runtime error. gcd :: (Integral a) => a -> a -> a gcd 0 0 = error "Prelude.gcd: gcd 0 0 is undefined" gcd x y = gcd' (abs x) (abs y) where gcd' a 0 = a gcd' a b = gcd' b (a `rem` b) Why is gcd 0 0 undefined? http://en.wikipedia.org/wiki/Greatest_common_divisor says: "It is useful to define gcd(0, 0) = 0 and lcm(0, 0) = 0 because then the natural numbers become a complete distributive lattice with gcd as meet and lcm as join operation. This extension of the definition is also compatible with the generalization for commutative rings given below." An added advantage, for haskell, of defining gcd 0 0 = 0 is that gcd would change from being a partial function to a total function. Regards, Steve

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gcd
by Steve 01 May '09

01 May '09

I had a look at the gcd definition in GHC 6.10.1 ghc-6.10.1/libraries/base/GHC/Real.lhs -- | @'gcd' x y@ is the greatest (positive) integer that divides both @x@ -- and @y@; for example @'gcd' (-3) 6@ = @3@, @'gcd' (-3) (-6)@ = @3@, -- @'gcd' 0 4@ = @4@. @'gcd' 0 0@ raises a runtime error. gcd :: (Integral a) => a -> a -> a gcd 0 0 = error "Prelude.gcd: gcd 0 0 is undefined" gcd x y = gcd' (abs x) (abs y) where gcd' a 0 = a gcd' a b = gcd' b (a `rem` b) Why is gcd 0 0 undefined? http://en.wikipedia.org/wiki/Greatest_common_divisor says: "It is useful to define gcd(0, 0) = 0 and lcm(0, 0) = 0 because then the natural numbers become a complete distributive lattice with gcd as meet and lcm as join operation. This extension of the definition is also compatible with the generalization for commutative rings given below." An added advantage, for haskell, of defining gcd 0 0 = 0 is that gcd would change from being a partial function to a total function. Regards, Steve

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