December 2009 - Beginners

Pure Regex.PCRE Possible? (PLEAC, Fix Style)
by Volkan YAZICI 23 Dec '09

23 Dec '09

Hi, I'm trying to implement FixStyle[1] program of PLEAC[2] for Haskell. (It's missing in the Haskell version of PLEAC.) For this purpose, I needed regular expressions. (Now, I have two problems!) Despite, my code is, IMHO, quite purely functional, Regex.PCRE.String functions totally mess up the design with impure functions they introduce even for really simple string matching stuff. If you'd check out the sources[3], you'll see below impure functions. transDictRegex :: IO PCRE.Regex matchRegex :: String -> IO (String, String, String) translate :: String -> IO String However, if I'm not mistaken, above functions _should_ be absolutely pure. Is it possible to encapsulate impure PCRE functions into their pure equivalents? BTW, why are PCRE functions impure? Regards. [1] http://pleac.sourceforge.net/pleac_perl/strings.html [2] http://pleac.sourceforge.net/ [3] http://hpaste.org/fastcgi/hpaste.fcgi/view?id=14422

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expression parser
by John Moore 22 Dec '09

22 Dec '09

Hi, Could some one point out what I'm doing wrong below. This is a parser which takes an arithmetic expression built up from single digits. Turns 3*4+2 into (3*4)+2 etc.It giving me the last expression in a do statement or parse error on -> (+++) :: Parser a -> Parser a -> Parser a expr :: term( '+' expr |"") term -> factor('*' term |"") expr :: Parser Int expr = do t <- term do char '+' e <- expr return (t + e) +++ return t term :: Parser Int term = do f <- factor do char '*' t <- term return (f * t) +++ return f factor :: Parser Int factor = do d <- digit return (digitToInt d) +++ do char '(' e <- expr char ')' return e eval :: String -> Int eval xs = fst(head(parse expr xs)) John

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inserting tupels inside list as tupels in list
by kane96＠gmx.de 22 Dec '09

22 Dec '09

maybe the subject is a little bit confusing :p I have a organiser list like that and have to add events (like panic, xmas, ...) which is also a type of tupel for a specific date: aliceOrg = [ ( (21,Dec,2009), [panic,aliceBDay] ) ( (25,Dec,2009), [xmas] ) ( (10,Jan,2010), [bobBDay] ) What function(s) can I use to add these nested lists and tupels here? -- Jetzt kostenlos herunterladen: Internet Explorer 8 und Mozilla Firefox 3.5 - sicherer, schneller und einfacher! http://portal.gmx.net/de/go/chbrowser

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print all days of calendar
by kane96＠gmx.de 21 Dec '09

21 Dec '09

I have to create a calendar like this: type Calendar = [Date] Date is: type Day = Int data Month = Jan | Feb | Mar | Apr | May | Jun | Jul | Ago | Sep | Oct | Nov | Dec deriving (Eq,Enum,Show) type Year = Int type Date = (Day,Month,Year) I shall create a function: calendar :: Year -> Calendar that return a calendar for the given year like: [(1, Jan, 2009), (2, Jan, 2009), ..., (31, Dec, 2009)] I also have a function from a previous exercise which checks if a given date is valid. Is there a function for a loop that iterates from 1 to n and checks if the date is valid. If it's valid it should return the date otherwise it should jump to the next month or end at the end of the year? Or is it better to do it on another way with this data I have? -- Jetzt kostenlos herunterladen: Internet Explorer 8 und Mozilla Firefox 3.5 - sicherer, schneller und einfacher! http://portal.gmx.net/de/go/chbrowser

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Enum for natural numbers
by kane96＠gmx.de 21 Dec '09

21 Dec '09

Hi, I have data Nat = Z | S Nat deriving (Eq,Ord,Show) toEnum should return Z for negative numbers. I did something like this but the problem is I don't know how to set "less than". I tried > and lt: instance Enum Nat where toEnum (gt 0) = S Z toEnum (lt 1) = Z -- GRATIS für alle GMX-Mitglieder: Die maxdome Movie-FLAT! Jetzt freischalten unter http://portal.gmx.net/de/go/maxdome01

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Type constraints question
by David Gordon 21 Dec '09

21 Dec '09

Hi Folks, Total newbie here. I don't know if I am having a syntactic problem or a conceptual problem. This code: data Test = Test data Test2 = Test2 class MyClass a where getChild :: (MyClass b) => a -> b instance MyClass Test where getChild = Test2 instance MyClass Test2 where getChild = Test results in: [1 of 1] Compiling Main ( test.hs, interpreted ) test.hs:10:15: Couldn't match expected type `Test -> b' against inferred type `Test2' In the expression: Test2 In the definition of `getChild': getChild = Test2 In the instance declaration for `MyClass Test' test.hs:13:15: Couldn't match expected type `Test2 -> b' against inferred type `Test' In the expression: Test In the definition of `getChild': getChild = Test In the instance declaration for `MyClass Test2' Failed, modules loaded: none. Is this a reasonable thing to try and do in Haskell? If not, I have a lot more questions... ;) many thanks, David

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`coarbitrary' is not a (visible) method of class `Arbitrary'
by Amy de Buitléir 19 Dec '09

19 Dec '09

I'm trying to follow the instructions in http://haskell.org/haskellwiki/How_to_write_a_Haskell_program. I'm at the step where we've just written a QuickCheck test: amy@localhost ~/forbairt/haq $ cat Tests.hs import Char import List import Test.QuickCheck import Text.Printf main = mapM_ (\(s,a) -> printf "%-25s: " s >> a) tests instance Arbitrary Char where arbitrary = choose ('\0', '\128') coarbitrary c = variant (ord c `rem` 4) -- reversing twice a finite list, is the same as identity prop_reversereverse s = (reverse . reverse) s == id s where _ = s :: [Int] -- and add this to the tests list tests = [("reverse.reverse/id", test prop_reversereverse)] But when I try to run it, I get the following errors: amy@localhost ~/forbairt/haq $ runhaskell Tests.hs Tests.hs:10:4: `coarbitrary' is not a (visible) method of class `Arbitrary' Tests.hs:17:33: Not in scope: `test' I think this has something to do with the changes between QuickCheck v1 and v2. Can someone tell me what I need to do to make this example work? Thank you in advance! Amy

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Parallel mutation
by Jon Harrop 18 Dec '09

18 Dec '09

Can you mutate different parts of the same array from different threads in parallel using GHC? Someone once told me this was possible but I cannot see how... -- Dr Jon Harrop, Flying Frog Consultancy Ltd. http://www.ffconsultancy.com/?e

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find element of tupels
by kane96 18 Dec '09

18 Dec '09

Hi, I have a list of tuples: [(Jan, 31),(Feb, 28),(Mar, 31),...] called monthAndMaxDay Date is: type Day = Int data Month = Jan | Feb | Mar | Apr | May | Jun | Jul | Ago | Sep | Oct | Nov | Dec deriving (Eq,Enum,Show) type Year = Int type Date = (Day,Month,Year) Now I want to check if a date is legal: legalDate :: Date -> Bool legalDate (myDay, myMonth, myYear) = not (myDay <= 0) && myDay >= (find ((== myMonth) . fst) monthAndMaxDay) . snd the find works to search for the month in the first element of every tupel but at the end I have to check the second value of the tupel (the day) that it issn't higher than the maximal number of days in that month. Where do I have to set the "snd" correctly. Here I get the error: Couldn't match expected type `b -> c' against inferred type `Maybe (Month, Day)' -- Jetzt kostenlos herunterladen: Internet Explorer 8 und Mozilla Firefox 3.5 - sicherer, schneller und einfacher! http://portal.gmx.net/de/go/atbrowser

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Is this MR? Type restriction confusion
by Mike Erickson 17 Dec '09

17 Dec '09

I was trying to convert a function, f1 x l = map (\y -> y*x) l to so-called point-free style, but in reducing it I found two functions that seemed to work the same, until I discovered they have different types, and aren't equivalent: f1a x = map (*x) -- has type (Num a) => a -> [a] -> [a] f1b = \x -> map (*x) -- has type Integer -> [Integer] -> [Integer] Can someone help me understand why the latter has a more restrictive type? If I add the type signature "f1b :: (Num a) => a -> [a] -> [a]", they are once again equivalent, but am confused about what's going on. If this is MR, what exactly am I overloading here? I read through http://www.haskell.org/haskellwiki/Monomorphism_restriction and http://www.cs.auckland.ac.nz/references/haskell/haskell-intro-html/pitfalls…. If anyone has a good reading recommendation here, I'd love to see it. Because if this is MR, I'm not understanding what MR is correctly. Also, They don't generate a type error, which makes me think it is not MR, but something else. Thanks, Mike

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