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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 19 Dec '09

19 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 18 Dec '09

18 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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Large tables and GHC.
by Sherard D. 17 Dec '09

17 Dec '09

Hi, I have a couple of rather large byte tables that I'd like to use. I decided to use them as arrays because I figured such large tables wouldn't work well as lists. table1 = listArray (0, 25599) [...] table2 = listArray (0, 25599) [...] This is how my code looks, minus the large tables. However, the problem is that the large tables are loaded as a list first, and then converted to an array. This is indeed a very big problem, as trying to compile with GHC takes an unacceptably long time. Is there any way I can get around this? - Sherard

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Applicability of FP unchanging things to real world changing things
by Glurk 17 Dec '09

17 Dec '09

I come from an OO background, but have recently started to see the light and think that functional programming has a lot to offer in real world, general software development - not just some small specific niche. Having the OO background certainly has limited the way I view software development, and I guess in learning something new, I have a tendency to compare it with what I do know, looking for similarities and differences. So, perhaps that's why I am having difficulty in seeing how some functional programming ideas apply. What's got me a little puzzled at the moment is non mutable data. On the one hand, I think it's great that I can mostly forget about errors caused by my variables having unexpected values, and that I can reason about my program much more easily... However, on the other hand, I wonder how it ties in with real world systems that do in fact do deal with objects that change state. For example, if I was building a library system - I might have book objects, and if a book gets borrowed, there remains the same book object, but it now just has a different state - from "on shelf" say, to "borrowed". Now, in Haskell, I might have a function to lend a book :- lend book1 and this is going to return a new book, with the appropriate status change.. ..but what of the old book ? I don't want 2 book objects floating around.. ...to my way of thinking, it's really not a new book, it really is a change of state. How do people see this kind of situation ? Is it just the way of looking at it... ? Do I just accept that I toss out my old library object, and have a new one, which happens to be the same as the old one, except for the "new" book which is now borrowed ? Do I really need to get over thinking of objects in the first place, even though it's what I see exist in the real world ? Is there where we part company with non mutability and venture into the monads where we do have side effects ? Any thoughts appreciated ! :) Thanks !

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What about scratch pad for each library
by Marc Weber 13 Dec '09

13 Dec '09

You can read about the complete proposal here: http://haskell.org/haskellwiki/Hackage_wiki_page_per_project_discussion Benefits: If something doesn't work for you should contact the author/ maintainer or the mailinglist. But if you find a workaround you can put it online immediately. You should remove it again after having discussed the issue with maintainers. However they may be on holidays.. So if you've had minor trouble getting started with a package and if you think that some small examples would have helped you in the past you should register at the haskellwiki and write about your experiences so that I get a understanding of whether this idea improves your experience or makes it worse. Of course not all wiki pages will be maintained. So it's very likely that you find outdated information. But I personally think that one tip is worth 10 outdated wiki pages or such (?) Additionally are you interested in helping removing outdated content when you find such ? You'll find a link to the haskell-cafe mailinglist thread on the wiki page as well. Hopefully most wiki pages are empty or contain links to the project pages only because there are no issues and you understand everything immediately :) Thank you for your feedback Marc Weber

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Problem implementing a stack
by Jacob Faren 13 Dec '09

13 Dec '09

Hello. I'm implement data structures (a stack, here) to get familiar with Haskell. It's not going very well. I have the following code... -- test.hs data Stack a = Top (Stack a) | Layers [a] deriving (Show) stack :: Stack a stack = Top (Layers []) push :: Stack a -> a -> Stack a push (Top (Layers ls)) value = Top (Layers (value:ls)) And I run the following in ghci... Prelude> :l test [1 of 1] Compiling Main ( test.hs, interpreted ) Ok, modules loaded: Main. *Main> push stack 4 Top (Layers [4]) *Main> let st = stack *Main> push st 4 ^CInterrupted. The output of the last command never resolves. I have to kill it. Might anyone have an idea why the push function is failing with st, but not with stack? I know there are other stack implementations that work, but I really want to know what's going on here. Thanks

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How to create a monad instance
by Marco De Oliveira 13 Dec '09

13 Dec '09

Hi, Is it possible to create an instance of Monad for IOException? Each time a try I stay blocked by IO monad. with the definition: data IOException a = IOExceptionCons (IO (Exception a)) data Exception a = SuccessCons (Maybe Warning) a | ErrorCons Error data Warning = WarningCons1 | WarningCons2 data Error = ErrorCons1 | ErrorCons2 Regards

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