July 2013 - Beginners

Type signature question
by Louis-Guillaume Gagnon 23 Jul '13

23 Jul '13

Hello beginners, I'm working on the second set of exercises in ch.03 of Real World Haskell I wrote a palindromize function (:: [a] -> [a]) which transforms a list in a palindrome (eg. [1,2,3] -> [1,2,3,3,2,1]) . I wrote a isPalindrome function which checks whether a given list is such a palindrome. it reads: isPalindrome xs | odd (length xs) = False | firstHalf == secondHalf =True | otherwise = False where half = div (length xs) 2 firstHalf = take half xs secondHalf = reverse (drop half xs) I would expect the type signature to be: isPalindrome :: [a] -> Bool but ghci gives me is Eq a => [a] -> Bool and I don't undestand why the "Eq a =>" shows up. many thanks, glg

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Syntactic sugar to insert into a Map?
by martin 22 Jul '13

22 Jul '13

Hello all, What is the best way to enter several (key, value) pairs into a Map, such that the code "looks nice", kind of like the do-notation with monads. I suppose this question is not really specific to a Map. It is about chaining functions with a type of a1 ->...-> an -> b -> b. Is there any syntactic sugar available?

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Mihai, please add me to your LinkedIn network
by Anish Kumar via LinkedIn 19 Jul '13

19 Jul '13

LinkedIn ------------ Anish Kumar requested to add you as a connection on LinkedIn: ------------------------------------------ Mihai, I'd like to add you to my professional network on LinkedIn. - Anish Accept invitation from Anish Kumar http://www.linkedin.com/e/-3dpk6t-hjbhwpdp-3z/uyFflI7IwvrmSA8-0MFfTI7lXX8s3… View profile of Anish Kumar http://www.linkedin.com/e/-3dpk6t-hjbhwpdp-3z/rso/252360250/ewr7/name/25134… ------------------------------------------ You are receiving Invitation emails. This email was intended for Mihai Maruseac. Learn why this is included: http://www.linkedin.com/e/-3dpk6t-hjbhwpdp-3z/plh/http%3A%2F%2Fhelp%2Elinke… (c) 2012, LinkedIn Corporation. 2029 Stierlin Ct, Mountain View, CA 94043, USA.

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Question about problem "KAMIL" from SPOJ
by Nadav Chernin 18 Jul '13

18 Jul '13

Hi, I'm trying to solve problem "KAMIL" on SPOJ site The target is to write as small as possible code. My first solution was: main=interact$unlines.map(show.(2^).length.filter(`elem`"TDLF")).lines This code = 70 bytes and it accepted on SPOJ My second solution was: main=getLine>>=print.(2^).length.filter(`elem`"TDLF")>>main This code = 60 bytes, but i get Run-Time error on SPOJ I don't understand how this code can create RTE? Please help me Nadav

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Lens
by Emmanuel Surleau 16 Jul '13

16 Jul '13

Hello there, I'm starting to use a bit more extensively the lens package. It is clearly very powerful, but it feels like a maze of related components (Setting/Setter/Lens'/LensLike/Lens/Traversal/Traversal') which can be swapped for one another except when they can't. The first (concrete) problem I ran into is how to update the members of a set with the result of an IO action. I have managed to do this with a pure function (prefixName) but I'm not sure of how to do this with promptName. Full program below: {-# LANGUAGE TemplateHaskell #-} import Control.Lens import Data.Set import Data.Set.Lens data Dog = Dog { _name :: String, _legs :: Int } deriving (...) makeLenses ''Dog data Dogs = Dogs { _dogs :: Set Dog } deriving Show makeLenses ''Dogs fourLegs :: Traversal' Dog Dog fourLegs = filtered (λd -> d^.legs == 4) promptName :: String -> IO String promptName dogName = do putStr $ "New name for " ++ dogName getLine prefixName :: Dog -> Dog prefixName dog = set name ("PREFIXED: " ++ dog^.name) dog main :: IO () main = do let fido = Dog "fido" 4 let milou = Dog "milou" 4 let cripple = Dog "cripple" 3 let doggies = Dogs $ fromList [fido, milou, cripple] -- prefix dog names via a pure function let doggies' = over (dogs.setmapped) prefixName doggies print doggies' -- change dog names by prompting the user ? return () Help would be appreciated (in particular, 'cripple' would love to be renamed). Since I was struggling with the library, I had the idea to look at the internals, but got stuck at the definition of Lens: type Lens s t a b = forall f. Functor f => (a -> f b) -> s -> f t a) I'm not sure why the explicit forall is needed here (isn't this equivalent to just Functor f => ...)? b) My understanding is that a lens packs both getter and setters, but I don't know which is supposed to be which here... c) Is there any kind of in-depth guide to Control.Lens somewhere? I have found some examples and tutorials but nothing that seemed to do more than scratch the surface. Thanks, Emm

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Re: [Haskell-beginners] exception, not in IO
by Lyndon Maydwell 16 Jul '13

16 Jul '13

I agree that it seems uncomfortable for a seemingly pure function to require handling exceptions in the IO monad, but if the code in question really does have the capability of raising IO exceptions, then I think it is reasonable to consider it as impure and belonging in the IO monad and that it probably should not have been given a pure type signature to start with. I'd double check to see if the function in question has the capability of raising an exception in a more pure monad as per Kees's suggestion. The other option is to perform the actions I've suggested and then wrap the whole lot in unsafePerformIO to make it pure again but with a new signature of (a -> Either YourException b), however that just makes me feel even more queazy... Other than that, I guess we'd need a snippet of the code and what libraries are being used in order to provide better help. Good luck! On Mon, Jul 15, 2013 at 4:07 AM, Kees Bleijenberg < k.bleijenberg(a)lijbrandt.nl> wrote: > Lyndon,**** > > You wrote: > You can turn anything into an IO action with return, or you could catch > the exception at a level where you are performing IO. Would this be what > you're after?**** > > Wait until you perform IO, seems quit uneasy to me and why? Converting a > string to a typeable is not a IO action. Problem is that this function > lives in a dll. The IO is done by the calling program (not a Haskell > program). The function is not in a monad, it is a pure function. So I think > I can’t do a return.**** > > Kees**** > > On Sun, Jul 14, 2013 at 7:47 PM, Kees Bleijenberg < > k.bleijenberg(a)lijbrandt.nl> wrote:**** > > The app I’am working on, converts a jsonString to another string encoding. > **** > > The function I want to write is jsonString -> (encoding, errorMsg) so > String-> (String, String) **** > > For this purpose I have a typeable datastructure Glass. Because it is > typeable I can do (decodeJSON jsonString) :: Glass**** > > But sometimes the jsonString is not valid (misformed or wrong fields). > decodeJSON then throws a exception. I want to**** > > catch that exection and transform the result to something like (“” , > theErrorMsg). Unfortunately all catch functions want IO parameters. **** > > What can I do?**** > > **** > > Kees**** > > **** > > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners**** > > ** ** > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners > >

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Re: [Haskell-beginners] exception, not in IO
by Ertugrul Söylemez 15 Jul '13

15 Jul '13

"Kees Bleijenberg" <k.bleijenberg(a)lijbrandt.nl> wrote: > But sometimes the jsonString is not valid (misformed or wrong fields). > decodeJSON then throws a exception. I want to > > catch that exection and transform the result to something like ("" , > theErrorMsg). Unfortunately all catch functions want IO parameters. > > What can I do? IO is just one of the many monads with exception support. For your case, since JSON parsing is a pure process, you would want to use a pure exception monad like `Maybe` or `Either MyError`: data MyError = InvalidDateField | {- ... -} | UnknownError There is nothing wrong with using regular exception types, if you wish, in which case you might use `Either SomeException`. Then separate concerns: decode :: String -> Either MyError Glass encode :: Glass -> String Finally the conversion function is as simple as: convert :: String -> Either MyError String convert = fmap encode . decode If `encode` can fail as well and exceptions are regular Haskell exceptions: import Control.Exception import Control.Monad decode :: String -> Either SomeException Glass encode :: Glass -> Either SomeException String convert :: String -> Either SomeException String convert = encode <=< decode I hope this helps. Greets, Ertugrul -- Not to be or to be and (not to be or to be and (not to be or to be and (not to be or to be and ... that is the list monad.

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Re: [Haskell-beginners] exception, not in IO
by Lyndon Maydwell 14 Jul '13

14 Jul '13

You can turn anything into an IO action with return, or you could catch the exception at a level where you are performing IO. Would this be what you're after? On Sun, Jul 14, 2013 at 7:47 PM, Kees Bleijenberg < k.bleijenberg(a)lijbrandt.nl> wrote: > The app I’am working on, converts a jsonString to another string encoding. > **** > > The function I want to write is jsonString -> (encoding, errorMsg) so > String-> (String, String) **** > > For this purpose I have a typeable datastructure Glass. Because it is > typeable I can do (decodeJSON jsonString) :: Glass**** > > But sometimes the jsonString is not valid (misformed or wrong fields). > decodeJSON then throws a exception. I want to**** > > catch that exection and transform the result to something like (“” , > theErrorMsg). Unfortunately all catch functions want IO parameters. **** > > What can I do?**** > > ** ** > > Kees**** > > ** ** > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners > >

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exception, not in IO
by Kees Bleijenberg 14 Jul '13

14 Jul '13

The app I'am working on, converts a jsonString to another string encoding. The function I want to write is jsonString -> (encoding, errorMsg) so String-> (String, String) For this purpose I have a typeable datastructure Glass. Because it is typeable I can do (decodeJSON jsonString) :: Glass But sometimes the jsonString is not valid (misformed or wrong fields). decodeJSON then throws a exception. I want to catch that exection and transform the result to something like ("" , theErrorMsg). Unfortunately all catch functions want IO parameters. What can I do? Kees

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coding style: instead of let, return?
by Obscaenvs 14 Jul '13

14 Jul '13

Just a style question: What's the community's verdict when it comes to using `return` to bind names instead of `let`? E.g. if instead of ` let p1 <- mkName "p1" a <- newName "a" ` , we use ` p1 <- return $ mkName "p1" a <- newName "a" `. I have seen this a couple of times in other people's code now, and I am ambivalent about it: `let` states intention better, and refrains from redundancy, but when you know why the writer behind the code is using the `return`-variant, it's somewhat easier on the eyes (at least when some actions in do-notation follow). In my opinion, `let` *is* the better alternative, but there is code in the wild[1] that uses the `return`-variant, so I wanted to gauge the community's opinion on the matter. [1] http://monads.haskell.cz/html/statemonad.html -- fredrik

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