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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 15 Jul '13

15 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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cps in yaht
by Deng Wu 14 Jul '13

14 Jul '13

hi, everyone, I'm learning haskell by reading the book <<Yet Another Haskell Tutorial>>, and I encounter a problem when comes to the contiuation passing style. The book gives a cps fold like: cfold’ f z [] = z cfold’ f z (x:xs) = f x z (\y -> cfold’ f y xs) and gives the test result: CPS> cfold (+) 0 [1,2,3,4] 10 CPS> cfold (:) [] [1,2,3] [1,2,3] but, when I try to test this, I find there is a problem, the ghci gives: ----------------------------------------------------------------------- *Main> cfold (+) 0 [] <interactive>:8:7: Occurs check: cannot construct the infinite type: t10 = (t10 -> t10) -> t10 Expected type: t10 -> t10 -> (t10 -> t10) -> t10 Actual type: t10 -> t10 -> t10 In the first argument of `cfold', namely `(+)' In the expression: cfold (+) 0 [] In an equation for `it': it = cfold (+) 0 [] ------------------------------------------------------------------------ It makes sense to me, so I change the definition of cps to something like this: cfold f z [] = z cfold f z (x:xs) = (\y -> cfold f y xs) (f x z) And it works fine: *Main> cfold (+) 0 [1,2,3] 6 So my question comes, is it a bug in the book or something I miss here? Regards! - wudeng

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(no subject)
by Francisco Gutiérrez 14 Jul '13

14 Jul '13

I jio ONU O J Enviado desde mi iPhone

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Re: [Haskell-beginners] Example using gmapQ
by Adrian May 13 Jul '13

13 Jul '13

Well I meant it as (gmapQ show) thing and in the meantime I found out that I need to replace show with gshow, and AFAICT, type classes are incompatible with generic stuff. Adrian. On 13 July 2013 19:27, Dilawar Singh <dilawars(a)iitb.ac.in> wrote: > Comment out the last line (main = etc...) and do this in ghci, > :t (show thing) > > Type of (show thing) is String. This is input to gmapQ function which has > following type. > > :t gmapQ gmapQ :: Data a => (forall d. Data d => d -> u) -> a -> [u] > > First argument to this function has the type (d -> u). Surely, this can't > be String. > > -- > Dilawar EE, IITB > On Sat, Jul 13, 2013 at 04:48:49PM +0800, Adrian May wrote: > >> Hi All, >> >> I got this far: >> >> {-# LANGUAGE OverloadedStrings #-} >> {-# LANGUAGE DeriveDataTypeable #-} >> import Data.Text >> import Data.Typeable >> import Data.Data >> >> data Thing = Thing { foo :: Int, bar :: String} >> deriving (Read, Show, Typeable, Data) >> thing :: Thing >> thing = Thing 1 "wop" >> con = toConstr thing >> fields = constrFields con >> main = putStrLn $ show con ++ show fields ++ ( Prelude.concat $ gmapQ show >> thing ) >> >> But it's barfing like this: >> >> Could not deduce (Show d) arising from a use of `show' >> from the context (Data d) >> bound by a type expected by the context: Data d => d -> [Char] >> at w.hs:76:65-80 >> >> I can see why, but not how to fix it. >> >> Any help much appreciated, >> Adrian. >> > > ______________________________**_________________ >> Beginners mailing list >> Beginners(a)haskell.org >> http://www.haskell.org/**mailman/listinfo/beginners<http://www.haskell.org/mailman/listinfo/beginners> >> > > > ______________________________**_________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/**mailman/listinfo/beginners<http://www.haskell.org/mailman/listinfo/beginners> >

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Example using gmapQ
by Adrian May 13 Jul '13

13 Jul '13

Hi All, I got this far: {-# LANGUAGE OverloadedStrings #-} {-# LANGUAGE DeriveDataTypeable #-} import Data.Text import Data.Typeable import Data.Data data Thing = Thing { foo :: Int, bar :: String} deriving (Read, Show, Typeable, Data) thing :: Thing thing = Thing 1 "wop" con = toConstr thing fields = constrFields con main = putStrLn $ show con ++ show fields ++ ( Prelude.concat $ gmapQ show thing ) But it's barfing like this: Could not deduce (Show d) arising from a use of `show' from the context (Data d) bound by a type expected by the context: Data d => d -> [Char] at w.hs:76:65-80 I can see why, but not how to fix it. Any help much appreciated, Adrian.

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running shelly script from root's crontab
by Miguel Negrão 12 Jul '13

12 Jul '13

Hi I have a shelly script that runs fine from my user account which I would to run every night. It needs to sudo to call some other processes so I need to add it to the root's crontab (sudo crontab -e). Unfortunately the script will fail to run because it can't find several modules, since it's running from the root account. Is there a way to make this work without having to install the packages globally (sudo cabal install --globally) ? I just tried that and manage to completely ruin my haskell installation, so I had to revert to previous backup. thanks, -- Miguel Negrão http://www.friendlyvirus.org/miguelnegrao

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Up-to-date HaskellDB sample or tute
by Adrian May 10 Jul '13

10 Jul '13

Hi All, All the tutorials and samples I can find for HaskellDB seem to be bitrotten. Does anybody know of a newish one? I just want to connect to a postgres DB and run some simple queries. Or is HaskellDB superseded now? TIA, Adrian.

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