- Beginners - Haskell.org

Re: [Haskell-beginners] Beginners Digest, Vol 68, Issue 22
by KMandPJLynch 24 Feb '14

24 Feb '14

Good evening, I'm trying to learn how to implement Haskell on a web site. I've got the O'Reilly book: "Developing Web Applications with Haskell and Yesod". I'd appreciate some guidance in regard to this. I'm having a bit of a problem getting the helloWorld.hs app up and running on Yesod. Thank you On Feb 24, 2014, at 2:30 PM, beginners-request(a)haskell.org wrote: > Send Beginners mailing list submissions to > beginners(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://www.haskell.org/mailman/listinfo/beginners > or, via email, send a message with subject or body 'help' to > beginners-request(a)haskell.org > > You can reach the person managing the list at > beginners-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Beginners digest..." > > > Today's Topics: > > 1. Failure to install cabal-install. (Derek Mahar) > 2. Re: Failure to install cabal-install. (Derek Mahar) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Mon, 24 Feb 2014 13:20:45 -0500 > From: Derek Mahar <derek.mahar(a)gmail.com> > To: beginners(a)haskell.org > Subject: [Haskell-beginners] Failure to install cabal-install. > Message-ID: > <CABtfiZb7+p=DtGoz57GVXwgHCdK1vPL175UxmirRY1EXM4aEBA(a)mail.gmail.com> > Content-Type: text/plain; charset="utf-8" > > How can I tell the Haskell compiler where to find module > Control.Monad.Error? After building GHC 7.6.3 and Haskell Platform 2013.2 > on Red Hat Linux 6, I found that 'cabal install cabal-install' fails when > it attempts to build library HTTP-4000.2.11 because the Haskell compiler > cannot find module Control.Monad.Error. > > ******************************************************* > * Installation completed successfully. > * > * Programs installed into: > * /usr/local/bin > * > * Now do "cabal update" to initialize the package list > * > * Additional packages may be found at http://hackage.haskell.org > * or via "cabal list <pattern>" > * > * Use "cabal install <foo>" to install additional packages > * > ******************************************************* > [dmahar@dmahar haskell-platform-2013.2.0.0]$ cabal update > Downloading the latest package list from hackage.haskell.org > Note: there is a new version of cabal-install available. > To upgrade, run: cabal install cabal-install > [dmahar@dmahar haskell-platform-2013.2.0.0]$ cabal install cabal-install > Resolving dependencies... > Configuring HTTP-4000.2.11... > Building HTTP-4000.2.11... > Preprocessing library HTTP-4000.2.11... > > Network/Stream.hs:35:8: > Could not find module `Control.Monad.Error' > There are files missing in the `mtl-2.1.2' package, > try running 'ghc-pkg check'. > Use -v to see a list of the files searched for. > Failed to install HTTP-4000.2.11 > cabal: Error: some packages failed to install: > HTTP-4000.2.11 failed during the building phase. The exception was: > ExitFailure 1 > cabal-install-1.18.0.2 depends on HTTP-4000.2.11 which failed to install. > > Thank you, > > Derek >

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make https request with client certificate
by Stanisław Findeisen 23 Feb '14

23 Feb '14

Hi How can you issue a HTTPS request using client X.509 certificate for authentication? Is it supported? -- http://people.eisenbits.com/~stf/ http://www.eisenbits.com/ OpenPGP: 80FC 1824 2EA4 9223 A986 DB4E 934E FEA0 F492 A63B

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Dynamic Loading of Haskell Code!
by Gilberto Melfe 21 Feb '14

21 Feb '14

Hi there All! I would like to ask: Is it possible for a Haskell program to load code while it's running, in order for it to facilitate the implementation of Plugins? (like it's possible in Perl or Python) Can someone enlighten me, please? Thank You Gilberto

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Need help with HXT
by Philippe Sismondi 21 Feb '14

21 Feb '14

Greetings. I am trying to move some very old code from using HaXml to HXT. This is experimental; I have reasons which I think are not relevant to this post. The website that purports to document HXT has broken links. This is Uwe Schmidt's website. I have emailed Dr. Schmidt. Meanwhile, can anyone elucidate the use of readDocument with its withCurl and withHTTP options? What may be passed in the list to e.g. withHTTP? - Phil -

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Type Class Instance Question
by Philippe Sismondi 21 Feb '14

21 Feb '14

In the Haskell 98 and 2010 reports it states that one may declare something of the form (tyvar1 -> tyvar2) to be an instance of a type class. Is that a function or what is it? Can anyone point me to an example of this? TIA. Best, - Phil -

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Print values stored in an "Either" in the main method
by Mathew Phillips 20 Feb '14

20 Feb '14

I have a module that looks like this runEval :: (Show a) => Env -> Eval a -> Either String a runEval = ... data Value = IntVal Integer main :: IO () main = do let env = Map.fromList [("x",IntVal 3)] result <- runEval env (eval (Plus (Var "x") (Lit 2))) case result of Left err -> putStrLn "Error: "++err Right (IntVal i) -> print i when I call runEval from ghci I get back a result just fine let env = Map.fromList [("x",IntVal 3)] runEval env (eval (Plus (Var "x") (Lit 2))) This outputs "Right (IntVal 5)" as I would expect. But when I try and compile my main method I get the following error. Couldn't match expected type `Value' with actual type `Either t1 Value' In the pattern: Right (IntVal i) In a case alternative: Right (IntVal i) -> print i In a stmt of a 'do' block: case result of { Left err -> print err Right (IntVal i) -> print i } I also tried using (putStrLn . either show show) result, but this gave me the following error. Couldn't match expected type `Either a0 b0' with actual type `Value' In the first argument of `putStrLn . either show show', namely `result' In a stmt of a 'do' block: (putStrLn . either show show) result In the expression: do { let env = Map.fromList ...; result <- runEval3 env (eval3 (Plus (Var "x") (Lit 2))); (putStrLn . either show show) result } Why is it when I try and pattern match on Either String Value it says result is of type "Value", but when I try and use either it says result is of type "Either String Value"? Matt P.

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QuickCheck for testing a parsing function
by Thomas Bach 20 Feb '14

20 Feb '14

Hi there, I have the following snippet: data Volume = Volume { name :: String , mount :: String , size :: String , fs :: String , mbr :: Bool } deriving (Show, Eq) defaultVolume :: Volume defaultVolume = Volume { name = "" , mount = "" , size = "" , fs = "ext4" , mbr = False } parseOptVols :: String -> Volume parseOptVols s = parseChain (defaultVolume, s) where parseChain = parseMBR . parseFS . parseSize . parseMount . parseName parseName (vol, s) = (vol {name = getThis s}, getNext s) parseMount (vol, s) = (vol {mount = getThis s}, getNext s) parseSize (vol, s) = (vol {size = getThis s}, getNext s) -- fs and mbr are optional parseFS (vol, "") = (vol, "") parseFS (vol, ',':s) = (vol, s) parseFS (vol, s) = (vol {fs = getThis s}, getNext s) parseMBR (vol, "t") = vol {mbr = True} parseMBR (vol, _ ) = vol getThis = takeWhile (',' /=) getNext = saveTail . dropWhile (',' /=) saveTail :: [a] -> [a] saveTail [] = [] saveTail (_:xs) = xs What it basically does is parse a string (handed over from the command line) like "boot,/boot,256M,ext3,t" and populates a Volume instance with this. The last two fields are optional. I like testing and coming from an object-oriented background unit testing is the most comfortable to me. But I wonder if there is some smart way to make QuickCheck produce test cases. I would need some generator for this which produces a pattern like "name,mount,size,fs,mbr" eventually leaving "fs" and/or "mbr" empty and sometimes omitting both commas or just one of them. And I also need access to these fields in order to be able to compare them with the resulting Volume instance. I can see how to achieve the former somehow, but I don't have any clue how to achieve the latter. Any hints? Or is this simply not a nail for the QuickCheck-hammer? Regards, Thomas Bach. PS: Any advice on how to improve the code above is highly appreciated as well.

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Is a ghc package the same as a cabal package?
by Philippe Sismondi 18 Feb '14

18 Feb '14

Is a ghc package the same as a cabal package? As far as I can tell they are not. I mean "package" as defined by the documentation for each. In the ghc documentation it states that "A package specification is a Haskell record". A Cabal package seems to be a collection of stuff including the .cabal file etc. Finally, does Hackage use the term "package" differently again? Rant: package and/or build systems are the bane of my existence in any non-commercial language that I use. Sigh. I have been using Cabal and cabal-install naively (read "blindly") for long enough; time to come to grips with it. Thanks, if anyone has time to help. - P -

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combine concatMap and mapM
by Renah Scarowsky 17 Feb '14

17 Feb '14

Hi, How would I combine the functionalities of concatMap and mapM to define a function like: Monad m => (a -> m [b]) -> [a] -> m [b] I need to create a recursive function within the IO monad which results in a list of the original type. So something like: goX :: MonadIO m => x -> m [x] goX = do .... x includes within it a list of xs, let's call it (children x) I need to call goX on each of the children and get a modified list of children as result ys <- ? return $ x { children = ys } : zs Thanks, Renah Scarowsky Suite Solutions Create>Manage>Deploy http://www.suite-sol.com<http://www.suite-sol.com/>

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type error in sub-function
by James Toll 16 Feb '14

16 Feb '14

Hi, I am trying to write a simple function to determine the divisors of an integer. In its simplest form the type signature should be something like: divisors :: Int -> [Int] divisors x = 1 : lower ++ upper ++ x : [] where lower = filter (\y -> mod x y == 0) [2..(ceiling . sqrt) x] upper = sort $ map (div x) lower Although, I think my type signature isn’t complete as it ignores a lot of what’s going on in the function. Regardless, the function throws an error when evaluated. I don’t know if I just need a more accurate type signature, or if there is a bigger problem. From what I can tell, my problem is in the lower function, but when I deconstruct it into its parts, they work individually and as a whole, but not as a stand-alone function. For example, using x = 36: Prelude> [2..(ceiling . sqrt) 36] [2,3,4,5,6] Prelude> filter (\y -> mod 36 y == 0) it [2,3,4,6] Or as a whole: Prelude> filter (\y -> mod 36 y == 0) [2..(ceiling . sqrt) 36] [2,3,4,6] But when I define this as a function, it throws an error when evaluated. Prelude> let lower x = filter (\y -> mod x y == 0) [2..(ceiling . sqrt) x] Prelude> lower 36 <interactive>:6:1: No instance for (RealFrac b0) arising from a use of `lower' The type variable `b0' is ambiguous Possible fix: add a type signature that fixes these type variable(s) Note: there are several potential instances: instance RealFrac Double -- Defined in `GHC.Float' instance RealFrac Float -- Defined in `GHC.Float' instance Integral a => RealFrac (GHC.Real.Ratio a) -- Defined in `GHC.Real' In the expression: lower 36 In an equation for `it': it = lower 36 <interactive>:6:7: No instance for (Num b0) arising from the literal `36' The type variable `b0' is ambiguous Possible fix: add a type signature that fixes these type variable(s) Note: there are several potential instances: instance Num Double -- Defined in `GHC.Float' instance Num Float -- Defined in `GHC.Float' instance Integral a => Num (GHC.Real.Ratio a) -- Defined in `GHC.Real' ...plus three others In the first argument of `lower', namely `36' In the expression: lower 36 In an equation for `it': it = lower 36 At this point, I’m not sure what I need to do to get his working properly. It appears to be a type error, but I’m apparently not understanding the error message enough to fix the problem. Any suggestions would be appreciated. Thanks, James

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