- Beginners - Haskell.org

Backtrace when a certain location in the code is executed
by Nathan Hüsken 17 Aug '12

17 Aug '12

Hey, I am trying to understand haskell program not written by me. During runtime one function is called with parameters which makes the function throw an error. I want to know from where the function is called. In c++ I would set a breakpoint on the error throwing code, and let gdb print the backtrace. To I have similar options in haskell? What other options do I have? Thanks! Nathan

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99problems: different response from ghci <--> runhaskell / ghc
by GiGi 17 Aug '12

17 Aug '12

Hi all, trying to solve the problem 6 "reverse a list" I'm getting a different behaviour from ghci / runhaskell. main = do putStrLn $ concat (map show (myReverse [1,2,3])) -- putStrLn $ concat (map show (myReverse [])) myReverse :: [a] -> [a] myReverse [] = [] myReverse (x:xs) = go [] (x:xs) where go :: [a] -> [a] -> [a] go curr (x:xs) | null xs = [x] ++ curr go curr (x:xs) = go ([x] ++ curr) xs the second putStrLn (once uncommented) gives this error in the runhaskell/ghc: Ambiguous type variable `a0' in the constraint: (Show a0) arising from a use of `show' Probable fix: add a type signature that fixes these type variable(s) In the first argument of `map', namely `show' _but_ if I load the same source into ghci I can successfully enter&run the command: Prelude> :reload [1 of 1] Compiling Main ( 05.hs, interpreted ) Ok, modules loaded: Main. *Main> putStrLn $ concat (map show (myReverse [])) *Main> Then if I "help" the type inference changing the myReverse type declaration to: myReverse :: [Int] -> [Int] everything works well So, my question is: _where_ is the difference between ghci / runhaskell? thx

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Re: [Haskell-beginners] cabal install errors & issues
by Gregory Guthrie 17 Aug '12

17 Aug '12

Thanks for the advice and pointers, I will try to make this transition, but it looks like it is not so simple. Trying to bootstrap into cabal-dev seems to require some external installations as well; >>cabal install cabal-dev --force-reinstalls ... Configuring network-2.3.0.14... cabal: The package has a './configure' script. This requires a Unix compatibility toolchain such as MinGW+MSYS or Cygwin. cabal: Error: some packages failed to install: HTTP-4000.2.3 depends on network-2.3.0.14 which failed to install. cabal-dev-0.9.1 depends on network-2.3.0.14 which failed to install. network-2.3.0.14 failed during the configure step. The exception was: ExitFailure 1 I do have MinGW+MSYS installed, might be a PATH issue, I'll check further. I don't understand the details of Haskell package management, and the implications of this description of cabal-dev: For installed packages, the sandboxing means that packages are not registered into the user or global ghc package database. The global package db is used, so it is recommended that the global package db is only used for the ghc core libraries. This approach conflicts with using distribution packages for non-core libraries, because they are installed into the global db. It seems odd to me to say that "they are not registered.." into either database, and then say the global package db is used. Is there some distinction here between a "global ghc package db" & a "global package db"?. Cabal install hsenv shows "no such package", and I'm not sure why using it with installs would be good. Already removing shadowing packages is breaking things, so more cleanup will also be needed for that. I am trying to convert some SML classes and labs for my students to Haskell, and all of this overhead is certainly something that I couldn't wish on them! Perhaps if I have them all start with cabal-dev none of this would happen? ------------------------------------------- > > Please see this: > >> http://ivanmiljenovic.wordpress.com/2010/03/15/repeat-after-me-cabal- > >> is-not-a-package-manager/ > > > > thanks Benjamin, for the cabal-dev, hsenv tip though.

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Re: [Haskell-beginners] cabal install errors & issues
by Gregory Guthrie 17 Aug '12

17 Aug '12

Thanks for the clarification and info. >From this I then deduce: Using "cabal install" is dangerous, and leads to broken package structure (inconsistent package versions and shadowing). Better to use cabal-dev, But it requires network, which is external to Haskell, and a pain to build (on windows)... Thus, no good solution on Windows - is this right? Since Windows is 94%+ of computing environments today (all of our labs use Windows, and "standard" IDEs), then Haskell is not a good fit? We want to use Windows since that is what students will generally see in their professional environments. I hope not...!! :-) ------------------------------------------------------- >> Ahh, I think network is a pain to build on Windows. >> I don't as a rule do any haskell on windows because of the headaches of msys / mingw.

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convert to fixed length Int
by Christopher Howard 16 Aug '12

16 Aug '12

Hi. In Haskell, how would I go about converting from a Double (or Float) type to a Data.Int.Int8, Data.Int.Int16, and so forth? (Of course, expecting rounding and damage to out of range values.) I was playing around with signal generation using trig functions (hence the floating types) but the actual output has to be Int16. -- frigidcode.com indicium.us

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source code for "Generics for the Masses"
by Michael Litchard 16 Aug '12

16 Aug '12

In the paper by Ralf Hinze, "Generics for the Masses", there is reference to code one may play with. But the link, http://www.ralf-hinze.de/masses.tar.bz2 is dead. I know someone, somewhere has this file. Could you point me to where it lives?

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Re: [Haskell-beginners] cabal install errors
by Gregory Guthrie 15 Aug '12

15 Aug '12

I didn't get any concrete remedy for this, or its underlying cause. Does it mean that this package is incompatible with others currently installed, and I am supposed to make a choice to uninstall or break them by forcing this, or ...??? ------------------------------------------- > I tried this: > C:\Users\haskell>cabal install regex-tdfa > Resolving dependencies... > In order, the following would be installed: > regex-base-0.93.2 (reinstall) changes: array-0.3.0.2 -> 0.4.0.0, base-4.3.1.0 > -> 4.5.0.0, bytestring-0.9.1.10 -> 0.9.2.1, containers-0.4.0.0 -> 0.4.2.1, > mtl-2.0.1.0 -> 2.1.2 > regex-tdfa-1.1.8 (new package) > cabal: The following packages are likely to be broken by the reinstalls: > regex-posix-0.95.1 > regex-compat-0.95.1 > haskell-platform-2011.3.0.0 > regex-posix-0.94.4 > regex-compat-0.93.1 > haskell-platform-2011.2.0.1 > regex-posix-0.95.1 > regex-compat-0.95.1 > haskell-platform-2012.2.0.0 > Use --force-reinstalls if you want to install anyway. > > So what to do? > >>cabal -V > cabal-install version 0.14.0 > using version 1.14.0 of the Cabal library GHCi version 7.4.1 > HaskellPlatform-2012.2.0.0 > -------------------------------------------

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cleanest way to unwrap a list?
by Christopher Howard 14 Aug '12

14 Aug '12

Hi. Is the some generic, clean syntax to unwrap a nested list, modify the value, and put it back together? Say, for example, I have the list [[1,2],[3,4]] and want to add 1 to each inner element, resulting in [[2,3],[4,5]]. After reading about the list monad, I was rather excited, because I (mistakenly) thought something like this would work: code: -------- a = do b <- [[1,2],[3,4]] c <- b return (c + 1) -------- That would be awesome, because I would be able to modify the list at each level of unwrapping, while leaving the code very neat and readable. However, what the above example actually does is produce a /single/ list from the values: code: -------- *Main> a [2,3,4,5] -------- Obviously wishing won't change how the list monad works, but I thought it might be worth asking if there is some other monad or syntactic trick that does something along the lines of what I am looking for. -- frigidcode.com indicium.us

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"Couldn't match expected type" and simple "Couldn't match type"
by Ken Kawamoto 12 Aug '12

12 Aug '12

Hi Haskellers, I found GHC sometimes complains "Couldn't match expected type X with actual type Y" , and simply "Couldn't match type X with Y" in other cases. not a big deal, but was wondering what makes this difference. -- Ken

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Cabal error
by Gregory Guthrie 12 Aug '12

12 Aug '12

I get the following Cabal install error: C:\Users\haskell>cabal install buildwrapper Resolving dependencies... Configuring regex-tdfa-1.1.8... Warning: This package indirectly depends on multiple versions of the same package. This is highly likely to cause a compile failure. If true, doesn't this mean that the regex-tdfa package has misconfigured dependencies? -------------------------------------------

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