- Beginners - Haskell.org

Re: [Haskell-beginners] problem installing checkers
by Britt Anderson 25 Oct '11

25 Oct '11

> Message: 10 > Date: Sun, 23 Oct 2011 21:42:20 -0400 > From: Brent Yorgey <byorgey(a)seas.upenn.edu> > Subject: Re: [Haskell-beginners] problem installing checkers > To: beginners(a)haskell.org > Message-ID: <20111024014220.GB26246(a)seas.upenn.edu> > Content-Type: text/plain; charset=us-ascii > > On Sun, Oct 23, 2011 at 06:56:57PM -0400, Britt Anderson wrote: >> I wanted to try reactive, but cabal install fails on checkers (a >> dependency). Here is a portion of the error message, there are other >> overlapping instances that follow. Can someone advise me how to >> proceed. Thank you. >> >> [13 of 15] Compiling Test.QuickCheck.Classes ( >> src/Test/QuickCheck/Classes.hs, dist/build/Test/QuickCheck/Classes.o ) >> >> src/Test/QuickCheck/Classes.hs:137:38: >> Overlapping instances for Show (a -> b) >> arising from a use of `property' >> Matching instances: >> instance Show (a -> b) -- Defined in Text.Show.Functions >> instance Show base:System.Event.Manager.IOCallback >> -- Defined in base:System.Event.Manager > > This is very naughty of System.Event.Manager, to define IOCallback as > a *type synonym* for a certain function type, and then declare a Show > instance for it. However, the instance shouldn't come into play > unless System.Event.Manager is imported, and I don't think it is. So > this error message seems rather odd to me. What version of GHC do you > have? What is the output of 'ghc-pkg list' and 'ghc-pkg check'? > > -Brent > Here is the print out of the last part of the error message when doing sudo cabal install reactive --global followed by the ghc-pkg list and ghc-pkg check functions. Thank you, ... src/Test/QuickCheck/Classes.hs:514:37: Overlapping instances for Show (a -> m) arising from a use of `property' Matching instances: instance Show (a -> b) -- Defined in Text.Show.Functions instance Show base:System.Event.Manager.IOCallback -- Defined in base:System.Event.Manager (The choice depends on the instantiation of `a, m' To pick the first instance above, use -XIncoherentInstances when compiling the other instance declarations) In the expression: property foldMapP In the expression: ("foldMap", property foldMapP) In the expression: [("fmap", property fmapP), ("foldMap", property foldMapP)] cabal: Error: some packages failed to install: category-extras-0.53.5 failed during the building phase. The exception was: ExitFailure 1 checkers-0.2.8 failed during the building phase. The exception was: ExitFailure 1 reactive-0.11.5 depends on checkers-0.2.8 which failed to install. [britt@britt-fr07 ~]$ ghc-pkg list /usr/lib/ghc-7.0.3/package.conf.d Boolean-0.0.1 Cabal-1.10.1.0 GLFW-0.4.2 GLURaw-1.1.0.0 HTTP-4000.1.1 HUnit-1.2.2.3 MemoTrie-0.4.10 MissingH-1.1.0.3 ObjectName-1.0.0.0 OpenGL-2.4.0.1 OpenGLRaw-1.1.0.1 QuickCheck-2.4.1.1 StateVar-1.0.0.0 Stream-0.4.2 Tensor-1.0.0.1 TypeCompose-0.8.3 X11-1.5.0.0 array-0.3.0.2 base-4.3.1.0 base64-bytestring-0.1.0.3 bin-package-db-0.0.0.0 binary-0.5.0.2 bytestring-0.9.1.10 citeproc-hs-0.3.2 containers-0.4.0.0 deepseq-1.1.0.2 digest-0.0.0.9 directory-1.1.0.0 dlist-0.5 extensible-exceptions-0.1.1.2 ffi-1.0 filepath-1.2.0.0 ghc-7.0.3 ghc-binary-0.5.0.2 ghc-prim-0.2.0.0 haskell2010-1.0.0.0 haskell98-1.1.0.1 hpc-0.5.0.6 hs-bibutils-4.12 hslogger-1.1.5 integer-gmp-0.2.0.3 json-0.4.4 lazysmallcheck-0.5 mtl-2.0.1.0 network-2.3.0.5 old-locale-1.0.0.2 old-time-1.0.0.6 pandoc-1.8.2.1 pandoc-types-1.8.2 parsec-3.1.1 pretty-1.0.1.2 process-1.0.1.5 random-1.0.0.3 regex-base-0.93.2 regex-compat-0.95.1 regex-posix-0.95.1 rts-1.0 syb-0.3 tagsoup-0.12.2 template-haskell-2.5.0.0 texmath-0.5.0.1 text-0.11.1.5 time-1.2.0.3 transformers-0.2.2.0 unamb-0.2.4 unix-2.4.2.0 utf8-string-0.3.6 vector-space-0.7.3 xhtml-3000.2.0.1 xml-1.3.9 zip-archive-0.1.1.7 zlib-0.5.3.1 [britt@britt-fr07 ~]$ ghc-pkg check [britt@britt-fr07 ~]$

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Are these statements about Haskell correct?
by Costello, Roger L. 25 Oct '11

25 Oct '11

Hi Folks, Are these statements accurate: 1. The Haskell language is a layer of abstraction on top of Lambda Calculus. 2. Haskell programs may be expressed entirely using Lambda expressions. 3. The non-Lambda stuff in Haskell is syntactic sugar to make Haskell programs easier to write. 4. The field of Lambda Calculus is formal and rich and enables powerful properties of Lambda expressions to be proven. 5. A Haskell program may be translated to consist entirely of Lambda expressions. Then, Lambda Calculus may be employed to prove powerful properties about the program. /Roger

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List of all prelude function types?
by Patrick LeBoutillier 24 Oct '11

24 Oct '11

Hi, Does anyone know where I can find a list of types for all Prelude functions? Something like: all :: (a -> Bool) -> [a] -> Bool and :: [Bool] -> Bool ... zip :: [a] -> [b] -> [(a, b)] Thanks, Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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problem installing checkers
by Britt Anderson 24 Oct '11

24 Oct '11

I wanted to try reactive, but cabal install fails on checkers (a dependency). Here is a portion of the error message, there are other overlapping instances that follow. Can someone advise me how to proceed. Thank you. [13 of 15] Compiling Test.QuickCheck.Classes ( src/Test/QuickCheck/Classes.hs, dist/build/Test/QuickCheck/Classes.o ) src/Test/QuickCheck/Classes.hs:137:38: Overlapping instances for Show (a -> b) arising from a use of `property' Matching instances: instance Show (a -> b) -- Defined in Text.Show.Functions instance Show base:System.Event.Manager.IOCallback -- Defined in base:System.Event.Manager (The choice depends on the instantiation of `a, b' To pick the first instance above, use -XIncoherentInstances when compiling the other instance declarations) In the expression: property binopP In the expression: ("binop", property binopP) In the expression: [("identity", property identityP), ("binop", property binopP)]

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How do I determine if a list created using a lambda expression is null?
by Costello, Roger L. 24 Oct '11

24 Oct '11

Hi Folks, I have a lambda expression that can be used to construct a list of values: cons = (\a -> (\b -> (\f -> f a b))) Here is an example that uses cons to construct "hello" hello = cons 'h' (cons 'e' (cons 'l' (cons 'l' 'o'))) And here is a lambda expression that returns the tail of a list created using cons: tail' = (\c -> c (\a -> (\b -> b))) I would like to create a function that computes the length of a cons-created list: length' = \xs -> if ( null' xs ) then 0 else length' (tail' xs) + 1 How would I define null' to determine if a cons list is empty? null' = ??? /Roger

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Newbie unicode utf8 question
by Manfred Lotz 23 Oct '11

23 Oct '11

Hi there, I want to read a unicode file and print it to screen. My terminal was unintentionally set to POSIX locale and that was the reason that it didn't work properly. When I added two hSetEncoding statements it worked fine, even under POSIX. Test file uc.in contains a simple line Möbius Château Here is the small test program I tried: <-----------------------------snip----------------------------> module Main where import System.IO import Text.Printf main :: IO () main = do h <- openFile "uc.in" ReadMode hSetEncoding h utf8 hSetEncoding stdout utf8 s <- hGetContents h print s putStrLn $ "File contains line: " ++ s printf "File contains line: %s\n" s <-----------------------------snap----------------------------> Both putStrLn, and printf work fine. When setting a different locale, e.g. en_GB.iso88591 then both putStrLn and printf work fine even if I haven't set encoding to utf in the source. Then I discovered System.IO.UTF8. However, when I use this as below then the output is garbled. <-----------------------------snip----------------------------> module Main where import System.IO import System.IO.UTF8 as U import Text.Printf main :: IO () main = do h <- openFile "uc.in" ReadMode hSetEncoding stdout utf8 s <- U.hGetContents h U.print s U.putStrLn $ "File contains line: " ++ s printf "File contains line: %s\n" s <-----------------------------snap----------------------------> Any idea why this won't work. -- Manfred

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Placement in module hierarchy
by Moritz Fischer 23 Oct '11

23 Oct '11

Hi list, I've been working on bindings for pcsclite. The library allows for communicating with smartcard readers and RFID. (My final goal is to hook it up to talk to a Mifare DESFire RFID card.) Where would you place something like this? System.Smartcard? Cheers & happy hacking Moritz Btw: If you want to have a look at what I did so far (just basic functionality), check out http://github.com/mfischer/haskell-smartcard. I'm open for comments on possible improvements.

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slightly scramble list randomly
by Dennis Raddle 22 Oct '11

22 Oct '11

I want to take a list of size N that starts in ascending order and "slightly randomly scramble it"--- each element is moved within M steps of its original position (there can be some tolerance for slightly over M). The application is a backtracking algorithm that produces random solutions but tends to favor certain choices--at each node of the search tree I can evaluate the goodness of each possible next step, but I don't want the algorithm to always take the best choice. Just roughly, much of the time. Dennis

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[Yesod] How can I dynamically generate selectFields
by Michael Litchard 21 Oct '11

21 Oct '11

Link to details http://stackoverflow.com/questions/7854923/how-can-i-dynamically-generate-s…

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Unresponsive interactive GHC using Leksah
by Hugo Ferreira 21 Oct '11

21 Oct '11

Hello, I am trying out the code in the book http://nlpwp.org/. When testing the code in chapter 7 (POS tagger), the interactive session consumes much memory and the whole system becomes unresponsive (Ubuntu 10.04, Dual Core, 2 Gig memory). I even find it difficult to kill the process. If I execute a ghci session I see very large memory (1.6Gig) use and relative slow execution. However the system does not become as unresponsive. If I run the executable directly the execution goes from minutes to seconds. Anyone know if I can tweak GHCi and/or leksah to avoid or simple reduce this problem? TIA, Hugo F.

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