October 2011 - Beginners

Generating Code To Embed in an Event Loop
by Darrin Thompson 26 Oct '11

26 Oct '11

I've been thinking about how I could use Haskell for programming just the "business logic" for games. So for a mobile game I might use an existing library such as libgdx or cocos2d for input handling and sprite batching. Where I'd like to use Haskell is for writing the big function to process the inputs and output a sprite list and audio triggers. Why Haskell? In even a simple 2d platformer there's a lot of polish needed to get the game to feel good to a player. For instance, if I was using box2d physics to take care of movement and collisions, I might still need to detect that inputs indicate applying a force vector opposite to the player's current velocity and the player is "on the floor". A naive game would show the player moon walking. A more sophisticated game would show a skidding player sprite or animation until the player's velocity reaches 0. I was hoping to use Haskell to capture a lot of little details like that more elegantly than I can in C or Java. However, my program needs to be compiled for a mobile device. I'm wary of trying to use GHC to generate arm machine code. I've had bad luck with cross compilers. It seemed to me that I could design a GADT to model my game actors and their relationships. I would then (misused term I'm sure) "lift" my behavior functions into some context where they can produce Java or C code for use in Xcode or Eclipse. That's my favored approach. Is there some background I could look at to save myself some dead ends pursuing this? -- Darrin

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Forcing output of show
by Hugo Ferreira 26 Oct '11

26 Oct '11

Hello, Apologies for this newbie question. I have a test function: testLearnRules train = do h <- IO.openFile train IO.ReadMode c <- IO.hGetContents h let proposedRules = instRules $ words c let rs = take 10 $ M.assocs proposedRules let _ = show rs -- let _ = length rs return () I realize that nothing is executed due to Haskell's lazy evaluation. How can I force show to print something to the screen? Appreciate any pointers, TIA, Hugo F.

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When to use ST
by Alex Rozenshteyn 26 Oct '11

26 Oct '11

Does anyone have any guidelines on when to use ST in an algorithm vs looking for a non-monadic, pure solution? The reason I ask is that I have just implemented an Earley parser for class (in Python) and would like to re-implement it in Haskell. The algorithm makes heavy use of mutable state, some of which can clearly be refactored out, but some of which can't be done so clearly. Thanks. -- Alex R

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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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