December 2010 - Beginners

How to think in Haskell
by Jun HU 21 Dec '10

21 Dec '10

Dear everyone, My first language is C, and I've strong intention in learning a pure functional programming language. My very first question is how to think in the functional programming way, anyone has some ideas. Really appreciate.... Regards, Jun

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Forum vs mailing list
by Amy de Buitléir 21 Dec '10

21 Dec '10

On 21 December 2010 12:42, David Jameson <dhjdhj(a)gmail.com> wrote: > P.S. By the way, is is not possible to have a forum instead of a mailing list --- these mailing lists are so 19th century! If you use an face like GMANE, then you'll get the best of both worlds, plus a bit more. Youll be able to post messages using a forum-like interface. And you don't have to be subscribed to the mailing list to post through GMANE, so if you don't want your inbox filling up, simply unsubscribe. You'll still be able to read all the other posts, but you'll have to go to the GMANE page to do so. Look here for more information: http://dir.gmane.org/gmane.comp.lang.haskell.beginners

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Uncalled for apology.
by Sean Charles 21 Dec '10

21 Dec '10

I felt compelled to PM Paul about his email. In support. If this list is to be vetted and thought-controlled by self-imposed politically correct Ministry of Thought operatives then I am leaving today. It's a shame that the mindset required to understand Haskell is still open to petty thinking and the desire to impose its thoughts upon others. Freedom of speech, thought and expression. Well, that's ruined my Christmas I think. What is it about human beings that makes them unable to accept what somebody else has to say without wanting to pontificate ? Sigh, and it was such a nice list too... :(

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Standard Haskell Module Files
by Jane Ren 21 Dec '10

21 Dec '10

Hi, Could someone tell me where I can get the standard Haskell module files such as source code for the Data module? Thanks

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Equivalent of inheritance in Haskell
by C K Kashyap 21 Dec '10

21 Dec '10

Hi, In oops, one can do reuse code by extending a parent class. What is the equivalent in Haskell? Regards, Kashyap

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haskell / prolog
by dan portin 21 Dec '10

21 Dec '10

I recently discovered that class and instance declarations in Haskell are Horn clauses. So I encoded the arithmetic operations from the first chapter of *Art of Prolog* into Haskell, in the standar fac(0,s(0)). fac(s(N),F) :- fac(N,X), mult(s(N),X,F). class Fac x y | x -> y instance Fac Z (S Z) instance (Fac n x, Mult (S n) x f) => Fac (S n) f I don't understand, however, to make Haskell instantiate values for variables in a proof. I.e., I don't understand what the Haskell equivalent of ?- fac(X,Y) would be for some value of X.

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Include external libs on cabal project
by Marco De Oliveira 20 Dec '10

20 Dec '10

Hi, I try to include an external lib (and C files that I made) on my cabal file and when I launch "configure" I have errors. I have added on my file: extra-libraries: "winscard" include-dirs: "C:\Program Files\Microsoft SDKs\Windows\v7.1\Include" extra-lib-dirs: "C:\Program Files\Microsoft SDKs\Windows\v7.1\Lib" and it fails because of the space on the path. (see logs) Is the syntax correct? What I have to change so that cabal can understand the space on directory path? Thanks for help Cabal version: 1.8.0.6 log after "runhaskell setup.lhs configure -v3" ("C:\\Program Files\\Haskell Platform\\2010.2.0.0\\mingw\\bin\\gcc.exe",["C:\\Us ers\\E6400\\AppData\\Local\\Temp\\8104.c","-o","C:\\Users\\E6400\\AppData\\Local \\Temp\\8104","-D__GLASGOW_HASKELL__=612","-Idist\\build\\autogen","-ISmartCard/ libpcsc/inc",*"-I\"C:\\Program","-IFiles\\Microsoft","-ISDKs\\Windows\\v7.1\\Incl ude\"*","-I.","-IC:\\Program Files\\Haskell Platform\\2010.2.0.0\\lib\\base-4.2.0 .2\\include","-IC:\\Program Files\\Haskell Platform\\2010.2.0.0\\lib/include","- IC:\\Program Files\\Haskell Platform\\2010.2.0.0\\lib/include","-lwinscard",*"-L\ "C:\\Program","-LFiles\\Microsoft","-LSDKs\\Windows\\v7.1\\Lib\"*","-LC:\\Program Files\\Haskell Platform\\2010.2.0.0\\lib\\base-4.2.0.2","-LC:\\Program Files\\H askell Platform\\2010.2.0.0\\lib\\integer-gmp-0.2.0.1","-LC:\\Program Files\\Has kell Platform\\2010.2.0.0\\lib\\ghc-prim-0.2.0.0","-LC:\\Program Files\\Haskell Platform\\2010.2.0.0\\lib","-LC:\\Program Files\\Haskell Platform\\2010.2.0.0\\l ib/gcc-lib","-LC:\\Program Files\\Haskell Platform\\2010.2.0.0\\lib"]) C:\Program Files\Haskell Platform\2010.2.0.0\mingw\bin\gcc.exe returned ExitFailure 1 with error message: C:\Program Files\Haskell Platform\2010.2.0.0\mingw\bin\ld.exe: cannot find -lwinscard collect2: ld returned 1 exit status ("C:\\Program Files\\Haskell Platform\\2010.2.0.0\\mingw\\bin\\gcc.exe",["C:\\Us ers\\E6400\\AppData\\Local\\Temp\\8104.c","-o","C:\\Users\\E6400\\AppData\\Local \\Temp\\8104","-lwinscard","*-L\"C:\\Program","-LFiles\\Microsoft","-LSDKs\\Windo ws\\v7.1\\Lib\"",*"-LC:\\Program Files\\Haskell Platform\\2010.2.0.0\\lib\\base-4 .2.0.2","-LC:\\Program Files\\Haskell Platform\\2010.2.0.0\\lib\\integer-gmp-0.2 .0.1","-LC:\\Program Files\\Haskell Platform\\2010.2.0.0\\lib\\ghc-prim-0.2.0.0" ,"-LC:\\Program Files\\Haskell Platform\\2010.2.0.0\\lib","-LC:\\Program Files\\ Haskell Platform\\2010.2.0.0\\lib/gcc-lib","-LC:\\Program Files\\Haskell Platfor m\\2010.2.0.0\\lib"]) C:\Program Files\Haskell Platform\2010.2.0.0\mingw\bin\gcc.exe returned ExitFailure 1 with error message: C:\Program Files\Haskell Platform\2010.2.0.0\mingw\bin\ld.exe: cannot find -lwinscard collect2: ld returned 1 exit status ("C:\\Program Files\\Haskell Platform\\2010.2.0.0\\mingw\\bin\\gcc.exe",["C:\\Us ers\\E6400\\AppData\\Local\\Temp\\8104.c","-o","C:\\Users\\E6400\\AppData\\Local \\Temp\\8104","-lwinscard","-L\"C:\\Program","-LFiles\\Microsoft","-LSDKs\\Windo ws\\v7.1\\Lib\"","-LC:\\Program Files\\Haskell Platform\\2010.2.0.0\\lib\\base-4 .2.0.2","-LC:\\Program Files\\Haskell Platform\\2010.2.0.0\\lib\\integer-gmp-0.2 .0.1","-LC:\\Program Files\\Haskell Platform\\2010.2.0.0\\lib\\ghc-prim-0.2.0.0" ,"-LC:\\Program Files\\Haskell Platform\\2010.2.0.0\\lib","-LC:\\Program Files\\ Haskell Platform\\2010.2.0.0\\lib/gcc-lib","-LC:\\Program Files\\Haskell Platfor m\\2010.2.0.0\\lib"]) C:\Program Files\Haskell Platform\2010.2.0.0\mingw\bin\gcc.exe returned ExitFailure 1 with error message: C:\Program Files\Haskell Platform\2010.2.0.0\mingw\bin\ld.exe: cannot find -lwinscard collect2: ld returned 1 exit status ("C:\\Program Files\\Haskell Platform\\2010.2.0.0\\mingw\\bin\\gcc.exe",["C:\\Us ers\\E6400\\AppData\\Local\\Temp\\8104.c","-o","C:\\Users\\E6400\\AppData\\Local \\Temp\\8104","-c","-D__GLASGOW_HASKELL__=612","-Idist\\build\\autogen","-ISmart Card/libpcsc/inc",*"-I\"C:\\Program","-IFiles\\Microsoft","-ISDKs\\Windows\\v7.1\ \Include\*"","-I.","-IC:\\Program Files\\Haskell Platform\\2010.2.0.0\\lib\\base- 4.2.0.2\\include","-IC:\\Program Files\\Haskell Platform\\2010.2.0.0\\lib/includ e","-IC:\\Program Files\\Haskell Platform\\2010.2.0.0\\lib/include"]) Setup.lhs: Missing dependency on a foreign library: * Missing C library: winscard This problem can usually be solved by installing the system package that provides this library (you may need the "-dev" version). If the library is already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where it is.

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How to "insert" a cross reference into a tree structure
by Tim Baumgartner 19 Dec '10

19 Dec '10

Hi Haskellers, since I learned the basics of algebraic data types, I have a question. First I thought I could answer it after getting a little more used to Haskell's type system. But now I prefer to ask here. Given the following example code, how can I "insert" a cross reference from the track "Money for Nothing" to the artist "Sting" who sang the background vocals? I know it must be a new MusicCollection, but I'd like to know the shortest general code to construct it. data MusicCollection = MusicCollection [Artist] deriving (Show) data Artist = Artist String [Album] deriving (Show) data Album = Album String [Track] deriving (Show) data Track = Track String [Artist] -- featured Artists are cross referenced, so Show cannot be derived collection = MusicCollection [direStraits, sting] direStraits = Artist "Dire Straits" [brothersInArms] brothersInArms = Album "Brothers in Arms" [moneyForNothing] moneyForNothing = Track "Money for Nothing" [] Thanks in advance Tim

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RandT
by Amy de Buitléir 19 Dec '10

19 Dec '10

The example below shows part of the architecture I'm using for an alife project. Q1: My implementation of "simulateBrain" seems clumsy. Is there a better way to do this? Q2: The "Animal" type includes a "brain" component. I implemented that as a simple list, but perhaps it would be better to use a monad here? I tried doing that, but I couldn't figure out the syntax. The closest I got was when I defined a "type Brain g a = (RandomGen g) => RandT g (State [Neuron]) a". But that a specifies a result type, which doesn't make sense to me for a record component. Thank you, Amy ----- SAMPLE CODE ----- {-# LANGUAGE PackageImports, RankNTypes, FlexibleContexts #-} import "mtl" Control.Monad.State import Control.Monad.Random type Neuron = Int -- The real type is more complex -- An "Alife" animal data Animal = Animal { brain :: [Neuron] -- There are other fields too, of course } deriving (Show, Read) -- | Stimulates an animal's brain, and allows it to react. stimulateBrain :: (RandomGen g) -- | The number of cycles => Int -- | The signals to apply to the sensor neurons -> [Double] -- | The animal -> RandT g (State Animal) () stimulateBrain n xs = do c <- get g <- getSplit let b' = execState (evalRandT (stimulate n xs) g) (brain c) put $ c{brain=b'} -- | Feeds some input signals into a brain, and allow it to react. stimulate :: (RandomGen g) -- | The number of cycles => Int -- | The signals to apply to the sensor neurons -> [Double] -- | The neuron states -> RandT g (State [Neuron]) () stimulate k xs = return () -- The real implementation is more complex

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Can I force cabal to install a package always to be installed with --enable-library-profiling
by Daniel Seidel 18 Dec '10

18 Dec '10

Hi, I want to create a cabal package and want to enforce via the descriptions in the .cabal file that it is always installed with --enable-library-profiling. Unfortunately I couldn't figure this out from the cabal documentation. I found how I can enforce that my cabal always installs profiling versions, but not if and how I can enforce that one special package is installed that way by any cabal installation. Is this possible, and if how? Cheers, Daniel.

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