March 2012 - Beginners

constant set
by Ovidiu Deac 10 Mar '12

10 Mar '12

If I want to define a constant set in Python I would do this: class Colors: red = 1 blue=2 ... and then I use them like this: Colors.red, Colors.blue... Can I do something similar in Haskell? thanks

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Problem with developing NCurses program
by Aditya Manthramurthy 10 Mar '12

10 Mar '12

HI, I am learning haskell by writing small programs. In this instance I want to write a program using the haskell binding of the NCurses library. Here is the (rather simple) code: import UI.NCurses main = do putStrLn "Hello World" When I try to run it: $ runhaskell cursesprog.hs cursesprog.hs: <command line>: can't load .so/.DLL for: ncursesw (/usr/lib/libncursesw.so: file too short) The file certainly exists but is only 32 bytes. I installed from Ubuntu's package manager. I am working on a 64 bit machine (if that helps). Can someone help me fix this? Thanks, Aditya.

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tuple update
by Ovidiu Deac 09 Mar '12

09 Mar '12

Does anybody know if there are any functions like these in the Haskell libray? updateSnd f (x,y) -> (x, f y) updateFst f (x,y) -> (f x, y) Thanks, ovidiu

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String type hello-o-matic...
by Emacs The Viking 09 Mar '12

09 Mar '12

Aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaagh! Ok, that said, now I can explain my self a little more clearly. I am almost to the point of exasperation on trying to know when and why and how to convert between String, ByteString, ByteString.Laz, Char8 etc. For heavens sake how many bloody string types can you need? LOL I am trying to use Data.Digest.Pure.SHA but it takes a ByteString in the "sha1" function but from the command prompt of ghci I cannot seem to work out how to do it, I just get the usual daunting messages. sha1 :: ByteString -> Digest SHA1State I have a String, but the pack function wants a [Word8] to create a digest value. pack :: [Word8] -> ByteString IIUC, String is just [Char] and that Char is in fact a Unicode character and therefore NOT "8 bit" as such which is why the Char8 variants exist. I understand the difference between lazy and strict and why one is different from the other. But... it's little things like the foldr' function NOT being present in one library but present in the other, presumably a lazy implementation doesn't need a strict function or something like that. Who knows. I don't! I am sure to "seasoned" haskellers this is all elementary and I would love to eventually become a seasoned developer of it too but at times like this I want to scream in utter confusion as to how many different string types there are and the sometimes almost unnotcable differences between them. And trying to convert between them to actually get on and write useful working code. When do I use strict? When do I use lazy? When do I use the Char8 variant? And so the questions continue with barely any "beginner" level answers to be found. I'd write them myself but I don't know the answers yet! Speaking as a non-academic, non-mathemetician, but as a developer with 25+ years of using lots of languages inclusing Erlang, LISP and Scala, sometimes Haskell is very very hard to perservere with because everybody just seems to love using words and phrases like "mono-morphism", "monadic transformers" etc. just to show off how well they understand Haskell. That's fine for those in the club but those on the outside looking in would also like to be able to "get in" as well and be able to write serious looking blog posts about how damned funky haskell is. And it is... but not if you can't understand it! is there some kind of Free Masonry involved ;) Thanks again, Sean... now where's my RWH book again...

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Why am I having such weird dependency issues?
by Tom Murphy 08 Mar '12

08 Mar '12

Neither of these errors makes any sense to me. What's happening? $ cabal update Downloading the latest package list from hackage.haskell.org $ cabal install yi Resolving dependencies... cabal: dependencies conflict: ghc-7.0.3 requires containers ==0.4.0.0 however containers-0.4.0.0 was excluded because ghc-7.0.3 requires containers ==0.4.2.0 $ cabal install hpath Resolving dependencies... cabal: cannot configure HPath-0.0.2. It requires base >=2 && <=4 For the dependency on base >=2 && <=4 there are these packages: base-3.0.3.1 and base-3.0.3.2. However none of them are available. base-3.0.3.1 was excluded because of the top level dependency base -any base-3.0.3.2 was excluded because of the top level dependency base -any

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Installing Haskell on Lion with Homebrew...
by Stuart Hungerford 08 Mar '12

08 Mar '12

Hi, I'd like to start using Haskell on my OS/X Lion system. In preparation I've installed 10.7.3 and the latest XCode. I've also installed the XCode command line tools. I'm using Homebrew and run "brew doctor" first to make sure Homebrew is happy. Then a "brew install haskell-platform" generates thousands of "ld: warning: text reloc in _cZh_str to _stg_ap_pppppp_info" warnings then fails on C Compiler errors. I must have missed something important? Any advice from other OS/X Lion users much appreciated. Thanks, Stu

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Ambiguous MonadIO and Monad
by Ken KAWAMOTO 07 Mar '12

07 Mar '12

Hi, I'm trying to understand how Network.OAuth.Consumer works, and run sample code found here with Twitter API. (the full code is attached at the bottom) http://hackage.haskell.org/packages/archive/hoauth/0.3.3/doc/html/Network-O… When I load the test code with GHCi 7.0.2, it raises error: --- Error message begin TestOAuth.hs:21:103: Ambiguous type variable `m0' in the constraints: (MonadIO m0) arising from a use of `serviceRequest' at TestOAuth.hs:21:103-116 (Monad m0) arising from a use of `>>=' at TestOAuth.hs:21:99-101 Possible cause: the monomorphism restriction applied to the following: response :: m0 Response (bound at TestOAuth.hs:17:1) Probable fix: give these definition(s) an explicit type signature or use -XNoMonomorphismRestriction In the second argument of `(>>=)', namely `serviceRequest CurlClient' In the expression: signRq2 HMACSHA1 (Just $ Realm "realm") srvUrl >>= serviceRequest CurlClient In the second argument of `($)', namely `do { signRq2 PLAINTEXT Nothing reqUrl >>= oauthRequest CurlClient; cliAskAuthorization authUrl; signRq2 PLAINTEXT Nothing accUrl >>= oauthRequest CurlClient; signRq2 HMACSHA1 (Just $ Realm "realm") srvUrl >>= serviceRequest CurlClient }' --- Error message end I seem using NoMonomorphismRestriction is workaround, so I want to add a proper type signature, but I cannot figure out what it is. What confuses me is why GHC didn't just take MonadIO as MonadIO is a Monad according to the definition. http://hackage.haskell.org/packages/archive/transformers/0.2.2.0/doc/html/C… I tried to add a type signature by replacing the second line from the bottom with this, in vain. ((signRq2 HMACSHA1 (Just $ Realm "realm") srvUrl) :: (MonadIO m) => OAuthMonadT m OAuthRequest) >>= serviceRequest CurlClient Can you help me find what's wrong with this? Thanks in advance, Ken --- Test code (TestOAuth.hs) import Control.Monad.IO.Class (MonadIO) import Data.Maybe (fromJust) import Network.OAuth.Consumer import Network.OAuth.Http.CurlHttpClient import Network.OAuth.Http.Request import Network.OAuth.Http.Response consumerKey = "MY CONSUMER KEY" consumerSec = "MY CONSUMER SEC" reqUrl = fromJust . parseURL $ "https://api.twitter.com/oauth/request_token" accUrl = fromJust . parseURL $ "https://api.twitter.com/oauth/access_token" srvUrl = fromJust . parseURL $ "http://service/path/to/resource/" authUrl = ("https://api.twitter.com/oauth/authorize?oauth_token="++) . findWithDefault ("oauth_token","ERROR") . oauthParams app = Application consumerKey consumerSec OOB response = runOAuthM (fromApplication app) $ do { signRq2 PLAINTEXT Nothing reqUrl >>= oauthRequest CurlClient ; cliAskAuthorization authUrl ; signRq2 PLAINTEXT Nothing accUrl >>= oauthRequest CurlClient ; signRq2 HMACSHA1 (Just $ Realm "realm") srvUrl >>= serviceRequest CurlClient }

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More on trying to install Haskell platform on Lion...
by Stuart Hungerford 06 Mar '12

06 Mar '12

Hi, An update on my continuing attempts to install Haskell platform on OS/X Lion. Just in case there was some problem with my Homebrew settings I completely removed Homebrew and reinstalled a fresh version, but still get the same errors. Could someone confirm for me whether OS/X Lion is supported for Haskell platform (or more likely) there's a different way of going about installation? Thanks, Stu

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Converting bytes to numbers
by Emacs The Viking 06 Mar '12

06 Mar '12

OK, I have a situation....I have a little PIC microchip sending out 24 bit ADC values over TCP/IP... I want to know what is the nicest / cleanest / best / efficient / geekiest etc way tyo convert the three bytes it sends into an unsigned 24-bit number... main :: IO () main = do h

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A tree code
by bahadýr altan 06 Mar '12

06 Mar '12

Hello. There is a code below and I couldn't understand it myself. Could you help me with that please? I especially have no idea about "data Tree alpha = Empty | Node " part. data Tree alpha = Empty | Node ( alpha ,Tree alpha , Tree alpha ) x = Node (1,Node (2,Empty ,Empty),Node(3,Empty ,Empty)) y = Node(3,Empty ,Empty) Thanks in advance

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