December 2009 - Beginners

using the current time
by Torsten Otto 08 Dec '09

08 Dec '09

Hi! We are trying to build a chatbot that can among many other things tell the partner what the current time is. Now obviously current time and pure functional programming don't mix well, but Haskell CAN do it. We have played around with it for a while and have figured out quite a bit, but the final icing is missing on the cake. It would be cool to have a dictionary like so > wissen = [ Einfache Begrüßung -> Frage nach dem Wohlbefinden > (["hallo", "hi", "hey"], "Hi, wie geht's?"), > (["guten tag", "tag"], "Wie geht es dir?")] (yes, the bot speaks German!) that could include a line like > (["Uhr","Zeit","spät"], time) where time is a function evaluating to a String including the current time. Once we have the time, we can get it translated into German by > formatiereZeit :: ClockTime -> String > formatiereZeit t > | (head(words (calendarTimeToString (toUTCTime t))) == "Mon") = "Montag" > | (head(words (calendarTimeToString (toUTCTime t))) == "Tue") = "Dienstag" > | (head(words (calendarTimeToString (toUTCTime t))) == "Wed") = "Mittwoch" > | (head(words (calendarTimeToString (toUTCTime t))) == "Thu") = "Donnerstag" > | (head(words (calendarTimeToString (toUTCTime t))) == "Fri") = "Freitag" > | (head(words (calendarTimeToString (toUTCTime t))) == "Sat") = "Samstag" > | (head(words (calendarTimeToString (toUTCTime t))) == "Sun") = "Sonntag" > | otherwise ="keine Uhr heute" which results in a string. Now how do we access the system time in a useful way? > time = formatiereZeit (NOW WHAT?) Would the current time step out of the IO monad, please? Regards, Torsten

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recursively building lists
by Torsten Otto 08 Dec '09

08 Dec '09

Hi! While trying to implement "words", we ran into the question of how to build lists of lists. The trouble boils down to this: test = []:[] is no problem, just as Prelude> []:[]:[]:[] [[],[],[]] works fine. Now trying to put the two together _is_ a problem: testlist 1 = []:[] testlist n = []:(testlist n-1) No instance for (Num [[a]]) arising from a use of `testlist' at <interactive>:1:0-9 Possible fix: add an instance declaration for (Num [[a]]) In the expression: testlist 2 In the definition of `it': it = testlist 2 Can someone please explain, what is going on here? Regards, Torsten

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Keyboard input
by legajid 07 Dec '09

07 Dec '09

Hello, in order to get familiar with keyboard data entry, i'm writing a program for the well-known Hangman game. Each entry must change several lists (letters found and remaining letters). I have three main difficulties with data entry : First When i enter a letter (A) , i get a good response (displaying old word and letters, message "guess ok") and an unattended one that says "guess false", thus displaying the result from my entry. I don't understand why the process seems to execute twice, the fisrt one being correct, the second not. Second difficulty : the "procedure" for data entry is written twice ("guess a letter ..." prompt). I think it would be a good idea to write it once as a "function" that would display the prompt then get a character from the keyboard. Third difficulty : in both cases (guess ok or false), i have to ask for a new entry; to avoid writing this twice again, i write it before the recursive calls of process_guess. Writing a function would perhaps allow to call it directly as a parameter of process_guess ? Being new to haskell, recursive functions now seem clear to me but i feel that IOs make writing a program more difficult. And i've not yet tried data base access... Please, say me if the way i wrote my program is a good or bad approach. Below an example of a run : Here's my program. Could you help me. Thanks, Didier solution="HANGMAN" word="H-----N" letters=['A'..'Z'] hangman = do -- enter a letter putStrLn "Guess a letter (9 to end):" guess <- getChar -- process process_guess guess letters word process_guess pg pletters pword =do let pguess = toUpper pg putStrLn pword putStrLn pletters if pguess == '9' then do putStrLn "Done" else do if pguess `elem` pletters then do putStrLn "Guess OK \n" else do putStrLn "Guess false \n" -- Enter a new letter before going on, in both cases putStrLn "Guess a letter (9 to end):" guess <- getChar if pguess `elem` pletters -- guess ok -> remove guess from available letters then add guess to word then do process_guess guess (newletters pletters pguess) (newword pword pguess solution) -- guess false -> remove guess from available letters, leave word unchanged else do process_guess guess (newletters pletters pguess) pword newletters l g = filter (/= g) l newword [] _ _ = [] newword (w:ws) g (s:ss) = if w == '-' then if s == g then g : newword ws g ss else w : newword ws g ss else w : newword ws g ss toUpper c | isLower c = toEnum (fromEnum c - fromEnum 'a' + fromEnum 'A') | otherwise = c isLower c = c >= 'a' && c <= 'z'

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freelancing jobs in haskell
by Vinod Parthasarathy 06 Dec '09

06 Dec '09

Hi, I recently learnt Haskell as part of my post graduate course work in Computer Science. I am looking for freelancing websites which offer online jobs in Haskell. If any of you know any such website where I could look for freelancing jobs, could you please let me know? I am from India, and am also looking for jobs in India in Haskell programming. Any help in that direction would also be helpful. Vinod.

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Re: [Haskell-beginners] Indenting
by Brent Yorgey 05 Dec '09

05 Dec '09

On Sat, Dec 05, 2009 at 05:58:25PM +0000, John Moore wrote: > Hi Brent, > Sorry about the confusion below is the whole program, which may help or not? > What i want to do is when the program prints out the answers , I would like > it to be able to automatically by use of a function indent the answers > depending how far down the answer goes. OK, I think I understand what you want to do, although I'm not exactly clear on how you want the indentation to work. Anyway, I've interspersed my comments in the code below. > import Maybe Just FYI, "Maybe" is the old Haskell-98 name for the module, but nowadays the standard name is "Data.Maybe". > data Expression = Val Float > | Add Expression Expression > | Subtract Expression Expression > | Multiply Expression Expression > | Divide Expression Expression > | Let String Expression Expression > | Var String > deriving Show > type Dict =[(String,Expression)] > emptyDict :: Dict > emptyDict = [] > addEntry :: String->Expression ->Dict -> Dict > addEntry n e d = (n,e): d > lookupEntry :: String -> Dict -> Maybe Expression > lookupEntry n [] = Nothing > lookupEntry n (x:xs) = if (n == k) > then (Just v) > else lookupEntry n xs > where (k,v) = x I would write lookupEntry as follows: lookupEntry _ [] = Nothing lookupEntry n ((k,v):xs) | n == k = Just v | otherwise = lookupEntry n xs Note that you can do a nested pattern match ((k,v):xs), and the use of guards instead of if...then...else. But actually, lookupEntry is already in the standard Prelude, it is called 'lookup'! So no need to reimplement it yourself. > evalIO :: Dict -> Expression -> IO Float This function is rather poor Haskell style, because it mixes up two separate things: evaluating the expression, and printing the result. Instead, I would do something like this: eval :: Dict -> Expression -> ([String], Double) where the output is the result paired with a "trace". (Note I have used Double, a double-precision floating point number, instead of Float, which is single-precision; there's usually very little reason to use Float instead of Double.) Given eval, you can recover evalIO as follows: evalIO :: Dict -> Expression -> IO Double evalIO d e = do let (trace, result) = eval d e mapM_ putStrLn trace return result But this is a lot nicer because it cleanly separates the evaluation from the IO, and you can now do anything you like with the trace --- print it to a file, process it further, etc; you are not tied down to printing it on the screen. Now, if you just want the indentation to increase at each step, like so xxxxx xxxxx xxxxx xxxxx xxxxx ... then you can also separate this out: just return a trace with no indentation, and then apply a function to the trace to indent successive lines, something like zipWith (++) (map (flip replicate ' ') [0..]) trace If, on the other hand, you want the indentation to correspond to how "deep" within the expression the evaluation is taking place, then you can recursively pass along an extra parameter to your evaluation function, like so: eval :: Dict -> Expression -> ([String], Double) eval d e = evalIndented 0 d e where evalIndented i d (Val x) = ([], x) evalIndented i d (Add x y) = let (tx, vx) = evalIndented (i+1) d x (ty, vy) = evalIndented (i+1) d y in (replicate i ' ' ++ "Add " ++ show vx ++ " and " ++ show vy, vx + vy) ...and so on. I hope this is helpful! -Brent

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Indenting
by John Moore 05 Dec '09

05 Dec '09

Hi, I'm a bit stuck. I want to indent my answer by using a count function to indent my answer. Say for example I want to show the levels in a simple arithmethic expression. like 3*(4+5) I want to be able to print out : 4+5 = 9 3*9 = 27 (notice the indentation as I go through an expression. I tried this but it a mess, I tried to use prettyprint but this seemed very complicated. Any suggestions greatly appreciated type Indent = (Int) spacing :: Int -> Indent spacing n = Nothing spacing n (x:xs) = if (n==1) then (Just n) else spacing n xs where (n > 1) John

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"reusable" data in Haskell
by John Velman 04 Dec '09

04 Dec '09

In prolog, one can create a database of facts and predicates, which, depending on the prolog (AFAIK) can be 'compiled' and loaded quickly. How does one do something comparable in Haskell? I guess I have in mind something like a function that accesses the data with a key. And ideally, the data should itself be able to contain functions. I know there is a hash table library, and interfaces to databases like postgresql. If using hash table, is there a way to 'pickle' the hash table, and unpickle it? I'd be happy to have some pointers to someplace that this has been done (or some documentation on how to do it. Thanks, John Velman

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Eq a => [a]->[a]
by I. J. Kennedy 03 Dec '09

03 Dec '09

I am looking for a function f::Eq a => [a]->[a] that takes a list and returns the longest initial segment of the list for which all the elements are distinct. For example f [2,3,6,4,3,5] = [2,3,6,4]. I didn't see anything that matched using Hoogle, but I thought this might be a common enough operation that this function might exists somewhere in the standard packages.

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partitionM
by Tom Tobin 03 Dec '09

03 Dec '09

While working on some filesystem traversal code, I found myself wanting to use the 'partition' function from Data.List, but with the function 'doesDirectoryExist' in System.Directory (with type Filepath -> IO Bool). I noticed that there was no 'partitionM' in Control.Monad, so I set out to write one. Here's what I ended up with: > import Control.Monad (foldM) > partitionMHelper :: Monad m => (a -> m Bool) -> ([a],[a]) -> a -> m ([a],[a]) > partitionMHelper p (ts,fs) x = do > b <- p x > return (if b then (x:ts,fs) else (ts,x:fs)) > partitionM :: (Monad m) => (a -> m Bool) -> [a] -> m ([a], [a]) > partitionM p xs = foldM (partitionMHelper p) ([],[]) $ reverse xs This works for my trivial cases, but can fail with extremely large lists (not to mention being unable to take an infinite list and work properly when passed to something like 'take 5'). Is there a way to change the function to avoid having to traverse to the end of the list (via reverse) just to get the output in the proper order? I started trying to write a "foldrM", but haven't gotten anywhere useful yet. Can someone point me in the right direction?

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fail: wxhaskell with cabal
by Josh Bronson 03 Dec '09

03 Dec '09

Greetings Haskellers, Happy Thanksgiving! I'm using Ubuntu 9.10. I've performed the following steps: # apt-get install ghc6 I then successfully did a local install of the Haskell platform in ~/mine/install Now I am trying to get wxhaskell to install: $ cabal install wx [snip] Configuring wxcore-0.12.1.2... setup: Missing dependencies on foreign libraries: * Missing C libraries: wx_baseu-2.8, wx_baseu_net-2.8, wx_baseu_xml-2.8 This problem can usually be solved by installing the system packages that provide these libraries (you may need the "-dev" versions). If the libraries are already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where they are. cabal: Error: some packages failed to install: wx-0.12.1.2 depends on wxcore-0.12.1.2 which failed to install. wxcore-0.12.1.2 failed during the configure step. The exception was: exit: ExitFailure 1 $ ls /usr/lib/libwx* /usr/lib/libwx_baseu-2.8.so /usr/lib/libwx_baseu_net-2.8.so.0.6.0 /usr/lib/libwx_baseu-2.8.so.0 /usr/lib/libwx_baseu_xml-2.8.so /usr/lib/libwx_baseu-2.8.so.0.6.0 /usr/lib/libwx_baseu_xml-2.8.so.0 /usr/lib/libwx_baseu_net-2.8.so /usr/lib/libwx_baseu_xml-2.8.so.0.6.0 /usr/lib/libwx_baseu_net-2.8.so.0 It looks like the libraries are there to me! any ideas?

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