- Haskell-Cafe - Haskell.org

monadic performance
by Chad Scherrer 31 Oct '06

31 Oct '06

Hello, I've written some code that does a foldl (or scanl, depending on my mood) kind of thing, and builds a huge tree structure as it goes along. I've been careful to make "insert"s as strict (and eager) as possible, since I know all the pieces will be evaluated eventually anyway. Now I'd like to be able to write various output as I traverse the structure, and I was thinking about using the WriterT monad transformer. For this, I could just make the base monad Identity, or I think it could be nice (from a code elegance perspective) to use State and rewrite the insert code to be State-ful. Sometimes when I have used State before, I have been bitten by laziness. Should I expect a monadic version to take a performance hit? What if I use some SPECIALIZE pragmas or somesuch? Is it more efficient to write one from scratch, or do specific type annotations give me the same thing anyway? Thanks, Chad

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mapAccumL - find max in-sequence subsequence
by jim burton 30 Oct '06

30 Oct '06

I need to find the length of the longest in-sequence section of a list of ints...I am guessing something like mapAccumL is the way but not sure how. --this doesn't work Prelude List> mapAccumL (\x y -> if y==(x+1) then (y, x) else (y,0)) 0 $ sort $ nub [9, 1, 2, 7, 3, 4, 5, 6,5] (9,[0,1,2,3,4,5,6,0]) -- View this message in context: http://www.nabble.com/mapAccumL---find-max-in-sequence-subsequence-tf253170… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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More documentation: languages written in Haskell
by dons＠cse.unsw.edu.au 30 Oct '06

30 Oct '06

I noticed today that although we have a list of most applications written in Haskell, nowhere was there collected a page of perhaps our best use case for Haskell: for implementing compilers and interpreters! So here's a new 'libraries and tools' category page: http://haskell.org/haskellwiki/Libraries_and_tools/Compilers_and_interprete… If you know of a compiler or interpreter written in Haskell, (I think at least a few people on this list have written one or two themselves ... ;) please add it to the list. -- Don

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Documenting subcommunities
by dons＠cse.unsw.edu.au 30 Oct '06

30 Oct '06

As the Haskell community grows and spreads its lambda-tipped tentacles into new domains, I've noticed that some distinct sub-communities are emerging. To try to document and collect information relevant to these groups, some new wiki pages have been created. Alongside the 'traditional' areas of: http://haskell.org/haskellwiki/Haskell_in_research http://haskell.org/haskellwiki/Haskell_in_education There is also a growing number of users of Haskell: http://haskell.org/haskellwiki/Haskell_in_industry http://haskell.org/haskellwiki/Haskell_and_mathematics If you're using Haskell for maths or in industry, and know of some relevant resources, please feel free to add the material to the above pages. Alternatively, if there are subcommunities that are missing a page (hmm, the open source community, perhaps?), then dive in and create a page documenting this. -- Don

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YAHT: Ex 9.2
by Magnus Therning 30 Oct '06

30 Oct '06

I've been slowly making my way through Yet Another Haskell Tutorial. As a first introduction to the language it doesn't seem bad at all. However, I'm wondering about the proposed solution to exercise 9.2. The text itself suggests using "instance Monad (Either String) where", so I arrived at instance Monad (Either String) where return a = Right a fail a = Left a Right a >>= f = f a Left a >>= _ = Left a However, when loading it in ghci 6.6 I get the following error message: Illegal instance declaration for `Monad (Either String)' (The instance type must be of form (T a b c) where T is not a synonym, and a,b,c are distinct type variables) In the instance declaration for `Monad (Either String)' The solution, according to the author is instance Monad (Either String) where return x = Right x fail s = Left s Right x >>= f = f x Left s >>= _ = Left s Which results in exactly the same error message. I'm suspecting this is a result of my limited grasp of Haskell's syntax (an area where YAHT is sorely lacking). Any tips/pointers appreciated. /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus(a)therning.org Jabber: magnus.therning(a)gmail.com http://therning.org/magnus Software is not manufactured, it is something you write and publish. Keep Europe free from software patents, we do not want censorship by patent law on written works. As far as the laws of mathematics refer to reality, they are not certain, and as far as they are certain, they do not refer to reality. -- Albert Einstein

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Rank N type tutorial?
by Greg Buchholz 30 Oct '06

30 Oct '06

Anyone know of a good source for learning about higher ranked types? I'm not quite sure why this is illegal... > foo :: Integer -> (forall a. Show a => a) > foo 2 = ["foo"] > foo x = x ...while this is just fine... > bar :: Integer -> (forall a. Show a => a->b) -> b > bar 2 k = k ["bar"] > bar x k = k x Thanks, Greg Buchholz

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The Read Class
by Tiago Veloso 29 Oct '06

29 Oct '06

Hi, I am trying to find out how to work with the Read Class for a school project, i need to declare instances of Read for a few data types. My problem is that i do not know how to do it, i mean i do not know how to build a Read instance, i do know about it for the Show Class. Can some one help? Thanks Tiago Veloso

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Partially applied type synonyms in instance heads
by Mathieu Boespflug 29 Oct '06

29 Oct '06

Hi everyone, I'm running into trouble with type synonyms in instance heads and I can't figure out what the resulting error message means. The case I'm considering is as follows: -- hoist a and b to class variables so that instances declarations may -- constrain them. class Map m a b where toAssoc :: m a b -> [(a, b)] fromAssoc :: [(a, b)] -> m a b type Assoc a b = [(a, b)] instance Map Assoc a b where toAssoc = id fromAssoc = id The class Map is used to allow translation from one map type to another (FiniteMap, arrays, etc...) by means of expressing the map as an association list. Useful for defining isomorphisms and so on. Now I'd like to define an association list as itself a trivial instance of Map class, but I cannot do so without wrapping the type of an association list, [(a, b)], behind a type synonym, as I'm not aware of any way of writing a type constructor of kind (* -> * -> *) constructing the type [(a, b)]. But when compiling the above code with GHC I get the following error: Map.hs:9:0: Type synonym `Assoc' should have 2 arguments, but has been given 0 In the instance declaration for `Map Assoc a b' Any idea what this means? Also, is there any other way of declaring an instance for [(a, b)] without using type synonyms? Many thanks, Mathieu

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Re: [Haskell] Animal guessing game - critique my code
by David Roundy 29 Oct '06

29 Oct '06

I haven't made anything more elegant, but have a few improvements that I found made it more fun to play (and yes, I'm rather zoned out after spending most of the day solving electromagnetism problems). It's nice (and quite easy) to save the tree, so that the game is more exciting every time you play. With a bit of work and integration with darcs, you could probably interface with a public server, so the guesser can be collaboratively improved by people who have too much time on their hands or are too tired to do anything productive. Here's the improved code: \begin{code} module Main where import Data.Char ( isUpper ) data GuessTree = Answer String | GuessTreeQuestion {guessTreeQuestion :: String, guessTreeNo, guessTreeYes ::GuessTree} deriving (Show, Read) askQuestion :: String -> IO Bool askQuestion str = do putStrLn str response <- getLine return $ isYes response where isYes ('y':xs) = True isYes _ = False a :: String -> String a s@(c:_) | isUpper c = s | c `elem` "aeiou" = "an " ++ s | otherwise = "a " ++ s a "" = "nonexistent" runTree :: GuessTree -> IO GuessTree runTree (Answer name) = do response <- askQuestion $ "Is it " ++ a name ++ "?" if response then do putStrLn "Ha!" return $ Answer name else do putStrLn "What is it?" animal <- getLine putStrLn $ "Enter a question to help distinguish between " ++ a name ++ " and " ++ a animal question <- getLine a <- askQuestion $ "Is the answer yes for " ++ animal ++ "?" (if a then id else flip) (\n y -> return $ GuessTreeQuestion {guessTreeQuestion = question, guessTreeNo = n, guessTreeYes = y}) (Answer animal) (Answer name) runTree GuessTreeQuestion {guessTreeQuestion = ques, guessTreeYes = yesTree, guessTreeNo = noTree} = do response <- askQuestion $ ques if response then do a <- runTree noTree return $ GuessTreeQuestion {guessTreeQuestion = ques, guessTreeNo = a, guessTreeYes = yesTree} else do a <- runTree yesTree return $ GuessTreeQuestion {guessTreeQuestion = ques, guessTreeNo = noTree, guessTreeYes = a} run :: GuessTree -> IO GuessTree run tree = do putStrLn "Think of an animal and press enter." getLine a <- runTree tree writeFile "animaldata" $ show a r <- askQuestion "Play again?" if r then run a else return a read_animal :: IO GuessTree read_animal = (read `fmap` readFile "animaldata") `catch` \_ -> return $ Answer "bear" main = read_animal >>= run \end{code} As you can see, I also modified it so that it'll be a bit smarter about not saying things like "a David Roundy" or "a elephant". -- David Roundy Dept. of Physics Oregon State University

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Self Study with SOE
by Aditya Siram 29 Oct '06

29 Oct '06

Hi all, I have been steadily working through Haskell SOE. However, as the exercises become more involved, I would like to know, not only that the answer I come up with works, but that I am doing it the right way (the elegant way?). For instance, Chapter 8 Exercise 8.3 requires me to modify the "area" and "perimeter" functions to accept negative arguments. My solution would be to change the functions to take the absolute value of the arguments as they come in. This would work but it doesn't seem all that elegant. Additionally, SOE exercises do not come with test data so I can test correctness. Is it part of my responsibililty as a student to come up with test data (boundary conditions etc) as I try to answer the questions? Is there some kind of solutions manual available? I promise I am not doing this as homework for a course. Thanks... Deech _________________________________________________________________ Find a local pizza place, music store, museum and morethen map the best route! http://local.live.com?FORM=MGA001

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Re: [Haskell-cafe] Rank N type tutorial?
by rossberg＠ps.uni-sb.de 28 Oct '06

28 Oct '06

"Greg Buchholz" <haskell(a)sleepingsquirrel.org> wrote: > I'm not quite sure why this is illegal... > >> foo :: Integer -> (forall a. Show a => a) >> foo 2 = ["foo"] >> foo x = x The type signature promises that foo returns a function that has type a *for all a* (that are in Show). But neither a string list nor an integer can have all types. Rather, they are very specific. That is, what you have implemented actually could have type foo :: Integer -> (exists a. Show a => a) (if GHC supported such types). Note that only bottom can have a type like forall a.a. > ...while this is just fine... > >> bar :: Integer -> (forall a. Show a => a->b) -> b >> bar 2 k = k ["bar"] >> bar x k = k x Here, you declare bar to *take* a function that delivers some b for any a. That is, the burden of defining such a function now is on the caller, not bar itself. However, in this case, defining such an argument function actually is easy, for another reason: the signature says k returns a result of type b, but b is fixed. So it is always possible to come up with a trivial instance of such a function (which will just ignore its argument). For instance, bar 2 (\_ -> "boo") But try to call this for a different example: ;-) baz :: Integer -> (forall a b. Show a => a->b) -> c baz 2 k = k ["baz"] baz x k = k x Hope this helps, - Andreas

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UTF and Parsec
by John Ky 27 Oct '06

27 Oct '06

Hi, I'm interested in using Parsec to parse utf-8 files and do some processing and output a utf-8 document. Is there anything I should be aware of? Does GHC handle all the utf-8 stuff automatically? What does it mean when I find that parsec is picking up "ï»¿" at the beginning of the file. Thanks -John

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Solitaire cipher
by jim burton 27 Oct '06

27 Oct '06

I'm a beginner having a go at implementing the Solitaire cipher (http://www.rubyquiz.com/quiz1.html as mentioned in another post) and I'd be really grateful if you could help me improve the code to be neater & use more functions from the Prelude etc, or errors (eg at the moment I can't work out why padding accumulates after encrypting, decrypting?)...Thanks. *Main> decrypt $ encrypt "haskell is better by miles" "HASKE LLISB ETTER BYMIL ESAYP XXXXX " *Main> decrypt $ encrypt $ decrypt $ encrypt "haskell is better by miles" "HASKE LLISB ETTER BYMIL ESAYP XXXXX BFCRK XXXXX " *Main> ------------------------------------------- import Char import Random import List import Foreign import Maybe data Card = Clubs Int | Spades Int | Diamonds Int | Hearts Int | JokerA | JokerB deriving (Show, Eq) type Deck = [Card] --cardval - clubs are face value, diamonds plus 13, and so on - Jokers are both 53 cardval :: Card -> Int cardval (Clubs i) = i cardval (Diamonds i) = i+13 cardval (Hearts i) = i+26 cardval (Spades i) = i+39 cardval _ = 53 isJoker :: Card -> Bool isJoker JokerA = True isJoker JokerB = True isJoker _ = False --take a card to a letter card2char :: Card -> Char card2char c = case c of (Clubs i) -> int2alpha $ cardval c --can case fall through in haskell? (Diamonds i) -> int2alpha $ cardval c (Hearts i) -> int2alpha $ (cardval c-26) (Spades i) -> int2alpha $ (cardval c-26) _ -> error ("Can't make " ++ show c ++ "into alpha") --take a letter to int, A=1, Z=26 char2int :: Char -> Int char2int = (64 `subtract`) . (ord) --take a letter to int, 1=A, Z=26 int2alpha :: Int -> Char int2alpha = (chr) . (+64) splitAtMb n l = let p = splitAt n l in if null $ fst p then Nothing else Just p in_fives l = foldr (\x y -> x++" "++y) [] $ unfoldr (splitAtMb 5) (l ++ replicate (5 - length l `mod` 5) 'X') --get an ordered deck newdeck :: Deck newdeck = suit 'c' ++ suit 'd' ++ suit 'h' ++ suit 's' ++ JokerA : JokerB : [] where suit s = case s of 'c' -> [Clubs i | i <- [1..13]] 's' -> [Spades i | i <- [1..13]] 'd' -> [Diamonds i | i <- [1..13]] 'h' -> [Hearts i | i <- [1..13]] --key the deck ready to provide a keystream - move JokerA down one place, --JokerB down 2 places, perform a triplecut then a countcut keydeck :: Deck -> Deck keydeck = countcut. triplecut . (movedown JokerB) . (movedown JokerB) . (movedown JokerA) --bump a card down by one place in a deck, treating the deck as circular so if the card is -- last in the deck it becomes 2nd to front not 1st movedown :: Eq a => a -> [a] -> [a] movedown c d = if c == last d then head d : c : init (tail d) else top ++ bot!!1 : c : (tail (tail bot)) where splt = splitAt (locate c d) d top = fst splt bot = snd splt --substitute the cards above the first joker for those below the 2nd one triplecut :: Deck -> Deck triplecut d = afterLastJoker d ++ center d ++ beforeFirstJoker d where beforeFirstJoker = takeWhile (not . isJoker) afterLastJoker = reverse . beforeFirstJoker . reverse center = reverse . dropWhile (not . isJoker) . reverse . dropWhile (not . isJoker) --get the value of the last card and move that many cards from the top of deck to above the last card countcut :: Deck -> Deck countcut d = init (drop n d) ++ take n d ++ [last d] where n = cardval (last d) --key the deck, read the value of the top card as n, add the nth card to stream, repeat keystream :: Deck -> String keystream d = if isJoker c then keystream d' else card2char c : keystream d' where d' = keydeck d c = d'!!(cardval $ d'!!0) locate :: Eq a => a -> [a] -> Int locate x xs = fromJust (elemIndex x xs) clean :: String -> String clean = map toUpper . filter isAlpha encrypt, decrypt :: String -> String --encrypt a string using an unshuffled deck to start encrypt = process (\x y -> max26 (x+y)) where max26 x = if x > 26 then x-26 else x --decrypt a string using an unshuffled deck to start decrypt = process (\x y -> if x <= y then (x+26)-y else x-y) process :: (Int -> Int -> Int) -> String -> String process f s = in_fives $ map int2alpha $ zipWith f ints1 ints2 where str1 = clean s str2 = take (length str1) (keystream newdeck) ints1 = map char2int str1 ints2 = map char2int str2 -- View this message in context: http://www.nabble.com/Solitaire-cipher-tf2500700.html#a6971077 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Ruby quiz, Haskell wiki
by dons＠cse.unsw.edu.au 27 Oct '06

27 Oct '06

I've created a page to document haskell solutions to the ruby quiz puzzle series. http://haskell.org/haskellwiki/Haskell_Quiz Those of you working on them, please upload your solutions, and create sub pages for new puzzles as they appear. Cheers, Don

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Glade Gtk2Hs Getting Started
by Hans van Thiel 26 Oct '06

26 Oct '06

Hello, My adaptation, with the author's permission, of the 'Developing Gnome Apps with Glade' tutorial by Eddy Ahmed for Gtk2Hs and Haskell can now be accessed on: http://j-van-thiel.speedlinq.nl/EddyAhmed/GladeGtk2Hs.html It's for the absolute beginner in both Glade and Gtk2Hs and covers the basics of Glade and how to access a .glade file and widgets in Gtk2Hs. Estimated learning time: 2 hours. Thanks to Duncan Coutts and Eddy Ahmed for their help and cooperation. The tutorial will be made available on the Gtk2Hs web site and on Eddy's homepage as well. Regards, Hans van Thiel

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Re: Re: Read a single Char
by Hans van Thiel 26 Oct '06

26 Oct '06

>That is normal Unix behaviour. Try the same input with head -1c | od -c; head -1c | od -c The terminal buffer is only discarded if a program changes the terminal mode. Hugs and ghci do (because they use readline), but runhugs doesn't. > Ah, so that's it! Thanks, all. Hans van Thiel

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Haskell custom search engine
by dons＠cse.unsw.edu.au 26 Oct '06

26 Oct '06

Google now lets us create our own custom search engine pages, so I whipped one up for Haskell, http://www.google.com/coop/cse?cx=015832023690232952875%3Acunmubfghzq also, as a demo, embedded http://www.cse.unsw.edu.au/~dons/search.html Seems to do a reasonable job of targetting just Haskell sites. Feel free to add more Haskell material, or refine the search. In fact, search for mailing list items seems rather easy this way. Neil, I wonder if we could integrate this with Hoogle somehow? -- Don

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Read a single char
by Maurício 26 Oct '06

26 Oct '06

Hi, How can I read a single character from standard output? I would like the user to press a single key and the reading function return imediately after that key is pressed. Thanks, Maurício

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Re: Re: Read a single char
by Hans van Thiel 26 Oct '06

26 Oct '06

On Tue, Oct 24, 2006 at 06:14:14PM +0200, Hans van Thiel wrote: > I have a similar question. When I use getChar with Hugs the newline > (caused by pressing 'Enter') seems to be carried over into the next > call of main, where it is treated as a single input character. > The workaround is getLine and head, but it would be nice to drop the > newline right away. I suppose the solution offered here: >>that is the correct behavior, input is never discarded, it is just >>saved >>up until you press enter for the first time. if you type >>foo<enter>, >>your next four getChars will get 'f' 'o' 'o' and '\n'. it is not >>haskell >>that is treating enter as specially, it is your terminal, getChar >>just returns exactly what was passed to it. Yes, but the problem is with two subsequent runs of main. I'd have thought the buffer would flush automatically and the second run would start with an empty buffer. Now I enter 'b', then press 'Enter' and it works as expected. Then I 'main' again, and it terminates right away with a blank line. If I enter 'a', 'b' and then Enter, the second run takes the second character. If I'm not doing anything wrong (always a possibility :-), then it seems more like a (small) bug in Hugs, as Neil suggested. I'll have to check what chgi does. John -- John Meacham - ⑆repetae.net⑆john⑈ Regards, Hans

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Fwd: [Haskell-cafe] Haskell custom search engine
by Diego Navarro 25 Oct '06

25 Oct '06

> Google now lets us create our own custom search engine pages, so I > whipped one up for Haskell, I volunteered. Are you planning to add just sites for Haskell-related software, or are research papers included in the scope of this? (Dude, where's my english grammar.) -- syntaxfree

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Re: Configurations proposal
by Brian Smith 25 Oct '06

25 Oct '06

On 10/24/06, Duncan Coutts <duncan.coutts(a)worc.ox.ac.uk> wrote: > > On the other hand, in Gtk2Hs I know one case where we do this. We have a > Graphics.UI.Gtk.Cairo api module that is only included if Gtk was built > against Cairo. In any case it could be faked by using cpp to just not > export anything rather than not having the module exposed at all. So > it's not clear that it's worth banning. Or maybe making it slightly > harder is worth it so that people don't get in the habit. Couldn't you split this into Gtk and Gtk-Cairo packages, where the latter is only built if Cairo is available? Similarly, in your GUI example, couldn't you have seperate foo and foo-gui packages, and only build the foo-gui package if the GUI libraries are available? Otherwise, how can you say "I depend on the Gtk package being built with Cairo support" and "I depend on the GUI portion of the foo package?" In general, optional groups of modules should be split off into separate packages, and there should be a way of building a bundle of related packages together (just like one can build a group of related executables together already). Regards, Brian

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Re: Read a single Char
by Hans van Thiel 25 Oct '06

25 Oct '06

Hello, I have a similar question. When I use getChar with Hugs the newline (caused by pressing 'Enter') seems to be carried over into the next call of main, where it is treated as a single input character. The workaround is getLine and head, but it would be nice to drop the newline right away. I suppose the solution offered here: hSetBuffering stdin NoBuffering will have the desired effect, but is there another way? Thanks, Hans van Thiel

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Guards with do notation?
by Misha Aizatulin 25 Oct '06

25 Oct '06

hello all, why is it not possible to use guards in do-expressions like do (a, b) | a == b <- getPair return "a and b are equal" Cheers, Misha

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Exception: Too many open files
by Bas van Dijk 25 Oct '06

25 Oct '06

Hello Haskellers, I'm wondering how to get the following to work: I need to parse about 18.000 files. I would like to have a function 'parseFiles' that parses all these files and returns the result in a list. When I execute 'test' from the simplified code below I get the error: *** Exception: file_0014994.txt: openFile: resource exhausted (Too many open files) So it seems that 'parseFiles' tries to open all the ~18.000 files and gets exhausted when opening the 14994 file. What I would like is 'take 3 =<< parseFiles' to read only the first 3 files. Is this possible, and if so, what is the best way to do this? Note that the code below is a bit simplified: ------------------------------------------------------------------------------ module Main where import Text.ParserCombinators.Parsec data T = ... test = print . take 3 =<< parseFiles parseFiles :: IO [T] parseFiles = mapM parseFile =<< getFileFPs getFileFPs :: IO [FilePath] getFileFPs = ... -- returns a large list of about 18.000 FilePaths parseFile :: FilePath -> IO T parseFile fp = liftM getRight $ parseFromFile someParser fp getRight (Right r) = r someParser :: Parser T someParser = ... ------------------------------------------------------------------------------ Greetings, Bas van Dijk

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split string into n parts
by Paul Brown 24 Oct '06

24 Oct '06

> I want to split a string into 5 parts of equal length, with the last fifth > padded if necessary, but can't get it right - here's what I've got - > > fifths :: String -> String > > fifths s = fifths' "" 0 s > > where l = (length s) `div` 5 > [... snip ...] > Any thoughts? Thanks! This isn't homework BTW, I'm having a go at the ruby > quiz puzzles in haskell, which seems to be a nice way to learn. Cool idea! Can you post a link for the puzzles? As for this one, don't you want the first multiple of five larger than the length of the string? You should be able to make things simpler if you auto-pad the string from the get-go (forgive any syntax errors, as I'm just composing in a browser here...): prepad :: Integer -> String -> String prepad n s | (length s) `mod` n == 0 = s prepad n s = prepad n (s ++ ' ') And now you can be sneaky: nths :: Integer -> String -> String nths n s = [first_nth n s] ++ nths (n-1) (after_first_nth n s) nths 1 s = s Where first takes the first n chars (e.g., take ((length s) `div` n)) and after_first_nth returns the tail of the list.. -- paulrbrown(a)gmail.com http://mult.ifario.us/ -- paulrbrown(a)gmail.com http://mult.ifario.us/

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Re: [Haskell-cafe] Haskell beginner questions
by jim burton 24 Oct '06

24 Oct '06

Cybertronic wrote: > > Hi all, I'm pretty much new to Haskell however I'm stuck on something > which is that I'm trying to create a function called display where I type > in a DVD name, e.g. dvd1, it returns d (String) and the multiplication of > q (Int) and i (Double) > > Here's what I've done so far: > > type Film = (Int,String,Int,Double) > > dvd1 :: Film > dvd1 = (1, "Space", 5, 9.99) > > display :: Product -> String > display (c,d,q,i) = d > > My display function only shows the string but unfortunately I'm stuck on > how to get the display function to multiply q (Int) and i (Double) > together and display it next to d. > > Can someone help me out please? :) > I'd recommend a tutorial like http://www.cs.utah.edu/~hal/docs/daume02yaht.pdf > display :: Film -> String > display (c,d,q,i) = d ++ " " ++ show (fromIntegral q * i) q needs to be converted first and show will convert the result to a string. -- View this message in context: http://www.nabble.com/Haskell-beginner-questions-tf2500050.html#a6971248 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Re: [Haskell-cafe] Exception: Too many open files
by Bas van Dijk 24 Oct '06

24 Oct '06

On Tuesday 24 October 2006 00:58, Greg Fitzgerald wrote: > > test = print . take 3 =<< parseFiles > > I haven't had time to double-check this code, but something like this ought > to work (no 'unsafe' operations!): > test = sequence . take 3 . map (print . parseFile) =<< getFileFPs > > Let me know how it goes. Indeed, the following works: test = sequence . take 3 . map (\fp -> print =<< parseFile fp) =<< getFileFPs Thanks, Bas.

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Segmentation fault with realdine library
by Maurício 24 Oct '06

24 Oct '06

Hi, This small program says "Segmentation fault": module Main (Main.main) where import Data.Char import System.Time import System.Console.Readline main :: IO () main = do readKey return () I don't understand anything about readline, so I probably should have to call some function before readKey (by they way, I don't know what readKey does). But I thought a segmentation fault is always something to be reported. Best, Maurício

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split string into n parts
by jim burton 24 Oct '06

24 Oct '06

I want to split a string into 5 parts of equal length, with the last fifth padded if necessary, but can't get it right - here's what I've got - > fifths :: String -> String > fifths s = fifths' "" 0 s > where l = (length s) `div` 5 > fifths' xs c [] = xs ++ (replicate (l-c) 'X') > fifths' xs c (y:ys) = if c == l > then fifths' (xs++[' ',y]) 0 > ys > else fifths' (xs++[y]) (c+1) > ys which, apart from surely being uglier than need be, doesn't work: *Main> fifths "IDOLIKETOBEBESIDETHESEASIDE" "IDOLI KETOBE BESIDE THESEA SIDEXX" *Main> fifths "12345" "1 23 45" Any thoughts? Thanks! This isn't homework BTW, I'm having a go at the ruby quiz puzzles in haskell, which seems to be a nice way to learn. -- View this message in context: http://www.nabble.com/split-string-into-n-parts-tf2496941.html#a6960346 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Re: type level functions (type of decreasing list)
by oleg＠pobox.com 24 Oct '06

24 Oct '06

Greg Buchholz wrote: > I'm wondering about creating a data structure that has the type of > decreasing numbers. > I can try... > > class Pre a b | a -> b > instance Pre (Succ a) a > instance Pre Zero Zero > > data (Pre a b) => Seq a = Cons a (Seq b) | Nil > > decreasing = Cons three (Cons two (Cons one Nil)) > > ...but that complains about "Not in scope: type variable `b'" One way is to use existentials: > data Seq1 a = forall b. (Pre a b, Show b) => Cons1 a (Seq1 b) | Nil1 > > decreasing1 = Cons1 three (Cons1 two (Cons1 one Nil1)) which perhaps not entirely satisfactory because we have to specify all the needed classes in the definition of Seq1. Perhaps a better -- and a more general alternative -- is to use type-level programming to its fullest. The following defines a list whose elements are constrained to be in the decreasing, increasing, or any other (defined in the future) order. This example shows that Haskell truly has more kinds than it is commonly acknowledged. For more details on constrained lists (list of odd numerals, even numerals, etc), please see the following implementation of Tim Sheard's `Half' example http://pobox.com/~oleg/ftp/Haskell/Half.hs > data Succ a = S a > data Zero = Z > > zero = Z > one = S zero > two = S one > three = S two > > class KIND l a b > > data Cons a b = Cons a b > data Nil = Nil > > > data Increasing = Increasing > instance KIND Increasing a (Succ a) > data Decreasing = Decreasing > instance KIND Decreasing (Succ a) a > -- one can add non-increasing, non-decreasing, etc. > > class ConstrainedL c l > instance ConstrainedL c Nil > instance ConstrainedL c (Cons a Nil) > instance (KIND c a b, ConstrainedL c (Cons b l)) > => ConstrainedL c (Cons a (Cons b l)) > > -- smart constructors. > -- Alternatively, define > as_increasing :: ConstrainedL Increasing l => l -> l > consi :: ConstrainedL Increasing (Cons a l) => a -> l -> Cons a l > consi = Cons > consd :: ConstrainedL Decreasing (Cons a l) => a -> l -> Cons a l > consd = Cons > incr = consi one (consi two (consi three Nil)) > -- No instance for (KIND Increasing (Succ Zero) (Succ (Succ (Succ Zero)))) > -- incr1 = consi one (consi three Nil) > decr = consd three (consd two (consd one Nil)) -- No instance for (KIND Decreasing (Succ (Succ Zero)) (Succ (Succ (Succ Zero)) -- decr1 = consd three (consd two (consd three Nil))

2 2

Re[2]: [Haskell] Pugs gains SMP parallelism support.
by Bulat Ziganshin 23 Oct '06

23 Oct '06

Hello Taral, Monday, October 23, 2006, 1:12:38 PM, you wrote: >> They probably are. However you get the overhead of creating the array >> (when you don't really need O(1) random access) and every thread >> signals the same semaphore which may lead to some congestion which >> could slow things down. > You do need O(1) random access for the writers to put their results in > efficiently. And newArray_ should be faster than N copies of > newEmptyMVar. It is true that I have one congestion point (the > semaphone) instead of N (the mvars). are you sure that writeArray calls may be overlapped? :) they may change some technical fields, such as 'dirty' bit -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

1 0

memory, garbage collection and other newbie's issues
by Andrea Rossato 23 Oct '06

23 Oct '06

Hi! I'm a newbie and, as a learning experience, I'm writing a feed reader with hscurses and hxt. For the present time the feed reader just reads a Liferea cache but, as you can imagine, I'm running into the usual newbie problems of memory consumption and garbage collection, probably (I'm not sure) related to strictness/laziness. Even though I spent a couple of hours search the mailing list archives, I did not come up with something I can relate to, so I'll try to explain my problem. The feed reader, that should be compatible with Liferea, takes an opml (1.0) file, that stores information on folders and subscribed feeds. It uses it as the major component of the state of a ST monad, after adding some attributes used by the reader UI. The UI, that uses the widget library of hscurses and is derived from the Contact Manager example, will just display this opml file, and every UI event (collapsing/expanding of folders, displaying feeds, tagging, flagging, and so on) is just an XML transformation of this opml state component. So, when the feed reader boots, only the layout of folders and subscribed feeds is presented to the user. When the user selects a feed do be displayed, the cached file containing up to 100 saved posts, is read and transformed into a data type (called Feed, obviously). After that this data type is transformed into an opml (xml) tree, that is inserted as a child in the appropriate place of the opml state component. Moreover the parent element of the opml state component (which holds the original information of the subscribed feed) is edited for adding general feed information (such as last update, feed's attributes, and so on) retrieved by reading the file. When the user collapses the feed, the added opml chunk is deleted from the state component (but not the added information to the parent of this chunk). Now, I would expect that after the opml chunk is deleted all the memory allocated for reading the cached file would be garbage collected. This is not happening, so, every time you open (or reopen) a feed, the used memory of the feed reader increases, and never decreases. After profiling I've seen that the problem is occurring in the function that reads the cached file: loadFeed :: String -> IO [Feed] readFeed id = do [a] <- runX $ readDocument [(a_validate, v_0)] (cachePath ++ id) return $ runLA toFeed a What this function does is reading the file with: h <- openFile ... hGetContents h and applying some XML filters to get the Feed type populated with the needed information. I tried making the function strict with $!. I tried using fps. It doesn't change this behaviour, obviously. Now, I know that this is a typical newbie problem: could you give me some direction on how to debug this problem and possibly find a solution? Or just some hints on the kind of problem I'm facing: is it related to strictness/laziness, or it's just that I did not understand a single bit of how garbage collection works in Haskell? Thanks for your kind attention. If I'll be able to solve this problem probably I'll be also able to share the feed reader (I know that you are not going to answer after reading this....;-) regards, Andrea

4 15

Exact Real Arithmetic
by Henning Thielemann 22 Oct '06

22 Oct '06

On http://www.haskell.org/hawiki/ExactRealArithmetic there is a module by David Lester mentioned, with a link to http://www.cs.man.ac.uk/arch/dlester/exact.html which does not work, and I also did not get a reply from the author. Does someone of you have this module?

2 1

[Haskell-cafe] Re: Strictness, order of IO operations: NewCGI & HDBC
by oleg＠pobox.com 22 Oct '06

22 Oct '06

Tim Smith wrote: > Has anyone found out how to lift bracket into another monad? Yes, please see the thread `Re: Control.Exceptions and MonadIO' staring at http://www.haskell.org/pipermail/haskell-cafe/2006-April/015444.html There is also a Haskell' ticket: http://hackage.haskell.org/trac/haskell-prime/ticket/110 We have used this CatchableMonadIO in takusen and the other project, with success. One wish is to have HUnit work in this CatchableMonadIO rather than just IO (currently Alistair Bayley has to maintain an adjusted version of HUnit for that purpose). Regarding the issue of proper disconnections from the database server: it was the thrust for the development of Takusen is to statically prevent errors like forgetting to disconnect from the database or to use the database connection after disconnecting (as well as related issues of using a cursor after closing it, or forgetting to close the cursor or to dispose the resultset). As to performance: Takusen can read and process 2 million rows from the database, in the same space it takes to fetch and process 10 rows. In fact, doing this is one of the performance tests.

2 1

Re: Best way to write endsWith (RESOLVED)
by John Ky 22 Oct '06

22 Oct '06

Hi, Thanks all. I learnt quite a few things: 1. There was already an equivalent builtin function. 2. That the best function argument order is the least surprising one. 3. That I can choose my preferred function order by changing the name of the function. 4. That the most efficient way of doing something in Haskell can be so short. 5. Operators really are awesome. -John On 10/22/06, John Ky <newhoggy(a)gmail.com> wrote: > > Hello, > > I have this function here: > > > endsWith :: Eq a => [a] -> [a] -> Bool > > endsWith suffix list > > | lengthDifference < 0 = False > > | otherwise = (drop lengthDifference list) == suffix > > where lengthDifference = (length list) - (length suffix) > > Would this be the preferred function argument order? Or is the reverse > (ie. endsWith list suffix) better? > > I like being able to say "abc" `endsWith` "c", but I also like to be able > to say map (endsWith 't') ["cat", dog"] but I can't have both. > > By the way, is there a better way to write this function to be clearer and > more efficient? > > Thanks > > -John > >

1 0

Best way to write endsWith
by John Ky 22 Oct '06

22 Oct '06

Hello, I have this function here: > endsWith :: Eq a => [a] -> [a] -> Bool > endsWith suffix list > | lengthDifference < 0 = False > | otherwise = (drop lengthDifference list) == suffix > where lengthDifference = (length list) - (length suffix) Would this be the preferred function argument order? Or is the reverse (ie. endsWith list suffix) better? I like being able to say "abc" `endsWith` "c", but I also like to be able to say map (endsWith 't') ["cat", dog"] but I can't have both. By the way, is there a better way to write this function to be clearer and more efficient? Thanks -John

4 3

Re: Data.List.join
by Stefan Holdermans 21 Oct '06

21 Oct '06

Samuel wrote: >> But wait! There is also a join in Control.Monad! Josef replied: > Good point. But I don't really see that as a problem. Don't you think > the two 'join's can co-exist? I do think it's a problem, for list are also monads. Instantiating the monadic join with [] we'd wind up with both join :: [[a]] -> [a] and join :: [a] -> [[a]] -> [a] which to me seems just a little too confusing. Cheers, Stefan

1 0

List comparisons and permutation group code
by Mikael Johansson 21 Oct '06

21 Oct '06

Comparing the code for permutationgropus at http://www.polyomino.f2s.com/david/haskell/codeindex.html with my own thoughts on the matter, I discover the one line to figure out whether a specific list represents the identity: isIdentity (PL xs) = all (\(i,j) -> i==j) (zip [1..] xs) Is there any sort of benefit to be won by using this construction instead of isIdentity (PL xs) = xs == [1..(length xs)] and if so, what? Best, -- Mikael Johansson | To see the world in a grain of sand mikael(a)johanssons.org | And heaven in a wild flower http://www.mikael.johanssons.org | To hold infinity in the palm of your hand | And eternity for an hour

12 20

getting a DB library working with ghc 6.6 and PostgreSQL 7.4.7
by Seth Gordon 21 Oct '06

21 Oct '06

I'm looking for an alternative to HSQL for database connectivity -- the lack of prepared statements in HSQL is particularly worrisome. I installed HDBC, but when I tried running a simple program that used it, I get the error message ghc-6.6: /usr/local/lib/HDBC-postgresql-1.0.1.0/ghc-6.6/HSHDBC-postgresql-1.0.1.0.o: unknown symbol `PQserverVersion' Looking more closely at the README.txt file there, it seems that the PostgreSQL driver is expecting 8.1, and I'm using 7.4.7. I installed the ODBC driver, and I know that in theory one can connect to my PostgreSQL server with ODBC, but as far as I can tell from the documentation, I would have to configure a separate DSN for every database instance that I would want to connect to, which makes it kind of useless for a utility that any user could pass database names to on the command line. The takusen package looks interesting, but when I checked it out and tried to build it (with "ghc --make -o setup Setup.hs"), I got the error message Setup.hs:6:7: Could not find module `Distribution.Compat.FilePath': it is hidden (in package Cabal-1.1.6) How do I work around these? Is there another library worth looking into?

2 2

[Haskell-cafe] Re: Strictness, order of IO operations: NewCGI & HDBC
by Tim Smith 21 Oct '06

21 Oct '06

Hello, Haskell Cafe. I posted a question a while ago about this, but didn't receive any responses. I'd like to try again. I've got a test case which uses John Goerzen's HDBC.ODBC. The problem I have is that it appears too lazy - using the results of a query after disconnecting causes an "unknown exception". If I use the results before disconnecting, it works fine. module Main where import Data.List (intersperse) import qualified Database.HDBC as DB import Database.HDBC.ODBC (connectODBC) main :: IO () main = do dbh <- connectODBC "DSN=test" res <- DB.getTables dbh -- print (show ((concat . intersperse ", ") res)) DB.disconnect dbh print (show ((concat . intersperse ", ") res)) Compiling and running this will show: $ ./db-discon db-discon: unknown exception If I uncomment the first 'print' line, then it works as expected: $ ./db-discon "\"d1, foo, odbctest\"" "\"d1, foo, odbctest\"" Am I just expecting the wrong thing from Haskell? Is there a technical reason why HDBC can't synchronize the IO so that everything is resolved before the disconnect? Or is this a bug in HDBC? Thanks, Timothy -- If you're not part of the solution, you're part of the precipitate.

4 6

Error building Edison 1.2.0.1
by Lyle Kopnicky 19 Oct '06

19 Oct '06

Hi folks, I tried to build edison-1.2.0.1-sources with the command 'make system' but got: *** Exception: Line 10: Unknown field 'hs-source-dirs' I am using GHC 6.4.1. Any idea how to fix this? Thanks, Lyle

3 13

A better syntax for qualified operators?
by Brian Hulley 19 Oct '06

19 Oct '06

Hi - Consider the scenario when you want to find a function that returns the i'th element of an array but all you know is that there is a module called Data.Array.IArray that will probably have such a function in it. So you start typing in your program: let ith = Data.Array.IArray. at this point, you'd hope the editor you're using would somehow display a list of avaliable values exported from Data.Array.IArray including the indexing function, so you could select it, thus I would *like* to be able to use the syntax: let ith = Data.Array.IArray.(!) because it's not the user's fault that the person who wrote Data.Array.IArray decided to use a symbol instead of an identifier for this function - the user of Data.Array.IArray in this case just wants to see normal identifiers to use with prefix application so the use of (!) at this point effectively gets rid of the unwanted operatorness associated with the function. However the current syntax of Haskell would not allow this. Instead you have to write: let ith = (Data.Array.IArray.!) The problem is that the user of Data.Array.IArray has to know already in advance, before typing the 'D' of "Data", that the indexing function has been named with a symbol instead of an identifier, but this knowledge is only available later, when the user has typed the '.' after "IArray", so the current syntax would be frustrating for the user because the user then has to go all the way back and insert an opening paren before the 'D'. Also, consider the appearance of: let ith = (Data.Array.IArray.!) arr i b = Data.Array.IArray.bounds arr vs let ith = Data.Array.IArray.(!) arr i b = Data.Array.IArray.bounds arr I'm not sure if I've managed to explain this problem clearly enough, but my proposal is that we might consider changing the lexical syntax of Haskell as follows: varId ::= id varOp ::= symbol varIdOp ::= ` varId varOpId ::= ( varOp ) varOpIdOp ::= ` varOpId qvarId ::= {conId .}+ varId -- { }+ denotes 1 or more times qvarIdOp ::= ` qvarId qvarOp ::= {conId .}+ varOp qvarOpId ::= {conId .}+ varOpId qvarOpIdOp ::= `qvarOpId In other words, to turn an operator symbol into an id, the parentheses would be put immediately around the symbol (with no spaces since this is lexical syntax), and to turn an id into an operator the backquote is put in front of the entire (qualified) id. (Also the trailing backquote in the existing syntax is redundant) The above syntax would have 3 advantages: 1) It allows the client of a module to write code without having to worry if the author of the module used symbols or identifiers to name functions - everything exported from the module can be made to appear as if it was named by an identifier (ie OpId) 2) Moving the parentheses to the lexical syntax makes syntax highlighting easier (because there are no comments to worry about inside the OpId) and also makes parsing simpler because all the mess associated with Ops versus Ids is handled by the lexer 3) It allows an editor to make a distinction between (+) -- an operator turned into an identifier - varOpId ( + ) -- an expression with 2 gaps in it which should be marked as incomplete (+ ) -- a section with 1 gap Some examples of the proposed syntax are: let ith = Data.Array.IArray.(!) arr i foo = k `Math.(+) 6 -- default precendence bar = k Math.+ 6 -- using precedence of + in module Math When you try to write an editor for Haskell (or some subset of it), you quickly discover these areas of Haskell syntax like the above which need to be changed to get an optimum interactive editing experience. I think it *is* possible to adjust the Haskell grammar so that it is LL(1) and the only reason it is not already LL(1) seems to be that the grammar has been designed with compilers (which only need to deal with complete modules) in mind rather than programmers interactively editing in mind. (The other change needed for LL(1) is to give contexts a marker before they appear eg: foo :: {MonadIO m} a -> m a ) By LL(1) I'm really meaning that the grammar for interactive editing needs to be adjusted so that it is possible to maintain the invariant that as code is entered from left to right constructs and identifiers can be highlighted according to their grammatical role and highlighting (modulo incompleteness) must remain unchanged regardless of whatever is typed afterwards to the right otherwise it can become more of a liability than a help, hence my hope that some future revision of Haskell grammar might consider taking the above points into account. Regards, Brian. -- Logic empowers us and Love gives us purpose. Yet still phantoms restless for eras long past, congealed in the present in unthought forms, strive mightily unseen to destroy us. http://www.metamilk.com

9 14

Thanks !! (was: Newbie and working with IO Int and Int)
by Víctor A. Rodríguez 19 Oct '06

19 Oct '06

I want to thanks you all for your helps regarding this issue. You've been very helpful and comprehensive. Salute !! -- Víctor A. Rodríguez (http://www.bit-man.com.ar) El bit Fantasma (Bit-Man) Perl Mongers Capital Federal (http://cafe.pm.org/) GNU/Linux User Group - FCEyN - UBA (http://glugcen.dc.uba.ar/)

1 0

Re: [Haskell] Lexically scoped type variables
by Bulat Ziganshin 19 Oct '06

19 Oct '06

Hello Simon, Wednesday, October 18, 2006, 12:09:40 PM, you wrote: > Other alternatives would be: do something about partial type > signatures; or make GADT type inference more sophisticated (and > thereby perhaps less predicatable). just one more problem is that this issue is too complicated. i'm not sure that i correctly understands details, but for me the situation seems like this: in 6.4 it was no distinction between declarations and usages of type variables - first use declared it, while in 6.6 we have exactly defined places to declare type variable. the problem is that sometimes we want to declare variable for one type but the place where it may be declared - function signature - requires to write full type so, if my understanding is correct, may be it's possible to use special syntax for explicit _declaration_ of type variable at any place where it can be used? i mean the following: f (x :: forall a. a) = undefined::a here, we use 'forall' to explicitly declare new type variable. so it is equivalent to the following: f :: forall a. a -> ... f x = undefined::a where '...' used to make partial type signature -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

2 1

beginner's problem about lists
by falseep＠gmail.com 19 Oct '06

19 Oct '06

Hi all, I'm trying to implement a function that returns the shorter one of two given lists, something like shorter :: [a] -> [a] -> [a] such that shorter [1..10] [1..5] returns [1..5], and it's okay for shorter [1..5] [2..6] to return either. Simple, right? However, it becomes difficult when dealing with infinite lists, for example, shorter [1..5] (shorter [2..] [3..]) Could this evaluate to [1..5]? I haven't found a proper implementation. Again it's ok for shorter [2..] [3..] to return whatever that can solve the above problem correctly. An infinite list could work, I guess, but I don't know how. Thanks for any help.

17 26

optimization help
by jeff p 18 Oct '06

18 Oct '06

Hello, I have been trying to do some CSV-style processing. My code works fine for small input (up to 10MB), but performs poorly for moderate to large input (it can't seem to finish 100MB of input with 700MB heap space). I have gone through several optimization passes with profiler help, and now I am hoping someone else can point out some other approaches to improving the code's performance (both space and time). The code breaks a large file into smaller files all of whose entries have the same date. thanks, Jeff ------------------------ module Main where import Debug.Trace import Control.Monad import Data.List import qualified Data.ByteString.Lazy.Char8 as B import qualified Data.Map as M import System.Environment (getArgs) dataDir = "dataH/" myRead file = do v <- B.readFile file let (cols' : rows) = map (B.split ',') $ B.lines v let cols = foldl' (\mp (k,v) -> M.insert k v mp) M.empty (zip cols' [0 ..]) return (cols, rows) getColId cols col = M.lookup col cols getCol cols col row = do i <- getColId cols col return $! row!!i dates file nRows = do (cols, rows) <- myRead file let addDate mp row | mp `seq` row `seq` False = undefined | otherwise = do date <- getCol cols (B.pack "\"Date\"") row let old = M.findWithDefault [] date mp return $ M.insert date (row:old) mp res <- foldM addDate M.empty $ take nRows rows mapM_ writeDate $ M.toList res where fmt = B.unpack . B.map (\x -> if x == '-' then '_' else x) . B.takeWhile (/= ' ') writeDate (date,rows) = B.writeFile (dataDir++fmt date) (B.unlines $ map (B.join (B.pack ",")) rows) main = do args <- getArgs case args of ["dates",file,nRows] -> dates file (read nRows)

4 16

Re: Lexically scoped type variables
by apfelmus＠quantentunnel.de 18 Oct '06

18 Oct '06

I think the problem comes from the fact that let (x :: a) = rhs in expr has two interpretations: either the programmer mainly specifies a judgment (\Gamma |- rhs :: a) (f.i. as a guide for type inference) or he just introduces a new variable and sees let as a sugar for (\(x :: a) -> expr) rhs with the viewpoint that the program is a huge expression and all top-level declarations are just (somewhat superflous) lambdas which lead to a final goal. In both cases, one would assume that the main variable binder is our good old lambda. Thus, the (::) in (\(x::a) -> ..) behaves like a constructor and the type variable on the right hand side is just a pattern: f (x :: [a]) = ... is not any different from f (Cons x [a]) = ... When f is applied to an argument, it's the type checker that performs "pattern matching". Of course, one needs nonlinear "type patterns": (f :: a -> b) $ (x :: a) = f x So the type variables in lambdas behaves like normal variables and things like g :: Int -> Int g (x :: a) = x + 1 :: a are entirely fine. All in all, type variables in lambdas are real variables. The first interpretation of let amounts to the Haskell98 intuition: the code x :: a -> a x = rhs means a judgment that x has type (forall . a -> a). This also means that this does not bring any new type variables in scope, only lambdas are capable of introducing new variables. GHC 6.6 rules are not compatible with that. They somehow admit variable bindings by judgments only to discover that this doesn't work for existential types. But note that we have (x :: ...) and (x=...) as separate statements, and we may well keep the second interpretation: f :: Int -> Int (f :: a -> a) = \x -> (x+1 :: a) The intuition is that stand-alone statements (x :: ...) are exactly what they look like: judgments for the type system. The only odd thing about them is that they are stand-alone and do not add to the "main program expression". IMHO, the most natural thing would be that type judgments appear on the rhs of a let or a lambda and that type variable bindings appear on the lhs of a let or a lambda. In the definition of f above, (f :: Int -> Int) and (x+1 :: a) are judgments and (f :: a -> a) is a variable binding. Any confusion between judgment and binding is just like a confusion between constructor application and pattern. Regards, apfelmus

1 0

Re: [Haskell-cafe] Newbie and working with IO Int and Int
by Víctor A. Rodríguez 18 Oct '06

18 Oct '06

> What's wrong with doing it this way? > > -- ** UNTESTED CODE ** > > verifyAdd :: Int -> Int -> Int -> Bool > verifyAdd a b sum | a + b == sum = True > otherwise = False > > testAddMundane :: Int -> Int -> Bool > testAddMundane a b = verifyAdd a b (a + b) > > -- all the IO-dependent stuff is below this line -- > > testAddRandom :: IO Bool > testAddRandom = do a <- randomIO > b <- randomIO > return verifyAdd a b (a + b) > I discovered something worst yet :-P Using the next code and calling verifyAdd or testAddMundane it says : Program error: verifyAdd: ERROR Instead calling testAddRandom only says : :: IO Bool (55 reductions, 92 cells) ---- CODE STARTS HERE, AND IS TESTED ----- import Random verifyAdd :: Int -> Int -> Int -> Bool verifyAdd a b sum = error "verifyAdd: ERROR" testAddMundane :: Int -> Int -> Bool testAddMundane a b = verifyAdd a b (a + b) -- all the IO-dependent stuff is below this line -- testAddRandom :: IO Bool testAddRandom = do a <- randomIO b <- randomIO return ( verifyAdd a b (a+b) ) -- Víctor A. Rodríguez (http://www.bit-man.com.ar) El bit Fantasma (Bit-Man) Perl Mongers Capital Federal (http://cafe.pm.org/) GNU/Linux User Group - FCEyN - UBA (http://glugcen.dc.uba.ar/)

4 4

Re: 6.4 -> 6.6, scoped type variables
by oleg＠pobox.com 18 Oct '06

18 Oct '06

I too miss the old way of handling local type variables. Previously, local type annotations worked a lot like an additional constraint, with type variables denoting some type. The same type variable denoted the same type. Here's how it works in OCaml, for example: # let pair x y = (x,y);; val pair : 'a -> 'b -> 'a * 'b = <fun> # let pair1 (x:'a) (y:'a) = (x,y);; val pair1 : 'a -> 'a -> 'a * 'a = <fun> # let pair2 x y = ((x:'a),(y:'a));; val pair2 : 'a -> 'a -> 'a * 'a = <fun> # let pair3 x (y:int) = ((x:'a),(y:'a));; val pair3 : int -> int -> int * int = <fun> # let pair4 x (y:int) = ((x:'a list),(y:'a));; val pair4 : int list -> int -> int list * int = <fun> # let pair5 (x:'a list) (y:int) = (x,(y:'a));; val pair5 : int list -> int -> int list * int = <fun> Which of the two occurrences of the type variable |'a| a binding one? Who cares... It just works: within the same expression, the same type variable stands for the same type. > I'm looking for some tips and references regarding prescribing > poly-/mono-morphism. I know of three ways to do it: > 1) rely on the monomorphism-restriction -- kind of scary in that its > not explicitly obvious > 2) introducing scoped-type variables of interest -- explicit and modular > 3) introducing all type variables with a forall -- explicit but not modular It seems that with GCH 6.6, we are forced to go back to the fourth method -- the one that was available back in Haskell98. Use useless expressions to hint the typechecker. > bop :: forall x y z m c v . > ( SubType x v, SubType y v, SubType z v > , SubType (Fun m c v) v > , Monad m, Comonad c > ) => (x -> y -> z) -> Fun m c v > bop op = Fun fn > where fn = wrap fn' > where fn' v0 = (inj . Fun) (wrap fn'') > where fn'' v1 = inj (lazyPrj v0 `op` lazyPrj v1) > wrap :: (v -> v) -> (c v -> m v) > wrap = (return .) . (. counit) > If I leave the types to inference (i.e. drop the forall and the type > ascription for |wrap|), then I end up with this type > > bop :: ( SubType x v2, SubType y v1, SubType z v1 > , SubType (Fun m c v1) v2 > , Monad m, Comonad c > ) => (x -> y -> z) -> Fun m c v2 > Note the multiple |v|s; they wreak havoc in the rest of my code. It seems that you wish that, for example, v0 and v1 be of the same type. We have to be direct then and write where fn'' v1 = inj (lazyPrj v0 `op` lazyPrj (v1 `asTypeOf` v0) or where fn'' v1 = inj (lazyPrj v0 `op` lazyPrj v1) where useless = if True then v0 else v1 or where fn'' v1 | const True [v0,v1] = inj (lazyPrj v0 `op` lazyPrj v1) or something similar...

1 0

Type level functions (type of decreasing list)
by Greg Buchholz 18 Oct '06

18 Oct '06

I'm wondering about creating a data structure that has the type of decreasing numbers. If I want an increasing list, it is easy... > {-# OPTIONS -fglasgow-exts #-} > > data Succ a = S a deriving Show > data Zero = Z deriving Show > > data Seq' a = Cons' a (Seq' (Succ a)) | Nil' deriving Show ...which can be used like... > > zero = Z > one = S zero > two = S one > three = S two > > increasing = Cons' zero (Cons' one (Cons' two (Cons' three Nil'))) ...on the other hand, if I want the decreasing list, I can try... > > class Pre a b | a -> b > instance Pre (Succ a) a > instance Pre Zero Zero > > data (Pre a b) => Seq a = Cons a (Seq b) | Nil > > decreasing = Cons three (Cons two (Cons one Nil)) ...but that complains about "Not in scope: type variable `b'". Of course that makes perfect sense, but I'd like to know if there is a Haskell idiom that I'm missing in order to get this to work. Thanks, Greg Buchholz

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