- Beginners - Haskell.org

Help with monad syntax?
by Arindam Roy 26 Aug '08

26 Aug '08

I'm trying to learn Haskell using Hutton's book and Hugs (Sep 06), and am trying to work the parsing examples (chapter 8, Functional parsers), but I can't understand the error I'm seeing when I try to load the program into Hugs. Following the book, I've defined a Parser type, then written the basic parsers, then a sequencing operator >>=, followed by the example p parser (bottom of page 77). However, trying to load this file in Hugs generates an error (caused by the definition of p). I've given below the entire file I've created and the error message I get on trying to load it. Any help much appreciated - Arindam ---------------------------------------- type Parser a = String -> [(a, String)] return :: a -> Parser a return v = \inp -> [(v, inp)] failure :: Parser a failure = \inp -> [] item :: Parser Char item = \inp -> case inp of [] -> [] (x:xs) -> [(x, xs)] parse :: Parser a -> String -> [(a, String)] parse p inp = p inp (>>=) :: Parser a -> (a -> Parser b) -> Parser b p >>= f = \inp -> case parse p inp of [] -> [] [(v, out)] -> parse (f v) out p :: Parser (Char, Char) p = do x <- item item y <- item Main.return (x, y) ------------------------------------------ Hugs> :r ERROR file:c:\parsers.hs:23 - Type error in explicitly typed binding *** Term : p *** Type : [Char] -> [(([(Char,[Char])],[(Char,[Char])]),[Char])] *** Does not match : Parser (Char,Char) Hugs> -- Arindam Roy arind_roy(a)fast-mail.org -- http://www.fastmail.fm - IMAP accessible web-mail

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How does the type of "ap = liftM2 id" work?
by Quergle Quergle 26 Aug '08

26 Aug '08

Hello, I'm a bit perplexed by what it means to do something like "ap = liftM2 id" liftM2 :: (Monad m) => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r id :: a -> a liftM2 id :: (Monad m) => m (a2 -> r) -> m a2 -> m r My intuitive understanding is that liftM2 is expecting a two-argument function as its first argument, so it seems a little unexpected to pass it "id", a one-argument function. And I can't really see how the type signature of "liftM2 id" is derived from "liftM2" and "id". Any help much appreciated! -- Matt

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Re:Re Puzzling type erros
by Logesh Pillay 25 Aug '08

25 Aug '08

Thanks, chaps, for the replies to my over-hasty post. I should have checked with a power of 10 value for scale which would have pointed to types to logBase as the error. Andrew's comment about floating point not having unlimited precision clears up a glitch that using fromInteger introduces. If you use it to typecast b, it appears to set a limit to the number of digits it generates. It only gives you 308 or so even if you set scale equal to 1000, say. If you test b against 10^scale, you get as many digits as you wish. Logesh Pillay > Message: 10 > Date: Sun, 24 Aug 2008 21:07:53 -0400 > From: ajb(a)spamcop.net > Subject: Re: [Haskell-beginners] Puzzling type error > To: beginners(a)haskell.org > Message-ID: <20080824210753.42c5u3ihw444gc8o-nwo(a)webmail.spamcop.net> > Content-Type: text/plain; charset=ISO-8859-1; DelSp="Yes"; > format="flowed" > > G'day all. > > Quoting Daniel Fischer <daniel.is.fischer(a)web.de>: > > >> > The good fix is to insert calls to the apprpriate conversion function where >> > necessary, for example >> > >> > sqRoot n scale = sqRoot' (5*n) 5 >> > where >> > sqRoot' a b >> > | floor (logBase 10 $ fromIntegral b) >= scale = div b 10 >> > | a >= b = sqRoot' (a-b) (b+10) >> > | otherwise = sqRoot' (a*100) ((100 * (div b 10)) + (mod b 10)) >> > > The problem here is that floating-point numbers do not have arbitrary > precision, whereas Integers do. For very large numbers, the > conversion might not be possible. > > Option #1: > > sqRoot n scale = sqRoot' (5*n) 5 > where > sqRoot' a b > | b >= invScale = div b 10 > | a >= b = sqRoot' (a-b) (b+10) > | otherwise = sqRoot' (a*100) ((100 * (div b 10)) + (mod b 10)) > invScale = 10^scale > > Option #2: > > sqRoot n scale = sqRoot' (5*n) 5 > where > sqRoot' a b > | length (show b) >= scale = div b 10 -- May be an off-by-one error here. > | a >= b = sqRoot' (a-b) (b+10) > | otherwise = sqRoot' (a*100) ((100 * (div b 10)) + (mod b 10)) > > Sadly, option #2 (and option #3, not shown, which is essentially to > implement your own integer-only floor-log-base-10) destroys the > pretty "mostly subtraction only" property of the algorithm. > > Cheers, > Andrew Bromage >

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Puzzling type error
by Logesh Pillay 25 Aug '08

25 Aug '08

I've written the following implementation of the algorithm in this article http://www.afjarvis.staff.shef.ac.uk/maths/jarvisspec02.pdf sqRoot n scale = sqRoot' (5*n) 5 where sqRoot' a b | floor (logBase 10 b) >= scale = div b 10 | a >= b = sqRoot' (a-b) (b+10) | otherwise = sqRoot' (a*100) ((100 * (div b 10)) + (mod b 10)) Since this involves whole numbers only, I was surprised by the following run-time error. *Main> sqRoot 2 5 <interactive>:1:0: Ambiguous type variable `t' in the constraints: `RealFrac t' arising from a use of `sqRoot' at <interactive>:1:0-9 `Floating t' arising from a use of `sqRoot' at <interactive>:1:0-9 `Integral t' arising from a use of `sqRoot' at <interactive>:1:0-9 Probable fix: add a type signature that fixes these type variable(s) What am I missing? Logesh Pillay

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plugins package
by Emmanuel 24 Aug '08

24 Aug '08

Hello I am having troubles bulding the plugins package : manu:~/Desktop/tmp manu$ cabal install plugins Resolving dependencies... cabal: cannot configure plugins-1.2. It requires ghc >=6.8 There is no available version of ghc that satisfies >=6.8 I am on a Mac PPC (OS X 10.4) with ghc-6.8.2 and Cabal-1.4.0.1 so the error message is puzzling... Can anyone help ? I'd like to install Lambdabot and plugins is one of the dependencies... Thanks Manu

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plugins package
by chomp 24 Aug '08

24 Aug '08

Hello I am having troubles bulding the plugins package : manu:~/Desktop/tmp manu$ cabal install plugins Resolving dependencies... cabal: cannot configure plugins-1.2. It requires ghc >=6.8 There is no available version of ghc that satisfies >=6.8 I am on a Mac PPC (OS X 10.4) with ghc-6.8.2 and Cabal-1.4.0.1 so the error message is puzzling... Can anyone help ? I'd like to install Lambdabot and plugins is one of the dependencies... Thanks Manu

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Running Yi 0.4.3
by Niels Aan de Brugh 23 Aug '08

23 Aug '08

Hi there, I'm having some trouble installing and running Yi 0.4.3. I just compiled and installed Yi, after disabling parsec-3.0.0 and recompiling regex-tdfa first (Yi requires a 2.x version, regex takes whatever it can get, including 3.0.0). Here's a short excerpt from my console (newlines inserted for readability). niels(~/src/yi-0.4.3): sudo runhaskell Setup.hs install Installing: /usr/local/lib/yi-0.4.3/ghc-6.8.2 Installing: /usr/local/bin Registering yi-0.4.3... Reading package info from "dist/installed-pkg-config" ... done. Saving old package config file... done. Writing new package config file... done. niels(~): mkdir .yi niels(~): cd .yi/ niels(~/.yi): cp /home/niels/src/yi-0.4.3/examples/yi.hs . niels(~/.yi): yi Launching custom yi: "/home/niels/.yi/yi-i386-linux" yi: exception :: System.Glib.GError.GError niels(~/.yi): yi --frontend=pango Launching custom yi: "/home/niels/.yi/yi-i386-linux" yi-i386-linux: exception :: System.Glib.GError.GError niels(~/.yi): yi --frontend=gtk Launching custom yi: "/home/niels/.yi/yi-i386-linux" yi-i386-linux: exception :: System.Glib.GError.GError niels(~/.yi): ghc-pkg list /usr/lib/ghc-6.8.2/package.conf: (Cabal-1.2.3.0), Cabal-1.4.0.1, (Cabal-1.5.1), Crypto-4.1.0, HTTP-3001.0.4, HUnit-1.2.0.0, HaXml-1.13.3, QuickCheck-1.1.0.0, Stream-0.2.2, X11-1.4.1, array-0.1.0.0, arrows-0.3, base-3.0.1.0, binary-0.4.1, bitset-0.6, bloomfilter-1.2.1, bytestring-0.9.0.1, cairo-0.9.12.1, cairo-0.9.13, containers-0.1.0.1, directory-1.0.0.0, fgl-5.4.1.1, filepath-1.1.0.0, fingertree-0.0, gconf-0.9.12.1, gconf-0.9.13, (ghc-6.8.2), glade-0.9.12.1, glade-0.9.13, glib-0.9.12.1, glib-0.9.13, gnomevfs-0.9.13, gstreamer-0.9.13, gtk-0.9.12.1, gtk-0.9.13, gtkglext-0.9.12.1, gtkglext-0.9.13, haddock-2.1.0, haskell98-1.0.1.0, hpc-0.5.0.0, html-1.0.1.1, hxt-8.0.0, mtl-1.1.0.0, network-2.1.0.0, old-locale-1.0.0.0, old-time-1.0.0.0, packedstring-0.1.0.0, pandoc-0.46, parallel-1.0.0.0, parsec-2.1.0.0, pretty-1.0.0.0, process-1.0.0.0, pureMD5-0.2.3, random-1.0.0.0, readline-1.0.1.0, regex-base-0.72.0.1, regex-base-0.93.1, regex-compat-0.71.0.1, regex-compat-0.91, regex-posix-0.72.0.2, regex-posix-0.93.1, regex-tdfa-0.94, rts-1.0, soegtk-0.9.13, sourceview-0.9.12.1, sourceview-0.9.13, svgcairo-0.9.13, tagsoup-0.6, template-haskell-2.2.0.0, time-1.1.2.0, unix-2.3.0.0, utf8-string-0.3.1, vty-3.0.1, wx-0.10.3, wxcore-0.10.3, xhtml-3000.0.2.1, xmonad-0.6, xmonad-contrib-0.6, yi-0.4.3 niels(~/.yi): dpkg -l | grep libghc6-gtk ii libghc6-gtk-dev 0.9.12.1-1ubuntu2 A GUI library for Haskell (Gtk2Hs) -- GTK+ b ii libghc6-gtkglext-dev 0.9.12.1-1ubuntu2 A GUI library for Haskell (Gtk2Hs) -- gtkgle I'm running a recent version of Ubuntu Linux and GHC 6.8.2. Does anyone know what mistake I'm making? I'd really like to replace Vim for Yi for Haskell source file editing some day but up until now this is actually the first time I got Yi to compile. Regards, Niels

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purely functional (non-monadic) alternatives for Haskell animations in _The Haskell School of Expression_?
by Benjamin L.Russell 22 Aug '08

22 Aug '08

Recently, I mentioned Paul Hudak's _The Haskell School of Expression_ (see http://www.haskell.org/soe/) as an example of using multimedia examples to motivate learning programming in another education-related programming mailing list, plt-scheme, in a thread there entitled "More pedagogic stuff" (see http://list.cs.brown.edu/pipermail/plt-scheme/2008-August/026441.html) In reply, a respondent, Matthias Felleisen, wrote the following (see http://list.cs.brown.edu/pipermail/plt-scheme/2008-August/026445.html) >P.S. Yes, I have read Paul's book and I think Haskell has something >to it. >You may be surprised to learn, however, that world.ss animations are >purely functional while Haskell animations (in Paul's book) are actually >quasi-imperative. That is, they are using monads and carry the >imperativeness >on their sleeves. In response to this claim, does anybody know how to rewrite Hudak's SOE animations so that they do not use monads and are "purely functional?" -- Benjamin L. Russell

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Reading input
by Barry Burd 21 Aug '08

21 Aug '08

In the last line of this program, I get the following error message (from ghci)... Couldn't match expected type `Int' against inferred type `Char' Expected type: [Int] Inferred type: String In the first argument of `median', namely `lst' In the expression: median lst Failed, modules loaded: none. Here's the program... module Main where import IO lessThan :: Int -> [Int] -> [Int] lessThan x lst = filter (< x) lst greaterThan :: Int -> [Int] -> [Int] greaterThan x lst = filter (> x) lst numLessThan :: Int -> [Int] -> Int numLessThan x lst = length (lessThan x lst) numGreaterThan :: Int -> [Int] -> Int numGreaterThan x lst = length (greaterThan x lst) numLessGreater :: Int -> [Int] -> (Int, Int) numLessGreater x lst = (numLessThan x lst, numGreaterThan x lst) isMedian :: (Int, Int) -> Bool isMedian (x, y) = x == y medians :: [Int] -> [Int] medians lst = [x | x <- lst, isMedian (numLessGreater x lst)] median :: [Int] -> Int median lst = case medians lst of -- All the values in the result are the same, we just pick the first one x:xs -> x main = do putStr "Enter a list: " lst <- getLine median lst I'm sure this is because Haskell isn't automatically changing a String to a List of numbers. But how can I do this? Thanks.

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ghci interactive behaviour.
by Venkat Ramanan 20 Aug '08

20 Aug '08

Hi all, I'm just beginning to learn haskell. I see some strange behaviour in ghci. Don't know if it is related to readline. Behaviour-1: Lets say after we enter an expression, we just hit "Enter" 3 times to insert 3 empty lines. To get back the expression, we need to do <C-p> ( or up-arrow ) 3 times. In other words, the empty lines seem to be stored in the history. Bash ( and tcsh ) seem to save only non-empty lines. Behaviour-2: It doesn't seem to do pruning of duplicates. If the same expression is entered twice, both of them are stored in the history instead of just one. Bash uses environmental variables (HISTCONTROL and HISTIGNORE ) to modify this behavior. Is there a way to get saner behaviour like in bash? I'm using ghc-6.8.2 which comes with ubuntu 8.04. Thanks, Venkat.

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