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Re: [Haskell-beginners] do not understand Semantics of Case Expressions in report
by John Dorsey 21 Oct '08

21 Oct '08

Michael, (Sorry for duplicates; I sent this from the wrong address initially.) > (m) case v of { K { f1 = p1 , f2 = p2 , ... } -> e ; _ -> e' } > = case e' of { > y -> > case v of { > K { f1 = p1 } -> > case v of { K { f2 = p2 , ... } -> e ; _ -> y }; > _ -> y }} > where f1, f2, ... are fields of constructor K; y is a new variable [...] > I'm totally confused with this identity. Especialy with " = case e'". > Can anyone explain a little what does this rule stand, please? > It is better to see some example. You got me curious, so I looked over this. The table you reference lists identities that should hold for all implementations of case expressions. (m) is about how case matches against constructors with named fields, and it shows that a case expression that matches against multiple fields (f1, f2, etc.) can be broken into nested case expressions which each match a single field (eg. f1). It looks like "case e' of { y -> ..." was introduced solely to give e' a name that can be used in two places, since the nested form may fail to match at each level. It seems to me that without this, you could get code explosion from duplicating the expression e' below; with this you get sharing. Here it is with a interim step added: -- Match v against a constructor with multiple fields case v of { K { f1 = p1 , f2 = p2 , ... } -> e ; _ -> e' } -- Same, but introduce y to represent e' without loss of sharing case e' of { y -> case v of { K { f1 = p1 , f2 = p2 , ... } -> e ; _ -> y }} -- Same, but match first field of K independent of the rest case e' of { y -> case v of { K { f1 = p1 } -> case v of { K { f2 = p2 , ... } -> e ; _ -> y }; _ -> y }} I hope this helps. Regards, John

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Weird compilation error while doing Euler problems?
by Ian Duncan 20 Oct '08

20 Oct '08

'm trying to compile my file that has my Euler problems in it to output the solution to problem four, but I'm getting a compile error. Here's my .hs file: ------------------------- module Euler1 where import Data.List import Data.Ord main = mapM_ putStrLn problem4 problem1 = foldl1' (+) $ nub $ (takeWhile (< 1000) [3,6..] ++ takeWhile (< 1000) [5,10..]) problem2 = sum $ takeWhile (<= 4000000) [x | x <- fibs, even x] where fibs = unfoldr (\(a,b) -> Just (a,(b,a+b))) (0,1) problem3 z = maximumBy compare (filter (\x -> z `mod` x == 0) (takeWhile (<= ceiling (sqrt (fromIntegral z))) primes)) problem4 = nub [ show $ y * z | y <- [100..999], z <- [100..999], show (y*z) == reverse (show $ y*z)] prime p = p `elem` primes primes = small ++ large where 1:p:candidates = roll $ mkWheel small small = [2,3,5,7] large = p : filter isPrime candidates isPrime n = all (not . divides n) $ takeWhile (\p -> p*p <= n) large divides n p = n `mod` p == 0 mkWheel ds = foldl nextSize w0 ds nextSize (Wheel n rs) p = Wheel (p*n) [r' | k <- [0..(p-1)], r <- rs, let r' = n*k+r, r' `mod` p /= 0] w0 = Wheel 1 [1] roll (Wheel n rs) = [n*k+r | k <- [0..], r <- rs] data Wheel = Wheel Integer [Integer] ------------------------ Here's my output when I try to compile: ian$ ghc ~/Documents/eulerProblem1.hs -o test Undefined symbols: "___stginit_ZCMain", referenced from: ___stginit_ZCMain$non_lazy_ptr in libHSrts.a(Main.o) "_ZCMain_main_closure", referenced from: _ZCMain_main_closure$non_lazy_ptr in libHSrts.a(Main.o) ld: symbol(s) not found collect2: ld returned 1 exit status What's going on? I'm running GHC 6.8.3, if that helps.

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do not understand Semantics of Case Expressions in report
by Michael Mekhanoshin 20 Oct '08

20 Oct '08

at http://www.haskell.org/onlinereport/exps.html on figure Figure 4 (Semantics of Case Expressions, Part 2) one can see: (m) case v of { K { f1 = p1 , f2 = p2 , ... } -> e ; _ - > e' } = case e' of { y -> case v of { K { f1 = p1 } -> case v of { K { f2 = p2 , ... } -> e ; _ -> y }; _ -> y }} where f1, f2, ... are fields of constructor K; y is a new variable (In Figures 3.1--3.2: e, e' and ei are expressions; g and gi are boolean-valued expressions; p and pi are patterns; v, x, and xi are variables; K and K' are algebraic datatype (data) constructors (including tuple constructors); and N is a newtype constructor.) I'm totally confused with this identity. Especialy with " = case e'". Can anyone explain a little what does this rule stand, please? It is better to see some example. Respect, Michael

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Re: [Haskell-beginners] requested module name differs from name found in interface file
by Larry Evans 20 Oct '08

20 Oct '08

On 10/20/08 11:38, Justin Bailey wrote: > On Mon, Oct 20, 2008 at 9:25 AM, Larry Evans <cppljevans(a)suddenlink.net> wrote: > [snip] > >> Do you mean install it manually as shown in the aforementioned >> How_to_install_a_Cabal_package? >> > > No, I mean go to http://hackage.haskell.org and download the .tar.gz > file directly (http://hackage.haskell.org/cgi-bin/hackage-scripts/package/category-extras) > Unzip/untar that package. I haven't looked at it, but there is > probably a "src" directory. Put your example file in there and load it > in ghci. Your source file should then use the source available in that > directory tree rather than the category-extras install which seems > broken. This works because the GHC will look at the import > "Control.Functor.Fix" and use that to look for > "Control/Functor/Fix.hs" from the current directory. > Tried that but: <--- cut here --- GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. Prelude> :load "/home/evansl/download/haskell/libs/category-extras-0.53.5/src/catamorphism.example.hs" Control/Category/Dual.hs:17:7: Could not find module `Control.Category': Use -v to see a list of the files searched for. Failed, modules loaded: none. Prelude> <--- cut here --- If it's any help, the directory list for src/Control/Category is: <--- cut here --- /home/evansl/download/haskell/libs/category-extras-0.53.5/src/Control/Category: total used in directory 52 available 148798604 drwxr-xr-x 3 evansl evansl 4096 Jun 30 08:08 . drwxr-xr-x 9 evansl evansl 4096 Jun 30 08:08 .. -rw-r--r-- 1 evansl evansl 2118 Jun 30 08:08 Associative.hs -rw-r--r-- 1 evansl evansl 1668 Jun 30 08:08 Braided.hs drwxr-xr-x 2 evansl evansl 4096 Jun 30 08:08 Cartesian -rw-r--r-- 1 evansl evansl 4973 Jun 30 08:08 Cartesian.hs -rw-r--r-- 1 evansl evansl 1020 Jun 30 08:08 Discrete.hs -rw-r--r-- 1 evansl evansl 1340 Jun 30 08:08 Distributive.hs -rw-r--r-- 1 evansl evansl 668 Jun 30 08:08 Dual.hs -rw-r--r-- 1 evansl evansl 578 Jun 30 08:08 Hask.hs -rw-r--r-- 1 evansl evansl 3254 Jun 30 08:08 Monoidal.hs -rw-r--r-- 1 evansl evansl 1746 Jun 30 08:08 Object.hs >--- cut here --- > You might have more luck finding out what's going on by emailing > Haskell-Cafe directly, too. > Yes, thanks. > Justin > >

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requested module name differs from name found in interface file
by Larry Evans 19 Oct '08

19 Oct '08

With a file containing: > module Main where > > import Array > import Control.Functor.Fix I get: > make > ghc -i/root/.cabal/lib/category-extras-0.53.5/ghc-6.8.2 -c > catamorphism.example.hs > > catamorphism.example.hs:19:0: > Bad interface file: > /root/.cabal/lib/category-extras-0.53.5/ghc-6.8.2/Control/Functor/Fix.hi > Something is amiss; requested module main:Control.Functor.Fix > differs from name found in the interface file > category-extras-0.53.5:Control.Functor.Fix > make: *** [all] Error 1 I used cabal to install category-extras: > /home/evansl/download/haskell/libs # cabal install category-extras > Resolving dependencies... > Downloading category-extras-0.53.5... > Configuring category-extras-0.53.5... > Preprocessing library category-extras-0.53.5... ... > /usr/bin/ar: creating dist/build/libHScategory-extras-0.53.5.a > Installing library in /root/.cabal/lib/category-extras-0.53.5/ghc-6.8.2 > Registering category-extras-0.53.5... > Reading package info from "dist/installed-pkg-config" ... done. > Saving old package config file... done. > Writing new package config file... done. > /home/evansl/download/haskell/libs # What should I do to import the Functor.Fix as shown here: http://comonad.com/reader/2008/recursion-schemes/ TIA. -Larry

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Mathematical Blundering
by Jeffrey Drake 17 Oct '08

17 Oct '08

I have defined myself a set of functions to test: fact 1 = 1 fact n = n * (fact $ n - 1) sine x = x - (x^3/(fact 3)) + (x^5/(fact 5)) - (x^7/(fact 7)) Where my code is 'sine' and the prelude's is sin: *Main> sine 1 0.841468253968254 *Main> sin 1 0.8414709848078965 *Main> sine 2 0.9079365079365079 *Main> sin 2 0.9092974268256817 *Main> sine 3 9.107142857142847e-2 *Main> sin 3 0.1411200080598672 *Main> sine 4 -1.3841269841269837 *Main> sin 4 -0.7568024953079282 After 2 they seem to diverge rather rapidly, and I am not sure why. Any ideas? I would have thought that 4 terms would have been enough. - Jeff.

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IO Problem
by Jamie McCloskey 17 Oct '08

17 Oct '08

Hi everyone, I have just started learning Haskell, it is a very interesting language. As a first project, I am building a brainfuck interpreter, but have hit a stumbling block: IO. My program source is located at http://hpaste.org/11219 My program handles the state of a brainfuck program by passing State(lists of ints) objects around. When I added my getIn fuction, however, it needed to return an IO State object. This broke the run function, as it returned a State. No problem, I thought, and changed run to return IO State. This broke my exec function, which calls run on each element in a list. As exec passes on the state returned by the run function to itself, it needs to take an IO State. This causes problems in the loop function, which calls exec on a subprogram. Because exec now takes IO State, loop also needs to. However, loop is called by run, so now run needs to take IO State. Now, all functions called by run need to take IO State. All this creates a whole load of functions with IO, even though it should not be necessary. What I would like to know, is how I can avoid this, possibly by modifying exec to just take a State. Whew! What a long-winded explanation! Any ideas? --Jamie P.S. Please ignore my long-winded list iteration functions -- I'm working on a cleaner version.

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HaXml.SAX successfully parses a malformed XML document
by David Frey 16 Oct '08

16 Oct '08

It seems that saxParse from Text.XML.HaXml.SAX will successfully parse a malformed XML document. Notice that the SubElem opening and closing tag are not matched in the XML document below. --- input.xml --- <RootNode> <ElemOne> <SubElem attr="foo">element data</SubElemBroken> <NoDataElem/> <ElemOne> </RootNode> --- end input.xml --- The Haskell code below runs without error. It prints out the type of elements found during the parse. --- Haskell Code --- module Main where import qualified Text.XML.HaXml.SAX as SAX import Data.Maybe main = let inputFilename = "input.xml" in do content <- readFile inputFilename let (elements, error) = SAX.saxParse inputFilename content if isNothing error then mapM_ putStrLn (summarizeElements elements) else putStrLn $ "ERROR:" ++ (fromJust error) summarizeElements :: [SAX.SaxElement] -> [String] summarizeElements elements = map summarizeElement elements summarizeElement :: SAX.SaxElement -> String summarizeElement element = case element of (SAX.SaxDocTypeDecl d) -> "DocType" (SAX.SaxProcessingInstruction p) -> "Processing Instruction" (SAX.SaxComment s) -> "Comment" (SAX.SaxElementOpen name attrs) -> "Element Open" (SAX.SaxElementClose name) -> "Element Close" (SAX.SaxElementTag name attrs) -> "No Content Element" (SAX.SaxCharData charData) -> "Character Data" (SAX.SaxReference reference) -> "Reference" --- End Haskell Code --- The Python code below throws an exception when parsing the same input document. --- Python Code --- from xml.sax import make_parser from xml.sax.handler import ContentHandler def main(): c = ContentHandler() p = make_parser() p.setContentHandler(c) p.parse(open('input.xml')) if __name__ == '__main__': main() -- End Python Code --- Is this a bug in the HaXml SAX parser? Thanks, David

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better way to create Array defined on all indices
by Larry Evans 16 Oct '08

16 Oct '08

I'd like to have an Array indices values (see http://haskell.org/onlinereport/array.html) defined on [minBound::indices ... maxBound::indices]. The attached does this; however, it also allows duplicate definitions (e.g. in the attached, the value for Term index is defined twice). Is there a better way to do this? Maybe a way that doesn't use all the contexts? Also, is there a way to do it so that the value associated with an index is not redefined? In effect, I'm making a map, but using an Array as the implementation because, I guess, it would be a bit faster. TIA.

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fix
by Matthew J. Williams 15 Oct '08

15 Oct '08

hello listers, a few days ago A fellow lister sent me the following link: http://en.wikibooks.org/wiki/Haskell/Fix_and_recursion The 'fix' function is interesting to say the least. There is one example that I've had difficulty expanding: fix (\rec n -> if n == 0 then 1 else n * rec (n-1)) 5 120 My interpretation: fix (\rec n -> if n == 0 then 1 else n * rec (n-1)) 5 ((\rec n -> if n == 0 then 1 else n * rec (n-1)) (fix (\rec n -> if n == 0 then 1 else n * rec (n-1)) )) 5 . . . Yet, it does not quite explain how 'fix' does not result in infinite recursion. Sincerely Matthew J. Williams

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