November 2007 - Haskell-Cafe

C++ vs. Haskell: Type-Level Death Match
by Don Stewart 10 Nov '07

10 Nov '07

http://www.rubinsteyn.com/template_insanity.html The C++ response to Conrad's 'instant insanity' type program from the last Monad.Reader ! Can we do better still with the planned type-level programming of type families? -- Don

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Some More Sinus Results
by Hans van Thiel 10 Nov '07

10 Nov '07

Thanks to all who've replied; Carl's explanation in particular was very interesting. So the precision, suggested by the many decimals in the 'show', is not the actual precision the user should 'count on'. If you take 1/60 of a degree to be approximately 0.0003 radians, you should not use sin for smaller values. In all cases the actual precision of sin appears to be 4 to 5 decimals, and results should be rounded to that before using them. Now I'm wondering about cos, tan and also the inverses, asin etc. :-) Regards, Hans van Thiel Hugs> sin (1.000 * pi) 1.22460635382238e-16 Hugs> sin (0.999 * pi) 0.00314158748587949 Hugs> sin (1.00001 * pi) -3.14159265309255e-05 Hugs> sin (0.99999 * pi) 3.14159265307264e-05 Hugs> sin (1.001 * pi) -0.00314158748587925 Hugs> sin (0.999 * pi) 0.00314158748587949 Hugs> sin (1.0001 * pi) -0.000314159260191213 Hugs> sin (0.9999 * pi) 0.000314159260191458 Hugs> sin (1.00001 * pi) -3.14159265309255e-05 Hugs> sin (1.0001 * pi) -0.000314159260191213 Hugs> sin (0.9999 * pi) 0.000314159260191458 Hugs> sin (1.00001 * pi) -3.14159265309255e-05 Hugs> sin (0.99999 * pi) 3.14159265307264e-05 Hugs> sin (6.0001 * pi) 0.000314159260189269 Hugs> sin (5.9999 * pi) -0.000314159260190738 Hugs> sin (6.00001 * pi) 3.14159265298692e-05 Hugs> sin (5.99999 * pi) -3.14159265313387e-05 Hugs> sin pi 1.22460635382238e-16

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FW: please help... small problem
by Ryan Bloor 10 Nov '07

10 Nov '07

hi I've attempted to cut down this module... but I cannot see where... can someone help... Ryan thanks From: ryanbloor(a)hotmail.comTo: Subject: FW: please help... small problemDate: Fri, 9 Nov 2007 21:57:30 +0000 sorry heres the code.... I always do that. From: ryanbloor(a)hotmail.comTo: Subject: please help... small problemDate: Fri, 9 Nov 2007 21:44:35 +0000 hi Is there anyway to cut down this code and to not use auxillary functons, but instead use pattern matching? The code basically splits up a list 'rslis' into a list of lists - but so each word is split up and the integers have been parsed. so.... ["hi ryan 1","hi jeff 2"] becomes [["hi","ryan" 1], ["hi","jeff", 2]].The code is far too long. I don't wanna use premade functions too much... pattern matching is required. Ryan The next generation of MSN Hotmail has arrived - Windows Live Hotmail Are you the Quizmaster? Play BrainBattle with a friend now! _________________________________________________________________ 100’s of Music vouchers to be won with MSN Music https://www.musicmashup.co.uk

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Doubly-linked zipper list w/ insert implementation
by Justin Bailey 10 Nov '07

10 Nov '07

The other day I decided to implement a ring buffer with a current element (i.e. a doubly-linked zipper list). In order to allow inserts (and, in the future, deletes and updates), I have a special sentinel element called "Join" in the structure. When inserting, I find the join first, insert and then rebuild the buffer using circular programming techniques. This also allows the buffer to be converted back to a list. The current element can be changed by rotating right or left, which never fails. Rotating n positions takes n steps. I'm posting it here for comments and feedback. How could the structure be smarter? Would storing a unique ID with each element make more sense? Any comments on the space behavior under insert and rotates? I wanted to "maximize" sharing. Thanks in advance. Justin p.s. The original motivation for writing this was to model cellular automata. The CA world is "circular", so that got me thinking about a structure that made connecting the ends easy to do. -- cut here --- module Ring (Ring, create, insert, current, rotR, rotL, toListL, toListR) -- Thanks to Keith Wansbrough for his posting describing a doubly-linked list -- in Haskell for inspiration here. -- -- http://groups.google.com/group/comp.lang.functional/msg/8c65fdd16f7e91e1 where import Data.List (foldl') -- For testing import Test.QuickCheck import Control.Monad (replicateM_) import System.Random (randomRIO) import System.Environment (getArgs) data Ring a = Ring (Ring a) a (Ring a) | Join (Ring a) (Ring a) instance (Show a) => Show (Ring a) where show r = "{" ++ show' (findLeftOfJoin r) where show' (Join _ _) = "}" show' (Ring l v _) | isJoin l = show v ++ show' l | otherwise = show v ++ "," ++ show' l create v = let me = Ring join v join join = Join me me in me insert r a = let (left, right) = insert' left right start a start = findLeftOfJoin r in left where insert' left right (Join _ _) val = let last = Ring join val right join = Join left last in (last, join) insert' left right (Ring l v _) val = let this = Ring newL val right (newL, newR) = insert' left this l v in (this, newR) fromList [] = error "Can't create empty ring" fromList ls = let (left, right) = fromList' left right ls in left where -- compute this ring, given left and right pointers. Return -- left and right pointers for this segment fromList' left right [] = let join = Join left right in (join, right) fromList' left right (x:xs) = let this = Ring l x right (l, r) = fromList' left this xs in (this, r) toList = toListL -- View of list from left hand side toListL = toList' . findLeftOfJoin where toList' (Join _ _) = [] toList' (Ring l v _) = v : toList' l toListR = toList' . findRightOfJoin where toList' (Join _ _) = [] toList' (Ring _ v r) = v : toList' r current (Ring r v l) = v current _ = error "Join is never current (curr)" rotR r amt | amt > 0 = goRight r amt | amt < 0 = goLeft r (negate amt) | otherwise = r rotL r amt | amt > 0 = goLeft r amt | amt < 0 = goRight r (negate amt) | otherwise = r goRight r 0 = r goRight (Ring _ _ r@(Ring _ _ _)) amt = goRight r (amt - 1) goRight (Ring _ _ (Join _ r)) amt = goRight r (amt - 1) goRight (Join _ _) _ = error "Join is never current (goRight)" goLeft r 0 = r goLeft (Ring l@(Ring _ _ _) _ _) amt = goLeft l (amt - 1) goLeft (Ring (Join l _) _ _) amt = goLeft l (amt - 1) goLeft (Join _ _) _ = error "Join is never current (goLeft)" isRing (Ring _ _ _) = True isRing _ = False isJoin (Join _ _) = True isJoin _ = False findLeftOfJoin (Join l _) = l findLeftOfJoin (Ring l _ _) = findLeftOfJoin l findRightOfJoin (Join _ r) = r findRightOfJoin (Ring _ _ r) = findRightOfJoin r

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Disable echo in POSIX terminal
by Taylor Venable 10 Nov '07

10 Nov '07

Hello all, I've written a little Haskell program to get information from a MySQL database (great thanks to anybody reading this who works on HSQL, by the way) but I want to keep the user's password concealed, obviously. Currently I prompt for it from the terminal, but the problem is that it's echoed just like all other input. I would like to disable input echo when asking for the password, but I'm not sure how. I notice there's a System.POSIX.Terminal module, but I have no idea how to use it, or whether it will do what I want. Any ideas would be greatly appreciated. Best regards. -- Taylor Venable taylor(a)metasyntax.net http://real.metasyntax.net:2357/

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checking regular expressions
by Uwe Schmidt 09 Nov '07

09 Nov '07

Hi all, what's the simplest way to check, whether a given string is a wellformed regular expression? In the API there's just a "mkRegex" which does not make any checks, and the "matchRegex" which throws an exception when the regex isn't wellformed. So do I need the IO monad for checking a regex? Uwe

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Odd error report against ghc-6.8.1
by ChrisK 09 Nov '07

09 Nov '07

thomas.hartman(a)db.com has sent me a new bug report. Apparently he can crash ghc-6.8.1 when compiling regex-tdfa-0.93 (darcs under http://darcs.haskell.org/packages/regex-unstable/regex-tdfa/ ). Should I open a ticket for this? > > I darcs got the newest regex-tdfa (0.93 iirc) and made all the obvious changes involving LANGUAGE/OPTION_GHC pragmas to get this to work on ghc-6.8.1 > > prereq regex_base 0.93 installs ok > regex-tdfa panics with > > hartthoma@linuxpt:~/installs/regexstuff/regex-newest/regex-tdfa>runghc Setup.hs build > Preprocessing library regex-tdfa-0.93... > Building regex-tdfa-0.93... > [ 1 of 21] Compiling Data.IntMap.CharMap ( Data/IntMap/CharMap.hs, dist/build/Data/IntMap/CharMap.o ) > > Data/IntMap/CharMap.hs:9:27: > Warning: Imported from `Data.Char' but not used: `chr' > [ 2 of 21] Compiling Data.IntSet.EnumSet ( Data/IntSet/EnumSet.hs, dist/build/Data/IntSet/EnumSet.o ) > [ 3 of 21] Compiling Data.IntMap.EnumMap ( Data/IntMap/EnumMap.hs, dist/build/Data/IntMap/EnumMap.o ) > [ 4 of 21] Compiling Text.Regex.TDFA.IntArrTrieSet ( Text/Regex/TDFA/IntArrTrieSet.hs, dist/build/Text/Regex/TDFA/IntArrTrieSet.o ) > [ 5 of 21] Compiling Text.Regex.TDFA.Common ( Text/Regex/TDFA/Common.hs, dist/build/Text/Regex/TDFA/Common.o ) > [ 6 of 21] Compiling Text.Regex.TDFA.Pattern ( Text/Regex/TDFA/Pattern.hs, dist/build/Text/Regex/TDFA/Pattern.o ) > [ 7 of 21] Compiling Text.Regex.TDFA.ReadRegex ( Text/Regex/TDFA/ReadRegex.hs, dist/build/Text/Regex/TDFA/ReadRegex.o ) > [ 8 of 21] Compiling Text.Regex.TDFA.CorePattern ( Text/Regex/TDFA/CorePattern.hs, dist/build/Text/Regex/TDFA/CorePattern.o ) > ghc-6.8.1: panic! (the 'impossible' happened) > (GHC version 6.8.1 for i386-unknown-linux): > splitFunTy > <nt>mtl-1.1.0.0:Control.Monad.RWS.Lazy.RWS{tc rsRo} > (base:Data.Maybe.Maybe{tc r6} > regex-tdfa-0.93:Text.Regex.TDFA.Common.GroupIndex{tc raaa}) > [base:Data.Either.Either{tc 344} > regex-tdfa-0.93:Text.Regex.TDFA.Common.Tag{tc raai} > regex-tdfa-0.93:Text.Regex.TDFA.Common.GroupInfo{tc raa4}] > ([regex-tdfa-0.93:Text.Regex.TDFA.Common.OP{tc raae}] > -> [regex-tdfa-0.93:Text.Regex.TDFA.Common.OP{tc raae}], > regex-tdfa-0.93:Text.Regex.TDFA.Common.Tag{tc raai}) > (base:GHC.Prim.Any{(w) tc 31N}, > [regex-tdfa-0.93:Text.Regex.TDFA.Common.Tag{tc raai}]) > > Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug Note that "Text.Regex.TDFA.Common.Tag" is "type Tag = Int" and compiling the CorePattern.hs file works fine under ghc-6.6.1. The only unusual thing in CorePattern is the use of "mdo" recursive-do syntax. His message continues with the steps he used, and the minimal patch that got it from ghc-6.6.1 to ghc-6.8.1 : > > hartthoma@linuxpt:~/installs/regexstuff/regex-newest/regex-tdfa>runghc Setup.hs configure > Configuring regex-tdfa-0.93... > hartthoma@linuxpt:~/installs/regexstuff/regex-newest/regex-tdfa>runghc Setup.hs build > Preprocessing library regex-tdfa-0.93... > Building regex-tdfa-0.93... > > Text/Regex/TDFA/MutRunBS.hs:12:17: > Could not find module `Data.ByteString.Base': > Use -v to see a list of the files searched for. > hartthoma@linuxpt:~/installs/regexstuff/regex-newest/regex-tdfa>darcs whatsnew > { > hunk ./Text/Regex/TDFA/CorePattern.hs 1 > +{-# OPTIONS_GHC -XRecursiveDo #-} > hunk ./Text/Regex/TDFA/RunMutState.hs 1 > -{-# LANGUAGE CPP #-} > +{-# OPTIONS_GHC -fglasgow-exts -XCPP #-} > + > hunk ./regex-tdfa.cabal 16 > -Build-Depends: regex-base >= 0.80, base >= 2.0, parsec, mtl > +Build-Depends: regex-base >= 0.80, base >= 2.0, parsec, mtl, containers, array, bytestring > hunk ./regex-tdfa.cabal 45 > -GHC-Options: -Wall -Werror -O2 -funbox-strict-fields > +GHC-Options: -Wall -Wwarn -O2 -funbox-strict-fields > } > hartthoma@linuxpt:~/installs/regexstuff/regex-newest/regex-tdfa> > >

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An exceptional exception
by Andrew Coppin 09 Nov '07

09 Nov '07

I've been doing some testing to try to figure out the tangled maze that is exception handling. (Is there a *reason* why half of all exceptions are dynamic, and the other half aren't? Couldn't we have just 1 system?) Anyway, I wrote a cute little "test" function for figuring out how each kind of exception works: > test (fail "go away") isAlreadyExistsError=False isDoesNotExistError=False isAlreadyInUseError=False isFullError=False isEOFError=False isIllegalOperation=False isPermissionError=False isUserError=True Next I tried this: > test (getDirectoryContents "banana") isAlreadyExistsError=False isDoesNotExistError=True isAlreadyInUseError=False isFullError=False isEOFError=False isIllegalOperation=False isPermissionError=False isUserError=False And now, this: > test (writeFile "C:\\C:\\x" "test") isAlreadyExistsError=False isDoesNotExistError=False isAlreadyInUseError=False isFullError=False isEOFError=False isIllegalOperation=False isPermissionError=False isUserError=False Erm... so this exception registers as "False" on every single predicate? How exactly am I meant to detect this error?

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FW: please help... small problem
by Ryan Bloor 09 Nov '07

09 Nov '07

sorry heres the code.... I always do that. From: ryanbloor(a)hotmail.comTo: Subject: please help... small problemDate: Fri, 9 Nov 2007 21:44:35 +0000 hi Is there anyway to cut down this code and to not use auxillary functons, but instead use pattern matching? The code basically splits up a list 'rslis' into a list of lists - but so each word is split up and the integers have been parsed. so.... ["hi ryan 1","hi jeff 2"] becomes [["hi","ryan" 1], ["hi","jeff", 2]].The code is far too long. I don't wanna use premade functions too much... pattern matching is required. Ryan The next generation of MSN Hotmail has arrived - Windows Live Hotmail _________________________________________________________________ Get free emoticon packs and customisation from Windows Live. http://www.pimpmylive.co.uk

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please help... small problem
by Ryan Bloor 09 Nov '07

09 Nov '07

hi Is there anyway to cut down this code and to not use auxillary functons, but instead use pattern matching? The code basically splits up a list 'rslis' into a list of lists - but so each word is split up and the integers have been parsed. so.... ["hi ryan 1","hi jeff 2"] becomes [["hi","ryan" 1], ["hi","jeff", 2]].The code is far too long. I don't wanna use premade functions too much... pattern matching is required. Ryan _________________________________________________________________ 100’s of Music vouchers to be won with MSN Music https://www.musicmashup.co.uk

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