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building a regular polygon
by dbastos＠toledo.com 20 Aug '07

20 Aug '07

I'm new to functional programming and to haskell. I'm reading ``The Haskell School of Expression'' by Paul Hudak and I'm studying how to solve exercise 2.2. My difficulty here is more algorithmic than Haskellian, but perhaps many here have solved this exercise already, or would know how to solve it; I hope it's okay to post. (*) Exercise 2.2 Define a function regularPolygon :: Int -> Side -> Shape such that regularPolygon n s is a regular polygon with n sides, each of length s. (Hint: consider using some of Haskell's trigonometric functions, such as sin :: Float -> Float, cos :: Float -> Float, and tan :: Float -> Float.) My idea is to start with the vertices (0,0) and (0,s). Then I ``connect a line'' from (0,0) to (0,s) and I then I need to compute the next vertex of the polygon. The angles of a regular polygon are alpha = ((n - 2) * pi)/n, where n is the number of sides of the polygon. So the third vertex of the polygon is x = cos (180 - alpha) * s y = sin (180 - alpha) * s, but I don't know how to teach the computer to get to the forth vertex. Once I figure that in a general way, I would just call myself until I get to the last vertex; if I start at n, we end at at n = 2 in which I return []. Something like: > data Shape = Polygon [Vertex] deriving Show > > type Side = Float > type Vertex = (Float, Float) > > regularPolygon :: Int -> Side -> Shape > regularPolygon n s = Polygon ((0,0) : (0,s) : buildList n s (fromIntegral n)) > where buildList :: Int -> Side -> Float -> [Vertex] > buildList 2 _ _ = [] > buildList n s m = let x = cos(pi - alpha) * s > y = sin(pi - alpha) * s > alpha = ((m - 2) * pi)/m > in (x,y) : buildList (n-1) s m but right now it just repeats all the vertices after the second. Any help is appreciated.

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trouble compiling "import GHC.Prim(MutableByteArray#, ....." (building regex-tdfa from darcs) -- what's that # sign doing?
by Thomas Hartman 20 Aug '07

20 Aug '07

trying to compile regex-tdfa, I ran into another issue. (earlier I had a cabal problem but that's resolved.) there's a line that won't compile, neither for ghc 6.6.1 nor 6.7 import GHC.Prim(MutableByteArray#,RealWorld,Int#,sizeofMutableByteArray#,unsafeCoerce#) so the fresh darcs regex tdfa package won't build. This line (line 16 below) causes this error for ghc -e '' RunMutState.hs for both ghc 6.1 and 6.7 Much obliged for any help, Thomas. ********************************* hartthoma@linuxpt:~/installs/regex_darcs/regex-tdfa>runghc Setup.hs build Preprocessing library regex-tdfa-0.93... Building regex-tdfa-0.93... Text/Regex/TDFA/RunMutState.hs:16:32: parse error on input `#' hartthoma@linuxpt:~/installs/regex_darcs/regex-tdfa>head -n20 Text/Regex/TDFA/RunMutState.hs | cat -n 1 {-# LANGUAGE CPP #-} 2 module Text.Regex.TDFA.RunMutState(TagEngine(..),newTagEngine,newTagEngine2 3 ,newScratch,tagsToGroupsST 4 ,toInstructions,compareWith,resetScratch 5 ,SScratch(..),MScratch,WScratch) where 6 7 import Control.Monad(forM_,liftM,liftM2,liftM3,foldM) 8 --import Control.Monad.ST.Strict as S (ST) 9 --import qualified Control.Monad.ST.Lazy as L (ST) 10 import Control.Monad.State(MonadState(..),execState) 11 12 import Data.Array.Base(unsafeRead,unsafeWrite,STUArray(..)) 13 #ifdef __GLASGOW_HASKELL__ 14 import GHC.Arr(STArray(..)) 15 import GHC.ST(ST(..)) *** 16 import GHC.Prim(MutableByteArray#,RealWorld,Int#,sizeofMutableByteArray#,unsafeCoerce#) 17 #else 18 import Control.Monad(when) 19 import Control.Monad.ST(ST) 20 import Data.Array.ST(STArray) hartthoma@linuxpt:~/installs/regex_darcs/regex-tdfa> --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.

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ghc 6.7 /6.8
by Luc TAESCH 20 Aug '07

20 Aug '07

Date: Sun, 19 Aug 2007 12:39:50 +0200 Subject: Fwd: ghc 6.7 /6.8 hello. will 6.7 be released, or will only 6.8 be ? ( i.e do you use an even/uneven relesing number convention) -in that case, does the 6.8 branch means the freeze is on, and what would be the target for 6.8 ? Q3/2007 ? Q4 ? ( context: I am impatient to test the new debbuger, but too newby to go and fix all the lib..hoping it may help me undertand haskell execution.. )

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Re: Interval Arithmetics
by Henning Thielemann 20 Aug '07

20 Aug '07

The page http://www.haskell.org/haskellwiki/Applications_and_libraries/Mathematics#N… lists several implementations of several flavours of computable reals, which may be useful for you.

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How do I simulate dependent types using phantom types?
by DavidA 19 Aug '07

19 Aug '07

Hi, I am trying to implement quadratic fields Q(sqrt d). These are numbers of the form a + b sqrt d, where a and b are rationals, and d is an integer. In an earlier attempt, I tried data QF = QF Integer Rational Rational (see http://www.polyomino.f2s.com/david/haskell/hs/QuadraticField.hs.txt) The problem with this approach is that it's not really type-safe: I can attempt to add a + b sqrt 2 to c + d sqrt 3, whereas this should be a type error because 2 /= 3. So I thought I'd have a go at doing it with phantom types. In effect I'd be using phantom types to simulate dependent types. Here's the code: {-# OPTIONS_GHC -fglasgow-exts #-} import Data.Ratio class IntegerType a where value :: Integer data Two instance IntegerType Two where value = 2 data Three instance IntegerType Three where value = 3 data QF d = QF Rational Rational deriving (Eq) instance IntegerType d => Show (QF d) where show (QF a b) = show a ++ " + " ++ show b ++ " sqrt " ++ show value instance IntegerType d => Num (QF d) where QF a b + QF a' b' = QF (a+a') (b+b') negate (QF a b) = QF (-a) (-b) QF a b * QF c d = QF (a*c + b*d*value) (a*d + b*c) fromInteger n = QF (fromInteger n) 0 The problem is, this doesn't work. GHC complains: The class method `value' mentions none of the type variables of the class IntegerType a When checking the class method: value :: Integer In the class declaration for `IntegerType' Is what I'm trying to do reasonable? If no, what should I be doing instead? If yes, why doesn't GHC like it? Thanks, David

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List comprehension desugaring
by Neil Mitchell 19 Aug '07

19 Aug '07

Hi, The Haskell desugaring for list comprehensions is given in: http://haskell.org/onlinereport/exps.html#list-comprehensions All the rules seem to be left to right rewrites, apart from the second one, which seems to be right to left. Is there some deep reason for this, or is this accidental. Thanks Neil

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Using Apple Spotlight to search for .hs and .lhs files
by Dan Piponi 19 Aug '07

19 Aug '07

This isn't really a Haskell question but I'm guessing some Haskell hackers have a solution. MacOS X's Spotlight doesn't seem to be able to search for text in .lhs and .hs files. But it can find text in .txt files. Is there a way of getting Spotlight to treat .lhs and .hs files like .txt files so I can instantly search all of my Haskell source? -- Dan

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Error building takusen with Cabal-1.1.6.2
by John Dell'Aquila 19 Aug '07

19 Aug '07

Setup.hs wants a module that Cabal hides. Am I doing something wrong (newbie :-) or should I try to fall back to Cabal-1.1.6.1? $ ghc --make -o setup Setup.hs Setup.hs:13:7: Could not find module `Distribution.Compat.FilePath': it is hidden (in package Cabal-1.1.6.2)

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Sudoku Solver
by Adrian Neumann 19 Aug '07

19 Aug '07

-----BEGIN PGP SIGNED MESSAGE----- Hash: RIPEMD160 Hi, I wrote a brute-force sudoku solver. It takes a [[Int]] and spits out the first solution it finds. Why is it, that [0,0,0,7,0,6,9,0,0] [9,0,0,0,0,4,7,0,0] [7,0,0,0,0,0,4,0,0] [0,2,7,0,3,5,8,0,0] [6,9,5,8,2,0,0,0,0] [0,8,0,0,0,0,5,0,0] [0,3,0,0,0,0,0,0,0] [8,7,0,9,0,0,0,0,0] [0,0,0,0,0,0,0,0,0] is solved instantly, but when I remove just one number my program takes forever? === import Array import List import System type Field = Array Int (Array Int Int) isEmpty :: Int -> Bool isEmpty = (==) 0 done :: Field -> Bool done a = isFull a && checkField a isFull::Field -> Bool isFull a = notElem (0) $ (concat.(map elems).elems) a readField :: [[Int]]->Field readField = (listArray (1,9).map (listArray (1,9))) checkField :: Field -> Bool checkField a = check (rows a) && check (columns a) && check (squares a) where check b = and $ ((map ((==1).length)).concat.(map group).clean) b clean = map (dropWhile isEmpty.sort) columns::Field -> [[Int]] columns a = [[a!i!j|i<-[1..9]]|j<-[1..9]] rows :: Field -> [[Int]] rows a = [[a!i!j|j<-[1..9]]|i<-[1..9]] squares :: Field -> [[Int]] squares a = [[a!(i+n)!(j+m)|i<-[1..3],j<-[1..3]] |n<-[0,3,6],m<-[0,3,6]] - -- creates a list of all allowed entries genMoves :: Field -> [Field] genMoves a = concat [update a i j|i<-[1..9],j<-[1..9],isEmpty (a!i!j)] where update b i j= [b //[(i,b!i //[(j,x)])]|x<-[1..9], checkField (b //[(i,b!i //[(j,x)])])] play :: [Field] -> Maybe Field play (f:a) | done f= Just f | isFull f= play a | otherwise = play ((genMoves f)++a) play [] = Nothing main :: IO () main = do x <- getArgs let field = read (x!!0) :: [[Int]] let erg=play [readField field] case erg of Just a -> putStrLn $ concat $ map ((++"\n").show) (rows a) Nothing -> putStrLn "Es gibt keine Loesung" -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.7 (MingW32) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFGtx0r11V8mqIQMRsRA7P5AJ9lG3mF3fHpUiyOqCeRVOOEGozp1wCeI80C RfD/U5y+yEgF0fe+kRwDbUI= =Ngss -----END PGP SIGNATURE-----

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Chessboard-building in Haskell
by Andrew Wagner 19 Aug '07

19 Aug '07

I've started a blog series on writing a chess engine in Haskell. I just posted the second blog entry today: http://sequence.complete.org/node/361 I suspect there's more work to be done on that function, though. It seems like there should be a nice way to remove that flip in apply. Any thoughts?

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