August 2007 - Haskell-Cafe

How can I pass IOUArrays to FFI functions?
by Ryan Ingram 20 Aug '07

20 Aug '07

I have a C function of type void f ( HsWord32* p0, HsWord32* p1, HsWord32 size ); along with the FFI declaration: foreign import ccall unsafe f :: Ptr Word32 -> Ptr Word32 -> Word32 -> IO () In my Haskell code I have an unboxed IO array of Word32; IOUArray Int Word32. I want to pass the pointer to this array to f(). How can I get the pointer out of the array? Or, is there a better way to declare f() to do this? I'm open to using GHC hackery; using v6.6.1 right now. -- ryan

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Diagnosing stack overflow
by Justin Bailey 20 Aug '07

20 Aug '07

I am trying to determine why my stack overflows in my medium sized program (it has several modules but maybe only 1000 LOC total). On Windows, at least, the ghcprof visualization tool doesn't work. Any suggestions besides an output trace? It may be the function below, which tries to determine if a list of strict bytestrings is longer than the count given. I have tried to make it strict but am not sure if it's too lazy. Any hints are appreciated. -- Determines if the length of the strings in the list is longer than the given -- count. If not, amount the list falls short is returned. Otherwise, -- -1 indicates the prefix list is at least that long. If the count is zero and -- the list is empty or just null strings, -1 is also returned. prefixesAtLeast :: Int -> [S.ByteString] -> Int prefixesAtLeast !0 !ss | null ss = 0 | all S.null ss = 0 | otherwise = -1 prefixesAtLeast !n !ss = prefixesAtLeast' n ss where prefixesAtLeast' !n ss | n < 0 = -1 | null ss = n | otherwise = let (!s : (!rest)) = ss in prefixesAtLeast' (n - (S.length s)) rest

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IO in HApps handler ?
by Luc TAESCH 19 Aug '07

19 Aug '07

Subject: IO in HApps handler ? I am trying to add a handler that would run an external command in HApps 0.8.8, and I got a type issue I do not know how to get around.. can we have IO in a handler ? testcmdpost.hs:52:8: Couldn't match expected type `Ev st Request' against inferred type `IO' Expected type: ServerPart (Ev st Request) Request IO Result Inferred type: ServerPart IO Request im Result In the expression: (h ["xxx"] GET) $ (ok $ (\ () () -> do (MySt val) <- get runCommand "ls" ["."] respond (show "dfdf"))) here is the handler iI am adding : ,h ["xxx"] GET $ ok $ \() () -> do (MySt val) <- get; runCommand "ls" ["."]; respond (show "dfdf" ) in there : import HAppS.Util.Common... ... main :: IO () main = stdHTTP [debugFilter -- we want to see debug messages in the console ,h [""] GET $ ok $ val "GETting root hello" -- ,h (Prefix ["s"]) GET $ respIO $ fileServe staticPath , hs (Prefix ["s"]) GET $ basicFileServe staticPath -- 0.8.8 -- /val shows us the current value ,h ["val"] GET $ ok $ \() () -> do (MySt val) <- get; respond (show val) -- /set with the POST data "val"=56 would set the value to 56 ,h ["xxx"] GET $ ok $ \() () -> do (MySt val) <- get; runCommand "ls" ["."]; respond (show "dfdf" ) ,h ["set"] POST $ ok $ \() newVal -> do put newVal; respond ("New value is " ++ show newVal) -- The first one is FromReqURI and the second one is FromMessage -- The cryptic comment about is referring to the arguments () and newVal -- to the method. The type of newVal being MyState is what -- invokes our custom FromMessage instance above. ] this is the runcommand from HAppS.Util.Common , not from defined as -- | Run an external command. Upon failure print status -- to stderr. runCommand :: String -> [String] -> IO () runCommand cmd args = do (_, outP, errP, pid) <- runInteractiveProcess cmd args Nothing Nothing let pGetContents h = do mv <- newEmptyMVar let put [] = putMVar mv [] put xs = last xs `seq` putMVar mv xs forkIO (hGetContents h >>= put) takeMVar mv os <- pGetContents outP es <- pGetContents errP ec <- waitForProcess pid case ec of ExitSuccess -> return () ExitFailure e -> do hPutStrLn stderr ("Running process "++unwords (cmd:args)++" FAILED ("++show e++")") hPutStrLn stderr os hPutStrLn stderr es hPutStrLn stderr ("Raising error...") fail "Running external command failed"

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building a regular polygon
by dbastos＠toledo.com 19 Aug '07

19 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 19 Aug '07

19 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 19 Aug '07

19 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 19 Aug '07

19 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

2 2

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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