May 2008 - Haskell-Cafe

ambiguous constraint errors
by Evan Laforge 30 May '08

30 May '08

I have two related questions: #1 I'm getting some annoying type errors that I don't fully understand, and wind up having to do a workaround that I don't totally like. Here's a simplified version of my situation: data Ambi m = Ambi { ambi_monad :: m Int , ambi_int :: Int } some_ambi :: Monad m => Ambi m some_ambi = Ambi (return 5) 10 ambi_table :: Monad m => [(String, Ambi m)] ambi_table = [("default", some_ambi)] get_int :: String -> Maybe Int get_int sym = fmap ambi_int (lookup sym ambi_table) ----------- get_int produces: Ambiguous type variable `m' in the constraint: `Monad m' arising from a use of `ambi_table' at ambi.hs:13:40-49 So I guess this means I'm not telling it which 'm', so it doesn't know how to resolve the 'return'... but the thing is, I'm not even using that value, so it doesn't matter what it resolves to. So it works if I pick some random monad: get_int sym = fmap ambi_int (lookup sym ambi_table :: Maybe (Ambi Maybe)) Note that I can't leave it as 'Monad m => Ambi m' because I still get an ambiguous type variable complaint. I'm a little disconcerted by having to pick some random dummy monad. Even worse, everything this type touches starts requiring explicit type declarations everywhere. Is there some easier way to do this? #2 This is somewhat related to another issue I've been having, which is that I have some kind of complicated type, e.g. '(SomeMonad some, Monad m) => some (SomethingM m Status)' that I use in a lot of places. It would be a lot less typing and easier to modify later if I wrote a type alias: type Command = (Monad some, Monad m) => some (State.StateT () m Status) but of course, this isn't allowed, since the type variables don't appear on the lhs, and if I put a context there, it's a syntax error. While I can write it with data: data (Monad some, Monad m) => Command some m = Command (some (State.StateT () m Status)) I've been told this doesn't mean what I expect it to, which is that the context constraints propagate up to and unify with the containing type (out of curiosity, since it's accepted, what *does* this do? I think I read it somewhere once, but now I forget and can't find it). And sure enough, using this type doesn't make my type declarations have the right contexts. So the first problem means that I have to declare types in various inconvenient places, and the second one means that I have to type out all the various class constraints (I can still alias away the non-polymorphic bits), and all my type declarations start looking much more complicated than they are. The "solution" I've been using for some of this is just to remove the polymorphism, so I can write a simple alias like type Command = SomethingM (State.StateT () Identity Status) and now I can think of "a command" and have various functions that take and return Commands, without caring that it's some kind of monad with context constraints. But of course, this isn't always possible since sometimes I need the type to remain polymorphic (i.e. while most of these I don't *think* will run in some other monad, some of them definitely get called in multiple contexts). Is there any nicer way around this? And what's the underlying issue that makes this necessary? I can live with all the context hair everywhere, but it sure would be nicer to be able to define it once and for all in one place.

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Question on type synonym definition and language extensions
by Olivier Boudry 30 May '08

30 May '08

Hi all, I'm trying to define a type synonym for 2D MArray instances. The goal is to keep the function signature simple and readable using type `Matrix` instead of something like `(Ix i, MArray a e m) => m (a i e)`. After some "read, guess, try, error" cycles I came up with this: type Matrix = forall m. forall a. forall i. forall n. (Ix i, MArray a n m, Num i, Num n) => m (a (i,i) n) it requires option -XRank2Types to work. But then I can write my function as: test :: Matrix test = do { a <- newArray ((0,0),(5,8)) 0; writeArray a (0,0) 1; return a } Then I wanted to be able to give the Index and Value types in the type synonym but keep it flexible in terms of which MArray instance is used. I changed it to: type Matrix i n = forall m. forall a. (MArray a n m) => m (a (i,i) n) and the type signature of the test function becomes: test :: Matrix Int Double For this one I had to add an extra -XFlexibleContexts option to build it. It works and I'm quite happy with the look of my new function type signatures, but I'm just wondering if I'm doing something bad or if there is a cleaner/simpler way to define the same type synonym without requiring language extensions. What are the risks associated with using these two language extensions, non compatibility with other compilers or more? Thanks, Olivier.

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Bug in parallel GHC runtime?
by Dusan Kolar 29 May '08

29 May '08

Hello all, The attached file was compiled by the following command: ghc -O2 --make -threaded ltest1pl.hs -o alall When run in a sequential mode, I get this result: ./alall Starting ... Lst1: 41666666650000 Lst2: 41669166700000 T1: 0m 1.0e-6s 36 End! On the other hand, when run in a threaded mode, I get the following error: ./alall +RTS -N2 Starting ... Lst1: 41666666650000 Lst2: 41669166700000 T1: 0m 0.0s Segmentation fault Is it fault of the GHC runtime, or is it something on my side? My machine: uname -a Linux pc 2.6.24-ARCH #1 SMP PREEMPT Sun Mar 30 10:50:22 CEST 2008 x86_64 Intel(R) Core(TM)2 CPU 6600 @ 2.40GHz GenuineIntel GNU/Linux My ghc: ghc --version The Glorious Glasgow Haskell Compilation System, version 6.8.2 Thanks and regards Dusan --import Control.Concurrent --import Control.Concurrent.MVar import System.Time import Control.Parallel.Strategies --import Data.List (foldl') import qualified Data.ByteString as B sumAllSums [] = 0 sumAllSums l@(_:xs) = sumlist 0 l + sumAllSums xs where sumlist res [] = res sumlist sr (v:vs) = sumlist (sr+v) vs wlist2wbs [] = B.pack [] wlist2wbs l@(_:_) = B.pack $ encode l where encode :: Integral a => [Int] -> [a] encode [] = [] encode (x:xs) = if x==0 then 0:0:encode xs else fromIntegral (x `mod` 256) : fromIntegral (x `div` 256) : encode xs main = do putStrLn $ "Starting ..." let lst1 = [0..49999] let lst2 = [0..50000] let bs1 = wlist2wbs lst1 let bs2 = wlist2wbs lst2 tm1 <- getClockTime let (v1:v2:_) = parMap rnf sumAllSums [lst1,lst2] tm1' <- getClockTime putStrLn ("Lst1: " ++ show v1) putStrLn ("Lst2: " ++ show v2) let tdiff1 = diffClockTimes tm1' tm1 --let tdiff2 = diffClockTimes tm2' tm2 putStrLn $ "T1: " ++ show (tdMin tdiff1) ++ "m " ++ show (fromIntegral(tdSec tdiff1) + fromIntegral(tdPicosec tdiff1)/1e12) ++ "s" --putStrLn $ "T2: " ++ show (tdMin tdiff2) ++ "m " ++ show (fromIntegral(tdSec tdiff2) + fromIntegral(tdPicosec tdiff2)/1e12) ++ "s" putStrLn $ show $ {-ibs1 +-} B.index bs1 99999 + B.index bs2 49999 {-((bs1 + fromIntegral (B.index bs2 99999)) :: Integer)-} putStrLn $ "End!" {- main = do tm1 <- getClockTime putStrLn $ "Starting ... " mv1 <- newEmptyMVar mv2 <- newEmptyMVar t1 <- forkIO (putMVar mv1 $! sumAllSums [0..49999]) t2 <- forkIO (putMVar mv2 $! sumAllSums [1..50000]) v1 <- takeMVar mv1 v2 <- takeMVar mv2 killThread t1 killThread t2 putStrLn $ "Result: " ++ show (v1+v2) tm2 <- getClockTime let tdiff = diffClockTimes tm2 tm1 putStrLn $ "End! " ++ show (tdMin tdiff) ++ "m " ++ show (fromIntegral(tdSec tdiff) + fromIntegral(tdPicosec tdiff)/1e12) ++ "s" -}

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An alternative break
by Pieter Laeremans 28 May '08

28 May '08

Hello, I need a break function that splits the list one element further than the ordinary break. This is the simplest solution I could imagine: breakI :: (a -> Bool) -> [a] -> ([a], [a]) breakI p s = case break p s of ([], []) -> ([], []) (x, []) -> (x, []) (x, l) -> (x ++ [head l], tail l ) Is there a better way to write this ? thanks in advance, Pieter -- Pieter Laeremans <pieter(a)laeremans.org>

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Type Coercion
by PR Stanley 28 May '08

28 May '08

Hi (16 :: Float) is a perfectly legitimate statement although I'm surprised that it's allowed in a type strong language such as Haskell. It's a bit like casting in good old C. What's going on here? Paul

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Benchmarking Framework
by Tom Harper 28 May '08

28 May '08

I am in the process of writing a library for my MSc dissertation and would like to do some benchmarking. In doing so I need to compare the time and space of my library with some other code. Is there a framework for doing so in Haskell, aside from the Profiling tools in GHC? Basically I'm looking for something like QuickCheck, but that helps with generating repeatable tests to measure performance. Is there anything out there that anyone would recommend? -- Tom Harper MSc Computer Science '08 University of Oxford Mobile: +44 (0)7533 998 591 Skype: +1 949 273 4627 (harpertom)

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Examples of using Haskell for mathematics
by Brent Yorgey 28 May '08

28 May '08

Hi all! In a couple weeks I will be giving a short (15-min.) talk to an audience of mostly mathematicians, entitled "Executable Mathematics: A Whirlwind Introduction to Haskell". The idea will be to give a flavor of Haskell, its uniquenesses, and why it is a great language for playing around with mathematics, by way of some well-chosen examples. There are definitely plenty of such examples out there, and I've already found quite a few that I might use, but I thought I would send an email to the cafe to ask whether anyone has any code which you think particularly exemplifies some aspect of why Haskell is a great language for mathematics. I'm looking to include a wide range of examples, so any length (from a few to hundreds of lines of code) and any level (from simple number theory to things only a few people in the world understand) are fair game. Thanks in advance! And yes, I will definitely make the slides available after the talk. I don't know if I can promise a video, I kind of doubt the sessions will be videotaped. I guess we'll see. -Brent

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Re: static linking
by Peter Gammie 28 May '08

28 May '08

Hello, Further on my static linking woes: debian ~$ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.8.2 -debian ~$ cat T.hs main :: IO () main = putStrLn "Hello world" -debian ~$ ghc -optl-static -static --make T.hs Linking T ... /usr/lib/gcc/i486-linux-gnu/4.2.3/../../../../lib/ librt.a(timer_create.o): In function `timer_create': (.text+0x107): undefined reference to `pthread_once' ... < references to missing pthread symbols > Does anyone have static linking working for GHC 6.8.2 and can inform me of the magic incantation? "Get the latest development version from darcs", perhaps? BTW when I said "move to the end", I meant "move to the end of -l flags". cheers peter On 27/05/2008, at 10:37 AM, Peter Gammie wrote: > Hello, > > I am having a bit of trouble static linking my program using GHC > 6.8.2. > > In brief: "rt" uses "pthread", but -lrt is the final thing on the > command line passed to the linker, and so the pthread symbols are > not resolved. Manually moving -lpthread to the end of the collect2 > command line does the trick. > > The question is how to do this using ghc. > > This is on Debian, using the 6.8.2-5 from unstable. > > I can provide the command lines if that is helpful. > > Note the same setup worked fine under GHC 6.6.1. > > cheers > peter

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Re: the Network.URI parser
by Neil Mitchell 28 May '08

28 May '08

Moving to haskell-cafe, as this almost certainly isn't a library issue. Hi > It most certainly is a security flaw. In the src of an img, yes, probably. In the href of a link, its a completely valid thing to do - and one that I've done loads of times. The URI is fine, its just the particular location that is dodgy. > whole pile of dodgy URIs. Most get culled (in my case) by the HaXml parser > and/or XHTML 1.0 Strict validation, and now I hope to eliminate the rest by > carefully handling the URIs. I don't think that's possible. A URI can validly have javascript, and can validly be a lot of things which are unsafe. > On that topic, does anyone have any good advice for handling these things? My advice is that you are targeting security at the wrong level. You shouldn't be cleaning the HTML to get a secure page, you should be having the level that interprets the HTML be secure regardless of the input. > If anyone knows of the state-of-the-art in this area, I'd appreciate a > pointer. > > http://htmlpurifier.org/live/smoketests/printDefinition.php > > doesn't seem to think the style attribute is unsafe. Have they not been > following the MySpace fiascos? Safety is a property of the HTML viewer, not of the HTML or CSS. Thanks Neil

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What's NaN?
by PR Stanley 28 May '08

28 May '08

Hello Why does sqrt (-16) return NaN? What is NaN? Thanks Paul

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