- Haskell-Cafe - Haskell.org

Importance of MonadRandom
by Yitzchak Gale 07 Feb '07

07 Feb '07

I would like to point out the importance of Cale Gibbard's MonadRandom, beyond what is currently mentioned on its wiki page: http://www.haskell.org/haskellwiki/New_monads/MonadRandom This monad makes it possible to write functions that use randomness without having to specify in advance whether the source of randomness will be a pure pseudorandom number generator, as in System.Random, or physical randomness via the IO monad, such as your operating system's source of physical randomness, or random.org, or a hardware random generator. Before use of MonadRandom becomes widespread - and I think it ought to - I would like to suggest a change to the interface. (I mentioned this once to Cale on #haskell, but I didn't say what change I meant.) Currently, the members of the MonadRandom class mimic the members of the Random class in System.Random. I think it would be better if instead they mimicked the members of RandomGen. Like this: \begin{code} class (Monad m) => MonadRandom m where nextR :: m Int splitR :: m (m ()) rangeR :: m (Int, Int) getR :: (forall g . RandomGen g => g -> a) -> m a \end{code} The extra function getR provides access not only to the member functions of Random, but to any function that generates random variables of any type. You would use getR random, getR $ randomR (a, b), etc. instead of getRandom, getRandomR (a, b), etc. Provide a default method for getR as follows: \begin{code} getR f = do r <- nextR (lo, hi) <- rangeR return $ f $ TrivialGen r lo hi data TrivialGen = TrivialGen Int Int Int instance RandomGen TrivialGen where next (TrivialGen r _ _) = r genRange (TrivialGen _ lo hi) = (lo, hi) split _ = undefined \end{code} We would use the default method of getR for MonadRandom instances of things like DevRandom, DevURandom, RandomDotOrg, etc. For the Rand and RandT instances we provide explicit methods: \begin{code} -- For RandT: getR f = RandT $ gets f --For Rand: getR =Rand $ getR f \end{code} I think this is better for several reasons: o We anyway need getR for general random variables o We could lose precision getting other random variables via getRandom in the case where genRange /= (minBound, maxBound) o I think it is a better semantic fit Regards, Yitz

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nested maybes
by Martin DeMello 07 Feb '07

07 Feb '07

I have a Data.Map.Map String -> (Layout, [String]) as follows: type Anagrams = [String] type Cell = (Layout, Anagrams) type WordMap = Map.Map String Cell exists str wmap = let a = Map.lookup (sort str) wmap in case a of Nothing -> False Just x -> case (find (== str) (snd x)) of Nothing -> False _ -> True the existence test looks ugly - any more compact way to write it? martin

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haskell and releasing resources
by Fawzi Mohamed 07 Feb '07

07 Feb '07

I am just coming to haskell, and I wrote a simple command to get some input from a pdf file I just wanted the output of the command so I did something like import System.Process (runInteractiveCommand) import IO (hGetContents) -- | returns the text of the first page of the pdf at the given path, needs pdftotext getTextOfPdf :: String -> IO String getTextOfPdf pdfPath = do (inp,out,err,pid) <- runInteractiveCommand ("pdftotext -l 1 "+ +pdfPath++" -") return (hGetContents out) I don't care about error handling, if something goes wrong it is ok to hang or crash, but knowing unix I wondered if this would do the right thing or if it would create a zombi process. I was about to ask, but then I thought "let's test it", and sure enough the zombi stays there. I tried to even to allocate more than one, wait, I even managed to exhaust the resources of my machine... So here is what I would have liked to happen: when the pid gets garbage collected it tries to wait for the process, and if that fails the pid stays around longer and will try to wait later. Too difficult? I don't know, but it is what I had expected from haskell. Failing that I would have expected clear hints that one should wait for external processes in the documentation, and I found none. So what is the way out? I could do a forkIO and wait for the process but then I wonder do I have to wait for the thread, or dead thread become zombi? I that is the case then the only way out would be to give back also the pid and make the waiting responsibility of the caller, not very nice, but probably the real solution. I have seen that in missingh there is a pipeout example but it seems to me that you are still responsible to wait for the process (with ensureSuccess). Maybe the correct thing is not being able to ignore the return code of the process.... but now I am becoming suspicious of other things, for example a file handle, you need to close it or you can expect that it will be closed when the handle is garbage collected? Are there other places where you need to pay attention to be sure that you are releasing the resources you acquired? I suppose problems can come only when an external resource is involved, or not So 1) documentation should specify if one should do some specific action to free resources, can someone fix this? 2) is there a clean (haskell;) way to deal with this? 3) other places apart external processes where you have to pay attention? What is your wisdom... thanks Fawzi

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Re: [Haskell-cafe] IO is not a monad
by Aaron McDaid 07 Feb '07

07 Feb '07

Hi, Apologies for referring to this old thread... I rearranged the code a little bit while experimenting but retained the same behaviour: Prelude> let f = undefined :: Int -> IO Int Prelude> (f 3 >> f 3) `seq` 42 42 Prelude> (f 3) `seq` 42 *** Exception: Prelude.undefined I think this makes it a little simpler for me to see the issue here. Simply, 'undefined >> undefined' is a bit more defined than simply 'undefined'. Just like 'undefined:undefined' is at least a non-empty list; which can be matched by (_:_) for example. This explains the differing behaviour of the two seemingly equivalent actions above. So I think the above behaviour is more to do with how shallow seq is, as others have probably already shown (but much of the rest of the thread is beyond me). Prelude> undefined `seq` 5 *** Exception: Prelude.undefined Prelude> (undefined:undefined) `seq` 5 5 Aaron (relative newbie) > On 1/23/07, Brian Hulley <brianh(a)metamilk.com> wrote: > > Brian Hulley wrote: > > > Brian Hulley wrote: > > >> Yitzchak Gale wrote: > > >>> I wrote: > > >>>>> Prelude> let f = undefined :: Int -> IO Int > > >>>>> Prelude> f `seq` 42 > > >>>>> *** Exception: Prelude.undefined > > >>>>> Prelude> ((>>= f) . return) `seq` 42 > > >>>>> 42 > > >>> The monad laws say that (>>= f) . return must be > > >>> identical to f. > > >> > > >> I thought it was: > > >> > > >> return x >>= f = f x > > >> > > >> so here the lhs is saturated, so will hit _|_ when the action is > > >> executed just as the rhs will. > > > > Ooops! But that does not mean the equation holds because for example > > > > Prelude> (return 3 >>= f) `seq` 42 > > 42 > > Prelude> (f 3) `seq` 42 > > *** Exception: Prelude.undefined > > > > In the lhs you only hit _|_ when the composite (>>=) action is actually > > being executed whereas in the rhs you hit _|_ when computing the function > > which will return the action to execute so there is difference. > > > > Brian. > > -- > > http://www.metamilk.com > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > >

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External Core
by Ricky Barefield 07 Feb '07

07 Feb '07

Dear All, For the last year I've been working on a program to translate between Haskell and Java. Originally this was for my third year dissertation to contribute towards my Masters in computer science at Sheffield University, UK. I have been given the opportunity to continue this project and have become interested in using the Core representation as an intermediary language. I understand that GHC can produce 'hcr' files which are specified in "An external representation for the ghc core language" 2001 and to this point I have been successful. I have also downloaded the tools for manipulating Core available from http://darcs.haskell.org/ghc/utils/ext-core/ I've tried running Happy on these files but get the error `Not enough type arguments for the type synonym "P"' when I try to run the resultant Parser.hs in Hugs and similar errors when run in GHC. What I'm trying to achieve is to read the hcr files into a Haskell data type which I could work with, if anyone could give me any help with using the files for manipulating Core I would be very grateful. Many Thanks, Ricky Barefield 4th Yr Undergraduate University of Sheffield

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Trouble representing a set of flags
by Stefan O'Rear 06 Feb '07

06 Feb '07

I have a structure: data Attr = Attr { fg :: !Color, bg :: !Color, bold :: !Bool, blink :: !Bool, rv :: !Bool, halfBright :: !Bool, underline :: !Bool } deriving(Eq,Show,Ord) newtype Color = Color Int deriving(Eq,Show,Ord) but ghc is doing a phenominally bad job of unboxing it; so I'm trying to do it manually using a bitfield: -- |Data type representing character attributes. newtype Attr = Attr Int deriving (Eq) -- | Set the foreground color of an `Attr'. setFG :: Color -> Attr -> Attr setFG (Color c) (Attr a) = Attr ((a .&. 0xFFFFFF00) .|. c) -- | Get the foreground color of an `Attr'. getFG :: Attr -> Color getFG (Attr a) = Color (a .&. 0xFF) -- | Set the background color of an `Attr'. setBG :: Color -> Attr -> Attr setBG (Color c) (Attr a) = Attr ((a .&. 0xFFFF00FF) .|. (c `uncheckedShiftL` 8)) -- | Get the background color of an `Attr'. getBG :: Attr -> Color getBG (Attr a) = Color ((a .&. 0xFF00) `uncheckedShiftR` 8) -- | Set bold attribute of an `Attr'. setBold :: Attr -> Attr setBold (Attr a) = Attr (a .|. 0x10000) -- | Clear bold attribute of an `Attr'. clearBold :: Attr -> Attr clearBold (Attr a) = Attr (a .&. 0xFFFEFFFF) -- | Examine bold attribute of an `Attr'. isBold :: Attr -> Bool isBold (Attr a) = (a .&. 0x10000) /= 0 Urk. this is NO FUN. How do I stop the copy&paste and still generate fast code? Is there a better way? (Also, GHC seems unable to unbox strict fields that are newtypes. Can this be done, or should I abandon type safety and use type synonyms?) (Yes, this is performance critical code, and I have profiler output to back up this claim.) (Should this be directed to glasgow-haskell-users@ ?)

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Newbie: generating a truth table
by phiroc＠free.fr 06 Feb '07

06 Feb '07

Hello, I would like to create a Haskell function that generates a truth table, for all Boolean values, say, using the following "and" function : and :: Bool -> Bool -> Bool and a b = a && b I have tried creating a second function called "loop", which repeatedly calls "and", but it did not work, because, for some reason unknown to me, "do" does not like repeated function calls loop = do and True True and True False and False True and False False Is there a better way to repeatedly call "and"? Furthermore, is there a way in Haskell to loop through the Boolean values (True and False) and call "and" each time? Last but not least, in the "loop" function above, assuming that there is a way to repeatedly call the "and" function, how could you intersperse "printStr"s between the "and" calls? loop = do printStr("Calling and True True returns ") and True True printStr("Calling and True False returns ") and True False printStr("Calling and False True returns ") and False True printStr("Calling and False False returns ") and False False Many thanks. Best regards, phiroc

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blog: Is C# becoming a functional language?
by Bulat Ziganshin 06 Feb '07

06 Feb '07

Hello haskell-cafe, this seems like a comprehensive overview of FP facilities in C# 3.0: http://blogs.msdn.com/madst/archive/2007/01/23/is-c-becoming-a-functional-l… -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Generalizing three programs
by Andrew Wagner 06 Feb '07

06 Feb '07

Hi everyone, I've got an interesting problem here I'm trying to solve. Actually, I've got several problems which seem to have a very similar structure. I want to find a way to abstract them to solve other problems which can be thought about in the same way. Here they are: http://hpaste.org/307 http://hpaste.org/308 http://hpaste.org/309 Note that these are just sketches, the programs aren't done yet. The general structure of the problem is that an object enters a system, interacts with different parts of the system, and eventually leaves, and we want to monitor some statistics about the interaction, so that we can then make some changes, and run it again, and hopefully improve it. Thanks in advance for any suggestions!

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Final issues to fix for the shootout entries (Was: no subject)
by dons＠cse.unsw.edu.au 06 Feb '07

06 Feb '07

Yeah, have a look on the shootout: http://shootout.alioth.debian.org/gp4/benchmark.php?test=recursive&lang=all Has an ackerman. More detailed comparisions, across the 19 benchmarks on the shootout, with notes on which of the 19 GHC Haskell is significantly slower (>4x) at: Haskell verus Python http://shootout.alioth.debian.org/gp4/benchmark.php?test=all&lang=ghc&lang2… Loses on: regex-dna Haskell verus Ruby http://shootout.alioth.debian.org/gp4/benchmark.php?test=all&lang=ghc&lang2… Loses on: regex-dna Haskell verus Perl http://shootout.alioth.debian.org/gp4/benchmark.php?test=all&lang=ghc&lang2… Loses on: regex-dna Haskell verus Lisp http://shootout.alioth.debian.org/gp4/benchmark.php?test=all&lang=ghc&lang2… Loses on: mandelbrot (Double math) spectral-norm (Double math) Haskell verus Clean http://shootout.alioth.debian.org/gp4/benchmark.php?test=all&lang=ghc&lang2… Loses on: k-nucleotide ( HashTable benchmark) mandelbrot spectral-norm Haskell verus SML http://shootout.alioth.debian.org/gp4/benchmark.php?test=all&lang=ghc&lang2… Loses on: k-nucleotide mandelbrot spectral-norm Haskell versus OCaml http://shootout.alioth.debian.org/gp4/benchmark.php?test=all&lang=ghc&lang2… Loses on: k-nucleotide mandelbrot regex-dna spectral-norm Haskell versus C Loses on: k-nucleotide mandelbrot regex-dna spectral-norm Fixing these last few programs: k-nucleotide: Needs a better hashtable library implementation. For example a Data.HashByteString based on a Trie of ByteString) regex-dna: Should use the regex-tre library instead of regex-posix (algorithm issue only) mandelbort/spectral-norm: Double math is slow. I don't know why. And note that strings aren't a bottleneck anymore! http://shootout.alioth.debian.org/gp4/benchmark.php?test=sumcol&lang=all -- Don seth: > GHC has profiling support. > > (By the way, many mail servers these days discard mail with no subject.) > > I've seen a number of papers comparing the speed of Haskell code to code of other functional languages; there is a periodic "shoot out" with ocaml. > > Some probably have comparisons with imperative languages, and, even if they do not, the methodology should help you. > > Seth Kurtzberg > > On Mon, 5 Feb 2007 11:28:03 -0800 (PST) > Tays Soares <tayscristina(a)yahoo.com.br> wrote: > > > Hello everyone, > > > > I did at my master thesis a compiler that generates Haskell code. Now I need to measure the execution time of my generated code and I've been searched and I don't know if I'm looking with the wrong keywords but I could not find anything. I just need to measure the time of simple functions, like Ackermann and Fibonacci. Does anyone know how to measure the execution time of a Haskell program or function? > > > > Thank you, > > Tays > > > > > > > > __________________________________________________ > > Fale com seus amigos de gra?a com o novo Yahoo! Messenger > > http://br.messenger.yahoo.com/ > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users(a)haskell.org > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

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