- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] HaXml and ghci unresolved symbol
by Andrea Rossato 10 Sep '06

10 Sep '06

Il Sat, Sep 09, 2006 at 04:34:04AM +0100, Marco André F. de Almeida ebbe a scrivere: > I know this will not solve your problem, but just so that you > know that with Hugs, you code works without any problems. > If you don't want/need to compile the program, I guess one interpreter > (Hugs) is as good as the other (GHCi) ;-) > > Marco Hi Marco! For the time being, that is to say, for learning xml transformation in haskell Hugs is just fine. But I'd like to start working on a project of a command line tool with hscurses that requires xml processing. I'm going to do that mostly as a learning exercise and I'm still in the preliminary phase, looking for libraries and tools. I'd like to start writing real code for the xml part, though. Thanks, Andrea

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NaN, Infinity literals
by Tamas K Papp 09 Sep '06

09 Sep '06

Hi, Is there a way to use NaN and Infinity as literals, or at least to test if a value is NaN or Infinity? I tried *Main> let nan=0/0 *Main> nan NaN *Main> nan==0/0 False so "storing" the value does not work... Thanks, Tamas

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Weak pointers and referential transparency???
by Brian Hulley 09 Sep '06

09 Sep '06

Hi, I have the following data structures: import System.Mem.Weak data Proxy = ... data Model = Model { _proxiesRef :: !(Ref.T [Weak Proxy]), ...} (Ref.T is just a lifted IORef) I was writing code like: createProxy :: MonadIO m => Model -> m Proxy createProxy Model{_proxiesRef = proxiesRef} = do proxies <- Ref.read proxiesRef let proxy = Proxy{ ... } weakProxy <- liftIO $ mkWeakPtr proxy Nothing Ref.write proxiesRef $! (weakProxy : proxies) return proxy where the intention is that whenever the returned proxy is no longer used, the table will no longer hold onto it (the table gets scavenged whenever I ask for the list of values in it) so there will be no space leak as proxies are created and discarded. However when I looked at the above code more closely, it struck me that Haskell gives no guarantees that the returned proxy is actually the same object that is referenced by the table, because of referential transparency. So my question is: will the above code work as expected and is this because we are allowed to assume non-referential transparency in situations like the above ie are we allowed to assume that all occurrences of the identifier "proxy" are always bound to a pointer to the same physical object and that all other uses of this value in the program are not going to cause it to be represented by multiple physical objects or otherwise split apart into registers etc? In other words, if the entry for the proxy in the table stored in the Model dies, can I be absolutely 100% sure that this means the proxy no longer exists in any shape or form in the running program? Thanks, Brian. -- Logic empowers us and Love gives us purpose. Yet still phantoms restless for eras long past, congealed in the present in unthought forms, strive mightily unseen to destroy us. http://www.metamilk.com

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How to round off frational number?
by Sara Kenedy 09 Sep '06

09 Sep '06

Hello all, I try to find some functions in Haskell library to deal with numeric such that the result in the following format (but I did not find) For example, >1/3 0.33 >1/6 0.17 >8/3 2.67 >9/3 3.00 It seems a so "baby" question, but I really did not find the answer after trying on that for some hours. Thanks for any help.

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sections of noncommutative operators
by Michael Shulman 09 Sep '06

09 Sep '06

A propos of sections of subtraction, and thence to sections of other noncommutative operators, as a Haskell newbie I was surprised to discover (the hard way!) that (< 0) and ((<) 0) mean different things. I had typed (< 0) when I meant to type ((<) 0). No compiler errors, of course, and I had a devil of a time finding that bug. My initial reaction was that (< 0) should be an error and you should have to write ((>) 0); now I realize that the section notation is more fundamental, since (<) itself is actually a "double section". And of course I should have written (> 0) anyway; it's probably my lisp background that tripped me up. But is there any way this could be made less confusing? Do any of the online tutorials warn against this sort of mistake, and I just missed it? Mike

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Derived Read instance for types with infix constructors (ghc 6.4.1)
by Misha Aizatulin 09 Sep '06

09 Sep '06

hi, the Haskell Report 10.4 says that "The result of show is readable by read if all component types are readable" however if I define a type like data T = A | T `And` T deriving (Read, Show) then *Main> show $ A `And` A "A And A" *Main> (read "A And A") :: T *** Exception: Prelude.read: no parse *Main> In fact, I wasn't able to guess, what I should type so that the value (A `And` A) gets parsed. I have ghc 6.4.1. Looking into the code of the derived instance I see that it expects Text.Read.Lex.lex to return (Symbol "And") for the constructor. If I understand the code for lex correctly, then it parses things as Symbol if they consist only of "!@#$%&*+./<=>?\\^|:-~" How then do I read values of type T defined above? Thanks in advance for any directions. Cheers, Misha

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[Newbie] Cannot derive Eq and Show. Why?
by Peter Arrenbrecht 09 Sep '06

09 Sep '06

Hi all I am stumped again. The following code generates the error "ERROR file:.\Cube.hs:12 - An instance of IArray UArray a is required to derive Eq (Cube a b)" in Hugs. But I did specify the IArray UArray k constraint. So what is wrong? module Cube( Cube(..) ) where import Data.Array.Unboxed data (Ord k, IArray UArray k, IArray UArray v) => Cube k v = Empty | Values (UArray Int k) (UArray Int v) | Subs (UArray Int k) (Array Int (Cube k v)) deriving (Show, Eq) Thanks for any help -peo -- View this message in context: http://www.nabble.com/-Newbie--Cannot-derive-Eq-and-Show.-Why--tf2239265.ht… Sent from the Haskell - Haskell-Cafe forum at Nabble.com.

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Reading integers
by Bertram Felgenhauer 08 Sep '06

08 Sep '06

Hi, prompted by a discussion about http://www.spoj.pl/problems/MUL/ on #haskell I implemented some faster routines for reading integers, and managed to produce a program that passes this problem. Currently I have a single module that provides reading operations for Integers and Ints. I'm not quite sure what to do with it. Ideally, read should use faster code for parsing numbers. This is hard to do though, because read for numbers is specified in terms of lex, which handles arbitrary tokens. One nasty side effect of this, besides bad performance, is that for example read ('"':reapeat 'a') :: Int diverges, even though there's obviously no possible parse. Incorporating faster integer reading code into read without breaking backward compatibility is not trivial. The next obvious place to put the functions is the Numeric module. This sounds like a good idea until one realizes that readSigned (which is the obvious thing to use for signed integers) again uses lex, with all the complications above. A third option is to put it all in a separate module. This would probably be hard to find, so few people would use it. I'm pondering breaking compatibility with Haskell 98 to some extent and implementing read instances for Int and Integer that don't diverge on weird inputs, but handle all valid numbers that Haskell 98 supports. This still requires some work to support octal and hexadecimal numbers, whitespace and parentheses. What do you think? The code (quite unpolished) is available with darcs get http://int-e.home.tlink.de/haskell/readinteger (browsing directly doesn't work) It also includes a simple benchmark for comparing reads, Numeric.readDec and my readInteger functions. Results for my computer are included as well. regards, Bertram

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Monad laws
by Deokhwan Kim 08 Sep '06

08 Sep '06

What is the practical meaning of monad laws? (M, return, >>=) is not qualified as a category-theoretical monad, if the following laws are not satisfied: 1. (return x) >>= f == f x 2. m >>= return == m 3. (m >>= f) >>= g == m >> (\x -> f x >>= g) But what practical problems can unsatisfying them cause? In other words, I wonder if declaring a instance of the Monad class but not checking it for monad laws may cause any problems, except for not being qualified as a theoretical monad? Cheers, -- Deokhwan Kim

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Does this function already exist in the libraries?
by Jason Dagit 08 Sep '06

08 Sep '06

Hello, Sometimes I find that I have two lists I want to combine, but I only want to combine them for as much as I have elements in both lists to combine and then the rest I just want to pass through unmodified. I've now found the following function useful in three different programs I've written: zipWith' :: (a -> a -> a) -> [a] -> [a] -> [a] zipWith' _ [] ys = ys zipWith' _ xs [] = xs zipWith' f (x:xs) (y:ys) = f x y : zipWith' f xs ys Does it already exist or maybe there is another natural solution to use in its place? Thanks, Jason

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