June 2007 - Haskell-Cafe

I saw this... and thought of you
by Andrew Coppin 06 Jun '07

06 Jun '07

http://dis.4chan.org/read/prog/1180896798/ (It's been a while since I touched Java, and I must confess I can't even comprehend this code...)

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(pre)compiled Haskell compiler for True64/Alpha
by Lutz Donnerhacke 06 Jun '07

06 Jun '07

Does anyone have a haskell compiler for True64 (formerly known as OSF/1) on Alpha hardware? I'm currently unable to compile the first bootstrap compiler. Any hint which compiler should I start with? I'll send detailed error reports only if I do not succeed in the next hours.

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compiling shared library with GHC on x86-64
by Alexander Vodomerov 06 Jun '07

06 Jun '07

Hello! I'm trying to build shared library from Haskell source to call it from external C program. Everything works fine on usual x86 machine. However, any attempt to compile code as position independent on x86-64 result in fatal error. This is simple example: $ cat adder.hs module Adder where f :: Int -> Int -> Int f x y = x + y $ ghc -fPIC -c adder.hs ghc-6.6.1: panic! (the 'impossible' happened) (GHC version 6.6.1 for x86_64-unknown-linux): initializePicBase Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug $ I've read source file compiler/nativeGen/PositionIndependentCode.hs and found that function initializePicBase is not implemented for x86-64 platform. Are there any plans to implement such a functionality? Are there any global difficulties with such an implementation or nobody just cared about it? What functions are needed to be implemented for building PIC code on x86-64? If it is not very difficult, may be I will try myself. What is best source of GHC code generator documentation? Any other ideas? With best regards, Alexander.

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What puts False before True?
by PR Stanley 06 Jun '07

06 Jun '07

Hi What is the basic philosophy for Bool being a member of Ord? What justifies False < True? many Thanks in advance, Paul

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Hardware
by Andrew Coppin 06 Jun '07

06 Jun '07

OK, so... If you were going to forget everything we humans know about digital computer design - the von Neuman architecture, the fetch/decode/execute loop, the whole shooting match - and design a computer *explicitly* for the purpose of executing Haskell programs... what would it look like? What design decisions would you make? Why?

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Failure in GHC compilation
by Monique Monteiro 05 Jun '07

05 Jun '07

Hi, has anyone any idea about what may cause the following error in GHC compilation? c:\ghc\ghc-6.6.1\bin\ar.exe: creating libHSbase.a xargs: c:/ghc/ghc-6.6.1/bin/ar: Argument list too long make[2]: *** [libHSbase.a] Error 126 make[2]: *** Deleting file `libHSbase.a' Failed making all in base: 1 make[1]: *** [all] Error 1 make[1]: Leaving directory `/cygdrive/c/GHC6.6.1/ghc-6.6.1/libraries' make: *** [stage1] Error 2 Thanks in advance, -- __________________________________________________________ Monique Monteiro, MSc MCP .NET Framework 2.0 / SCJP / IBM OOAD Project Manager Recife Microsoft Innovation Center +55 81 34198137 http://www.cin.ufpe.br/~mlbm http://thespoke.net/blogs/moniquelouise/default.aspx monique(a)qualiti.com.br MSN: monique_louise(a)msn.com

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Re: [Haskell-cafe] Idiomatically using lists
by kevin birch 05 Jun '07

05 Jun '07

On 火, 2007-6月-05, at 02:54, Greg Fitzgerald wrote: > > rotating the fourth element 2 positions would result in: [1, 2, > 4, 3, 5] > Seems odd. Should that be [4,1,2,3,5]? > Yes, I meant to use the 5 element in my second example. Sorry for the confusion. > > Is there an idomatic way to handle both of these cases in a > function? > Generally people like to see your attempt at a solution before > giving the idomatic one so that they are sure it's not a homework > question. What do you have so far? > Yeah, I only wish I had gone to a school that would be forward thinking enough to each FP. ;-) Here is my version: rotate :: Array Integer Card -> Integer -> Integer -> Array Integer Card rotate a i n | i <= u - n = a // [(i, a ! (i + 1)), (i + 1, a ! (i + 2)), (i + 2, a ! i)] | otherwise = a // zip [l..u] (h ++ [a ! i] ++ filter (not . (== (a ! i))) t) where (l, u) = bounds a (h, t) = splitAt (fromInteger ((i - u) + n)) $ elems a This function is part of my implementation of the Solitaire encryption algorithm, so that is why I have the reference to a Card data type. This does what I want, and seems basically idiomatic, but perhaps it could be better. Thanks, Kevin

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FOL
by Tony Morris 05 Jun '07

05 Jun '07

I would like to know if all 16 possible functions that accept two boolean arguments have names in First-Order Logic. I know they have Haskell function names (e.g. \p -> \_ -> id p, \_ -> \_ -> const True), but I'm hoping there is a more general name that is recognised by anyone with a feel for logic, but not necessarily Haskell. I have listed all sixteen of these functions below using Haskell (named a to p) along with the name of those that I am aware of having a name as a comment. Thanks for any tips. {- p q | a b c d e f g h i j k l m n o p 0 0 | 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 0 1 | 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 1 0 | 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 1 1 | 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 -} a :: Bool -> Bool -> Bool a _ _ = False b :: Bool -> Bool -> Bool -- conjunction AND b False _ = False b _ q = q c :: Bool -> Bool -> Bool c False _ = False c _ q = not q d :: Bool -> Bool -> Bool d p _ = p e :: Bool -> Bool -> Bool e False q = q e _ _ = False f :: Bool -> Bool -> Bool f _ q = q g :: Bool -> Bool -> Bool -- exclusive disjunction XOR g False q = q g _ q = not q h :: Bool -> Bool -> Bool -- disjunction OR h False False = False h _ _ = True i :: Bool -> Bool -> Bool -- negation of disjunction NOR i False False = True i _ _ = False j :: Bool -> Bool -> Bool -- biconditional j False q = not q j _ q = q k :: Bool -> Bool -> Bool k _ q = not q l :: Bool -> Bool -> Bool l False q = not q l _ _ = True m :: Bool -> Bool -> Bool m p _ = not p n :: Bool -> Bool -> Bool -- implication n False _ = True n _ q = q o :: Bool -> Bool -> Bool -- negation of conjunction NAND o False _ = True o _ q = not q p :: Bool -> Bool -> Bool p _ _ = True -- Tony Morris http://tmorris.net/

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Why does the class called "Real" support only rationals, and not all reals?
by bretm 05 Jun '07

05 Jun '07

I just got started learning Haskell a few days ago. I've implemented a numeric data type that can represent some irrational numbers exactly, and I'd like to instantiate the RealFrac class so I can do truncate, round, etc., in the most natural way in the language. Implementing properFraction (which is in RealFrac) would not a problem at all, but implementing RealFrac requires implementing Real, which has the function toRational, which Haskell 98 says "returns the rational equivalent of its real argument with full precision" which isn't possible for this number type (which includes irrationals). I'd also like to implement approxRational since this can be done in a straightforward manner with my data type, but this appears to be just a library function in the Ratio library and not a function defined in a class, and is dependent on the toRational function which doesn't make sense for a data type that can represent irrational values exactly. So my questions are: 1) Am I understanding the Haskell numeric classes correctly by thinking that to define properFraction I also have to define (incorrectly in this case) the toRational function? 2) Is a literal reading of Haskell 98 necessary for toRational or can it just be approximate? (Seems like not, since there's approxRational, and no other way to indicate the desired accuracy for toRational.) 3) If I'm right about toRational requiring "full precision", why was implementing this made a requirement for implementing properFraction? 4) Why on earth is there a class called "Real" that requires the ability to convert to Rational with full precision, which can't be done with all real numbers? As an example, consider a data type that can represent a quadratic surd, e.g. (1, 5, 2) might mean (1 + (sqrt 5)) / 2. With this data type it's straightforward to implement the Num and Fractional classes and get full arithmetic operations +, -, *, /, etc. The properFraction function also makes sense: there's an integer portion equal to 1 in this case, and a fractional portion equal to (-1, 5, 2), exactly. So truncate, round, ceiling, and floor make sense (in RealFrac). But toRational can't be implemented with "full precision". With this example you can also see how approxRational is a perfectly good idea, and would be something likely to be needed, but this function isn't a class function designed to be implemented by custom data types. It seems to me (with my meager few days of Haskell experience) that approxRational should have been part of the Real class, and that toRational should be relegated the Ratio library instead. Or else, that the functions that are in RealFrac should be in the Fractional class instead. So I'm guessing there are some good reasons for having RealFrac in addition to Fractional that I'm not yet aware of. -- View this message in context: http://www.nabble.com/Why-does-the-class-called-%22Real%22-support-only-rat… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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compiler implementation details
by José Miguel Vilaça 05 Jun '07

05 Jun '07

Hi, Im curious about how the Haskell compilers nowadays take care of a simple function definition like append append [] l = l append (h:t) l = h: (append t l) Do they take a rewriting way like append [1,2] [3,4] ==> 1: (append [2] [3,4]) ==> 1: (2 : (append [] [3,4]) ) ==> 1 : (2: [3,4]) ==> [1,2,3,4] Or do they converte the append definintion to some lambda-calculus flavour language append = Y (\f -> \l1 l2 -> case l1 of [] -> l2 (h:t) -> h: (f t l2)) And then apply this converted definition to the arguments? Best Miguel

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