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Newbie question about classes and class parameters
by Dmitry Astapov 17 Oct '01

17 Oct '01

Let's say we have infamous Set class defined this way: > class Set s a where > empty :: s a > isEmpty :: s a -> Bool Lets also say that we could define sets as lists: > data SetAsList a = SL [a] deriving (Eq,Ord,Show) > instance Set SetAsList a where > empty = SL [] > isEmpty (SL []) = True > isEmpty _ = False So far, so good. Lets define type synonim for set of integers: > type IntegerSet = SetAsList Integer And set of sets of integers: > data IntegerSuperSet = ISS (SetAsList IntegerSet) Testing in Hugs show that everything seems to be working: Main> :t ISS (SL [ (SL [1,2,3])::IntegerSet, (SL [4,5,6])::IntegerSet ]) ISS (SL [SL [1,2,3],SL [4,5,6]]) :: IntegerSuperSet However: Main> isEmpty (ISS (SL [(SL [1,2,3])::IntegerSet, (SL [4,5,6])::IntegerSet])) ERROR - Type error in application *** Expression : isEmpty (ISS (SL [SL [1,2,3],SL [4,5,6]])) *** Term : ISS (SL [SL [1,2,3],SL [4,5,6]]) *** Type : IntegerSuperSet *** Does not match : a b This is completely understood - I need to state that IntegerSuperSet is an instance of Set for isEmpty to be working. But: > instance Set IntegerSuperSet where > empty = ISS (SL []) > isEmpty (ISS (SL [])) = True > isEmpty _ = False gives me: Reading file "/tmp/bug.lhs": ERROR /tmp/bug.lhs:38 - Wrong number of arguments for class "Set" Obviously I am missing something very important here. Could someone enlighten me? -- Dmitry Astapov //ADEpt E-mail: adept(a)umc.com.ua Information Systems Expert Office: +380-50-110-3876 Ukrainian Mobile Communications Mobile: +380-50-330-2019 GPG KeyID/fprint: F5D7639D/CA36 E6C4 815D 434D 0498 2B08 7867 4860 F5D7 639D

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Re: Haskell 98 - Standard Prelude - Floating Class
by Dylan Thurston 17 Oct '01

17 Oct '01

On Mon, Oct 15, 2001 at 03:52:06PM +0200, Kent Karlsson wrote: > > Simon Peyton-Jones: > > > Russell O'Connor suggests: > > > | but sinh and cosh can easily be defined in terms of exp > > > | sinh x = (exp(x) - exp(-x))/2 > > > | cosh x = (exp(x) + exp(-x))/2 ... > > > This looks pretty reasonable to me. We should have default methods > > > for anything we can. > Mathematically, yes. Numerically, no. Even if 'exp' is implemented > with high accuracy, the suggested defaults may return a very > inaccurate (in ulps) result. Take sinh near zero. sinh(x) with x > very close to 0 should return x. With the above 'default' sinh(x) > will return exactly 0 for a relatively wide interval around 0, which > is the wrong result except for 0 itself. Hmm, on these grounds the current default definition for tanh x is even worse behaved: tanh x = sinh x / cosh x For moderately large floating point x, this will overflow. Frankly, I don't think the whole discussion matters very much; nobody who cares will use the default definitions. But remember to think about branch cuts. And why not go further? cos x = (exp (i*x) + exp (-i*x))/2 where i = sqrt (-1) etc. > In general, this is why LIA-2 (Language Independent Arithmetic, part > 2, "Elementary numerical functions", ISO/IEC 10967-2:2001) rarely > attempts to define one numerical operation in terms of other > numerical operations. That is done only when the relationship is > exact (even if the operations themselves are inexact). That is not > the case for the abovementioned operations. But it is the case for > the relationship between the complex sin operation and the complex > sinh operation, for instance. (Complex will be covered by LIA-3.) This sounds like a very interesting standard. I am constantly annoyed by ISO's attempts to hide their standards; one might wonder what the purpose is of having unavailable "standards". Is the content available somewhere? Best, Dylan Thurston

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ExitFailure 127
by Sebastian Schulz 16 Oct '01

16 Oct '01

hi. I'm trying to run an system command (wget) within a CGI script and get the following error: >Exitfailure 127 How can I get more information out of an ExitFailure x error? Can I use ioeGetErrorString? But this function doesn't work on ExitCode, which system returns. TIA sebastian

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fixed point
by Song Li 16 Oct '01

16 Oct '01

Hi, anybody can explain how Y combinator work in find fixed point ? say : Y = 'lam' f . ('lam' x . f(xx))('lam' x . f(xx)) X = Y F -> ( 'lam' x .F(xx))('lam' x . F(xx)) -> F(('lam' x . F(xx))('lam' x . F(xx))) <- How can this expression be derived ???????????? ->F X <- And why ? Thanks a lot! Song

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a newbie's question
by Song Li 15 Oct '01

15 Oct '01

Hi, there, Could anybody explain what does this type defination mean : data xxx a = xxx (a ->b) looks xxx can use itself as constructor(like tree) but change the type...... Thanks! Song

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Closed Classes
by Eric Allen Wohlstadter 13 Oct '01

13 Oct '01

Are there any plans to add closed classes as a hugs extension? This was described in the paper: Object-Oriented Style Overloading for Haskell, Mark Shields and Simon Peyton Jones; BABEL workshop '01. Eric Wohlstadter

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Re: = vs ->
by Ashley Yakeley 12 Oct '01

12 Oct '01

At 2001-10-09 11:55, Mark Carroll wrote: >What is the rationale for when Haskell demands a "=" and when it demands >a "->"? What? Example please... -- Ashley Yakeley, Seattle WA

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Re: performUnsafeIO
by Ashley Yakeley 12 Oct '01

12 Oct '01

At 2001-10-11 11:55, Sebastian Schulz wrote: >I need to extract an IO String to String. No!!! Don't do it. Make the final result of your Haskell calculation be an "IO String". Then when you have a look at it, it will be automatically executed and you'll see the String. -- Ashley Yakeley, Seattle WA

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help for exercise 4.10
by rock dwan 12 Oct '01

12 Oct '01

Iam having some difficulties doing exercise 4.10 from craft of functional programming book second edition ..is their a possible solution for this ? _________________________________________________________________ Get your FREE download of MSN Explorer at http://explorer.msn.com/intl.asp

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performUnsafeIO
by Sebastian Schulz 12 Oct '01

12 Oct '01

hi. I need to extract an IO String to String. Remembering some function like 'performUnsafeIO :: IO a -> a', I searched the hugs libs, but without a match. Does a function like this still exists? Or how can I transform IO a to a? regards seb

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