October 2004 - Haskell-Cafe

Newbie Question on type constructors
by Brian Beckman 04 Nov '04

04 Nov '04

This is a tiny question on the "data" syntax. Consider the following (cribbed from Paul Hudak's "School of Expression" book, page 22): data Shape = Circle Float | Square Float I read this something along the lines of "'Shape' is a type constructor, for use in other type-defining expressions, and 'Circle' and 'Sqare' are its two data constructors, which should be used like functions of type 'Float -> Shape'". Indeed, typing "Circle" at the Hugs prompt reveals that Haskell has a "function" named "Circle" with type "Float -> Shape." However, I don't know of other circumstances where (1) I can declare functions with capitalized names (Hugs groans about syntax errors if I attempt the following: Circle2 :: Float -> Shape Circle2 = Circle And (2) where the argument-types of a function can be declared on the function's right-hand side. So, in some sense, am I right to think of "data" as a special syntax for special kinds of function declarations, namely data-constructor function declarations, and that this syntax is different in appearance, but not terribly different in meaning, from the ordinary syntax, typified by circle2 :: Float -> Shape circle2 = Circle (this is just the uncialized version of the one above, and is perfectly legal) Apologies if this is just too nitpicky and pedantic for words, but I try to notice everything.

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installing HaXml
by Tom Spencer 02 Nov '04

02 Nov '04

Hi, I thought i had successfully installed HaXml, ghci loads up the package successfully etc. but then it complains that it can't find XmlLib. Using -v i get the following: Prelude> :load iChing.hs *** Chasing dependencies: iChing.hs: Can't find module `XmlLib' locations searched: XmlLib.hs XmlLib.lhs /usr/lib/ghc-6.2.1/imports/XmlLib.hi /usr/lib/ghc-6.2.1/imports/HaXml/XmlLib.hi /usr/lib/ghc-6.2.1/hslibs-imports/data/XmlLib.hi /usr/lib/ghc-6.2.1/hslibs-imports/util/XmlLib.hi /usr/lib/ghc-6.2.1/hslibs-imports/posix/XmlLib.hi /usr/lib/ghc-6.2.1/hslibs-imports/concurrent/XmlLib.hi /usr/lib/ghc-6.2.1/hslibs-imports/lang/XmlLib.hi i ran the install script again to see what was happening and noticed this line at the end of the install script: warning: can't find GHCi lib `HSHaXml.o' Saving old package config file... done. Writing new package config file... done. make[1]: Leaving directory `/usr/local/bin/HaXml-1.12/obj/ghc' it doesn't seem to work in ghc or hugs either any ideas? Thanks in advance Tom Spencer

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Problems with HaXml
by Tom Spencer 31 Oct '04

31 Oct '04

Hi, I thought i had successfully installed HaXml, ghci loads up the package successfully etc. but then it complains that it can't find XmlLib. Using -v i get the following: Prelude> :load iChing.hs *** Chasing dependencies: iChing.hs: Can't find module `XmlLib' locations searched: XmlLib.hs XmlLib.lhs /usr/lib/ghc-6.2.1/imports/XmlLib.hi /usr/lib/ghc-6.2.1/imports/HaXml/XmlLib.hi /usr/lib/ghc-6.2.1/hslibs-imports/data/XmlLib.hi /usr/lib/ghc-6.2.1/hslibs-imports/util/XmlLib.hi /usr/lib/ghc-6.2.1/hslibs-imports/posix/XmlLib.hi /usr/lib/ghc-6.2.1/hslibs-imports/concurrent/XmlLib.hi /usr/lib/ghc-6.2.1/hslibs-imports/lang/XmlLib.hi i ran the install script again to see what was happening and noticed this line at the end of the install script: warning: can't find GHCi lib `HSHaXml.o' Saving old package config file... done. Writing new package config file... done. make[1]: Leaving directory `/usr/local/bin/HaXml-1.12/obj/ghc' it doesn't seem to work in ghc or hugs either any ideas? Thanks in advance Tom Spencer P.S. apologies if i've accidentally posted this twice

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Re: [Haskell-cafe] Set of reals...?
by MR K P SCHUPKE 30 Oct '04

30 Oct '04

>Double already has +Inf and -Inf; it's just that Haskell doesn't have >(AFAIK) syntax to write them as constants. In the source for the GHC libraries it uses 1/0 for +Infinity and -1/0 for -Infinity, so I assume these are the "official" way to do it. Personally I would define nicer names: positiveInfinity :: Double positiveInfinity = 1/0 negativeInfinity :: Double negativeInfinity = -1/0 Keean.

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Set of reals...?
by Stijn De Saeger 29 Oct '04

29 Oct '04

Hi all, I'm new to this list, as well as to haskell, so this question probably has "newbie" written all over it. I'm thinking of a way to represent a set of reals, say the reals between 0.0 and 1.0. Right now I am just using a pair of Float to represent the lower and upper bounds of the set, but i have this dark throbbing feeling that there should be a more haskellish way to do this, using laziness. List comprehensions are out it seems, because they increment with integer steps... (obviously). In other words, 0.5 `inSet` (Set [0.0..1.0]) returns False. I'm sure someone must have hit this problem before me and found a way around it. any suggestions greatly appreciated, regards, stijn.

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RE: [Haskell-cafe] Set of reals...?
by Brian Beckman 29 Oct '04

29 Oct '04

Very pretty, Keean, though to get it to work in Hugs Nov 2002 I had to type the following uglier but equivalent syntax > myInterval = Interval { > isin = (\r -> > if r == 0.6 then True else > if r > 0.7 && r < 1.0 then True else > False ) > } -----Original Message----- From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Keean Schupke Sent: Wednesday, October 27, 2004 3:53 AM To: Stijn De Saeger Cc: haskell-cafe(a)haskell.org Subject: Re: [Haskell-cafe] Set of reals...? I think someone else mentioned using functions earlier, rather than a datatype why not define: data Interval = Interval { isin :: Float -> Bool } Then each range becomes a function definition, for example: myInterval = Interval { isin r | r == 0.6 = True | r > 0.7 && r < 1.0 = True | otherwise = False } Then you can test with: (isin myInterval 0.6) Keean _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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hugs segmentation fault
by Andrej Bauer 29 Oct '04

29 Oct '04

Hi, I am new to haskell, but otherwise experienced in programming languages. My first attempt at Haskell was this (on a Linux Debian) session with hugs: Type :? for help Prelude> :version -- Hugs Version November 2003 Prelude> let p = 1 : [2 * x | x <- p, x < 1] in p [1Segmentation fault Is it the norm for hugs to dump core like this? Or was I just very lucky to discover a bug? Best regards, Andrej

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Re: [Haskell-cafe] Set of reals...?
by MR K P SCHUPKE 29 Oct '04

29 Oct '04

>Keith Wansbrough wrote: >>Which brings me to a question: is there a better way to write -inf and >>+inf in Haskell than "-1/0" and "1/0"? Why not do it with types: data InfDbl = Dbl Double | PositiveInfinity | NegativeInfinity Keean.

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Re: [Haskell-cafe] Set of reals...?
by MR K P SCHUPKE 29 Oct '04

29 Oct '04

> | otherwise = contractSet (contract x0 y0:xs) ys I think you'll find the original is correct. The first two cases deal with non-overlapping ranges. The only remaining case is overlapping ranges, (partial and full overlap) both these cases are dealt with by contract, and as a result use up both the ranges at the head of both lists, sdo the merged range is prepended to the output list and the tail is calculated by passing the unused tails of both lists to contactSet... Keean.

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Re: [Haskell-cafe] Set of reals...?
by Keith Wansbrough 29 Oct '04

29 Oct '04

I forgot to mention: With regard to the dangerous practice of using "==" (as in your `elem` example) on Double, every computer scientist or programmer should read the following paper at some point in their training: David Goldberg, "What every computer scientist should know about floating-point arithmetic". ACM Computing Survey Volume 23 , Issue 1 (March 1991) Pages: 5 - 48. http://portal.acm.org/citation.cfm?id=103163 --KW 8-) -- Keith Wansbrough <kw217(a)cl.cam.ac.uk> http://www.cl.cam.ac.uk/users/kw217/ University of Cambridge Computer Laboratory.

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