- Haskell - Haskell.org

Re: Import Chasing Problems - hello.lhs
by Sigbjorn Finne 10 Oct '01

10 Oct '01

John Heron jheron(a)enteka.com writes: > > I just installed ghc 5.02 on my NT 4.0 machine, and I'm wondering if the > import chasing is broken for ghc-5.02 on Windows NT? Or perhaps there is > yet another thing I don't understand about Haskell's environment or > modules? I got the following complaints when trying to compile Sigborn > Finne's Win32 example hello.lhs > (http://www.dcs.gla.ac.uk/~sof/hello.lhs) > > F:\stuff>ghc -o hello hello.lhs > > hello.lhs:14: > failed to load interface for `Win32': > Could not find interface file for `Win32' ... Hi, have a look at the top of that file - it tells you what options you need to feed GHC to compile the example (well, the options it suggests are deprecated by The Powers That Be; you ought to use "-package" instead of "-syslib"). Simpler yet, try running it via ghci: c:\src\haskell> ghci -package win32 hello.lhs and then evaluate 'main'. hth --sigbjorn btw, notice that ghc-5.02 currently has got a problem or two with the Win32 library -- see http://www.haskell.org/pipermail/glasgow-haskell-bugs/2001-October/000816.ht ml

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help for excersise 5.13
by rock dwan 10 Oct '01

10 Oct '01

i have done the function for borrowers but having some trouble with the borrowed and numBorrowed function. I have listed my code below with the borrowed function not completed and numBorrowed also not completed.Can you plz try to fill in the borrowed and numBorrowed functions if you can THANKS. type Database = [(Person,Book)] type Person = String type Book = String exampleBase :: Database exampleBase = [("Alice","Tintin"),("Anna","little Women"),("Alice","Asterix"),("Rory","Tintin")] borrowers :: Database -> Book -> [Person] borrowers db findBook = [person|(person,bks)<-db,bks==findBook] borrowed :: Database -> Book -> Bool borrowed db bookName numBorrowed :: Database -> Person -> Int reference from the craft of func programming second edition book page 84 and 87 _________________________________________________________________ Get your FREE download of MSN Explorer at http://explorer.msn.com/intl.asp

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Re: Extensible downcasts impossible in Haskell? (was Re: Monomorphism, monomorphism...)
by Marcin 'Qrczak' Kowalczyk 09 Oct '01

09 Oct '01

Tue, 9 Oct 2001 10:50:19 +1300, Tom Pledger <Tom.Pledger(a)peace.com> pisze: > I'm curious about this impossibility. > > - Is it well known? If so, would someone please refer me to a paper > or posting which explains it? I don't know. I'm not even sure if some clever encoding couldn't express it, but I can't imagine how it could do it and I would guess that it's impossible. It's not easy to formulate the question precisely but I'm quite sure that it's a well defined problem. You want to be able to embed arbitrary Haskell types in "subtypes". So it doesn't suffice to make a universal type with many useful concrete types under constructors, which emulates dynamically typed languages with a fixed number of types like Lisp or Erlang (I think). Because an interface of abstract types not included in this set may have binary methods or alike, so you can't just wrap the interface in a tuple of functions with types taken from the included set only. > - Does it just affect Haskell 98, or does it have deep implications > for any future language extensions? IMHO it's independent from most extensions. The only extension I know of which could be used for implementing downcasts is Dynamic. Some extensible algebraic types (where definition of constructors is spread among an open set of modules) would allow downcasts too I think. > - How does it relate to the alternative record mechanism idea you > mentioned a while ago? > > http://haskell.org/pipermail/haskell/2000-December/000213.html My mechanism doesn't allow downcasts, although it promotes a style of programming which probably wants to use downcasts sometimes. It's described as a translation to Haskell 98 with multiparameter classes, fundeps, the ability to gather label names from all modules and associate each name with an unique class, and some small adjustments of the type system (related to the termination of typechecking). These extensions don't provide anything useful for implementing downcasting, so I couldn't implement downcasts without changing the compiler even if I specified them somehow. -- __("< Marcin Kowalczyk * qrczak(a)knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTÊPCZA QRCZAK

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Re: Unicode support
by Ashley Yakeley 09 Oct '01

09 Oct '01

At 2001-10-09 02:58, Kent Karlsson wrote: >In summary: > > code position (=code point): a value between 0000 and 10FFFF. Would this be a reasonable basis for Haskell's 'Char' type? At some point perhaps there should be a 'Unicode' standard library for Haskell. For instance: encodeUTF8 :: String -> [Word8]; decodeUTF8 :: [Word8] -> Maybe String; encodeUTF16 :: String -> [Word16]; decodeUTF16 :: [Word16] -> Maybe String; data GeneralCategory = Letter_Uppercase | Letter_Lowercase | ... getGeneralCategory :: Char -> Maybe GeneralCategory; ...sorting & searching... ...canonicalisation... etc. Lots of work for someone. -- Ashley Yakeley, Seattle WA

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Re: Monomorphism, monomorphism...
by cwitty＠newtonlabs.com 09 Oct '01

09 Oct '01

"Marcin 'Qrczak' Kowalczyk" <qrczak(a)knm.org.pl> writes: > Since OO languages often use subtypes to emulate constructors of > algebraic types, they need downcasts. In Haskell it's perhaps less > needed but it's a pity that it's impossible to translate an OO scheme > which makes use of downcasts into Haskell in an extensible way > (algebraic types are "closed"). I agree. The TREX paper from Mark Jones and Benedict Gaster (I hope I have the names right) had both extensible records and extensible variants (extensible variants being what you would need to implement downcasts), but only the extensible records part of the paper was implemented in Hugs. Carl Witty

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trouble with excercise 5.26 fibtable
by riz er 09 Oct '01

09 Oct '01

im having abit of difficulty doing excersise 5.26 .. possible for an assistance plz :) _________________________________________________________________ Get your FREE download of MSN Explorer at http://explorer.msn.com/intl.asp

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exercise 4.19
by riz er 09 Oct '01

09 Oct '01

howMany :: Int -> Int -> Int -> Int howMany n1 n2 n3 | (n1 > a) && (n2 > a) && (n3 > a) = 3 | (n1 > a) && (n2 > a) = 2 | (n1 > a) && (n3 > a) = 2 | otherwise = 1 where a = (n1 + n2 + n3)/3 i get an error message "ERROR C:\My Documents\Haskell programming\question4.19.txt:4 - Instance of Fract ional Int required for definition of howMany" how could i resolved this ? _________________________________________________________________ Get your FREE download of MSN Explorer at http://explorer.msn.com/intl.asp

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Re: Unicode support
by Ashley Yakeley 09 Oct '01

09 Oct '01

At 2001-10-09 03:37, Kent Karlsson wrote: >> > code position (=code point): a value between 0000 and 10FFFF. >> >> Would this be a reasonable basis for Haskell's 'Char' type? > >Yes. It's essentially UTF-32, but without the fixation to 32-bit >(21 bits suffice). UTF-32 (a.k.a. UCS-4 in 10646, yet to be limited >to 10FFFF instead of 31(!) bits) is the datatype used in some >implementations of C for wchar_t. As I said in another e-mail, >if one does not have high efficiency concerns, UTF-32 is a rather >straighforward way of representing characters. Would it be worthwhile restricting Char to the 0-10FFFF range, just as a Word8 is restricted to 0-FF even though in GHC at least it's stored 32-bit? ... >> data GeneralCategory = Letter_Uppercase | Letter_Lowercase | ... >> getGeneralCategory :: Char -> Maybe GeneralCategory; > >There is not really any "Maybe" just there. Yet unallocated code >positions have general category Cn (so do non-characters): > Cs Other, Surrogate > Co Other, Private Use > Cn Other, Not Assigned (yet) OK. It occured to me to put 'unassigned' as Nothing, since it might change -- so in a sense getGeneralCategory doesn't know what the GC is. I assume once a codepoint has a non-Cn GC, it cannot be changed. But confusingly, some of the GCs are 'normative', whereas others are merely 'informative' -- perhaps these last are subject to revision. -- Ashley Yakeley, Seattle WA

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Your Bid 4 Placement Account
by unknown＠example.com 09 Oct '01

09 Oct '01

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Re: Unicode support
by Ashley Yakeley 09 Oct '01

09 Oct '01

At 2001-09-30 07:29, Marcin 'Qrczak' Kowalczyk wrote: >Some time ago the Unicode Consortium slowly began switching to the >point of view that abstract characters are denoted by numbers in the >range U+0000..10FFFF. It's worth mentioning that these are 'codepoints', not 'characters'. Sometimes a character will be made up of two codepoints, for instance an 'a' with a dot above is a single character that can be made from the codepoints LATIN SMALL LETTER A and COMBINING DOT ABOVE. Perhaps this makes the UTF-16 'surrogate' problem a bit less serious, since there never was a one-to-one correspondence between any kind of n-bit unit and displayed characters. -- Ashley Yakeley, Seattle WA

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