- Haskell-Cafe - Haskell.org

11 Jul '02

��http://www.comdns.com��ʺ��л��տ��ʼ��ϵ�ϣ��ܳ�Ϊ��ڹ��Ҫ ��顣��Ϣ��ã��ɾ��ڴ��Ƕ��ʾǸ��!! �� http://www.comdns.com��һ��ר��Ϊ��ҵ��λ�ṩ��վ��衢��ƽ̨��Ϣ��… ˾��Ŀ��ǳ�Ϊ��缼��ƽ̨��֮һ��ǵķ��ɫ�ǣ�˫Ӳ��ʱ��ݣ��ʧ�� ˾�Ƴ��´��⽫��ֵ��ѡ�� 1.��200M��HTML�ռ�+1��150Ԫ/�꣨�ں��ݣ�� 2.��60Masp��վ�ռ�+60��ҵ�ʾ֣��п��䣩+1��246Ԫ/�꣨�ں��ݣ�� 3.��׼��200MASP��վ�ռ�+200��ҵ�ʾ֣��п��䣩+1��+30M Access��ݿ⣬��336Ԫ/�� ں��ݣ�� 4��ʵ��ע��ֻ��500��/�� 5.��Ż��վ��跽��ο�� Ŭ��ã��Ҳ��ʧ��ǲ��һ��Ĳ��ġ��ǳ�ŵ�� ʱ�ɰ�ʵ��˿ ��ף��ģ�˳�� webmaster of www.comdns.com webmaster(a)comdns.com ========================================================================================= ��ʼ��õ��˵��Ϣ��˵��û�ã��ֱ�Ǹ��ʼ�� Գ�ŵֻ��һ�Ρ��ϣ��Ǵ��б��ɾ��ĵ�ַ��Ÿ��ĵ�ַ��лл�� ========================================================================================== --------------------------------------------------------------- �е�Ⱥ��ʼ�,��ר��(http://www.21cmm.com) ��CMM��У(http://www.21cmm.com)��Ŀ��ר��

2 1

haskell.sty
by François Bédard 10 Jul '02

10 Jul '02

Hello, I find it interesting that Haskell should make native provision for a basic form of literate programming, and this immediately suggests, of course, the possibility of mixing code with LaTeX commands. The problem is that putting one's hand on a dedicated LaTeX extension doesn't seem so straightforward. The link to Andrew Cooke's haskell.sty is broken and I couldn't find the file at his new home (www.acooke.org/andrew/) Other possibilities such as Manuel Chakravarty's, lambdaTex and lhs2tex (which I, admittedly, have only looked at briefly) deviate from the \begin{code}...\end{code} suggested by the report and seem to require more (La)TeX proficiency on the part of the user. As my interest is to introduce the ideas of literate programming and LaTeX to people thorougly unfamiliar with such, I need to keep things as simple as possible - of special interest is the fact that a given file can be submitted as is to both hugs and latex, without the need for preprocessing. I've tried to address the question by redefining an existing environment \usepackage{moreverb} \newenvironment{code}{\begin{verbatimtab}[4]}{\end{verbatimtab}} but moreverb and the like depend on verbatim which is apparently the one environment you can't use like this and I'm no such LaTeX guru that I can come up with better alternatives. So the question is : would anyone know where to find haskell.sty (presuming that's what I need) or is there some fairly simple solution I've missed ? Thanks, François

3 4

Parsec question
by Eray Ozkural 10 Jul '02

10 Jul '02

Hi there, I'm hacking together a small parser for a subset of C++. I'm in trouble with left recursion as you may guess. What is the best way to write a parser for the part of grammar below in Parsec? Declarator -> Name -> PtrOper Declarator -> Declarator '(' ArgDeclarationList ')' [CvQualifierList] -> Declarator '[' [ConstantExp] ']' -> '(' Declarator ')' Here [X] denotes an optional non-terminal X, and all symbols starting with a capital letter are non-terminals. The remedies I thought of were using chainl or eliminating left recursion in the grammar. What do you think should be done in such a case? Regards, -- Eray Ozkural <erayo(a)bilkent.edu.tr> Comp. Sci. Dept., Bilkent University, Ankara www: http://www.cs.bilkent.edu.tr/~erayo GPG public key fingerprint: 360C 852F 88B0 A745 F31B EA0F 7C07 AE16 874D 539C

1 0

Fw: 2 Positions
by Manuel M. T. Chakravarty 06 Jul '02

06 Jul '02

Not 100% Haskell-related, but I thought some people on this list might be interested in these two open positions at UNSW. Manuel

1 0

Is there any algorithms for solving equation systems
by Daniel Andersson 04 Jul '02

04 Jul '02

Hi I looking for algorithms for solving equation systems and wonder if there is anyone that has seen an algorithm, either a numerical or an analytical, for solving equation systems in haskell. thanks Daniel _____________________________________________________ Följ VM på nära håll på Yahoo!s officielle VM-sajt www.yahoo.se/vm2002 Håll dig ajour med nyheter och resultat, med vinnare och förlorare...

1 0

Re: Writing a counter function
by Jon Cast 02 Jul '02

02 Jul '02

Mark Carroll <mark(a)chaos.x-philes.com> wrote: > On Sat, 29 Jun 2002, Samuel E. Moelius III wrote: > (snip) > > Here's another not-exactly-what-you-wanted solution. :) > (snip) > Do any of the experimental extensions to Haskell allow a > what-he-wanted solution? I couldn't arrange one in H98 without > something having an infinitely-recursive type signature. That's because the problem requires an infinitely-recursive type. > I'm sure it would have been easy in Lisp, and he already gave a Perl > equivalent, That's because both Lisp and Perl are weakly typed. Haskell is strongly typed. Consider Lisp: (defun self-apply (f) "Apply F to itself." (f f)) No problem (and I'm sure you can pull the same trick off in Perl). Consider Haskell: self-apply f = f f The *typing algorithm* (the thing that didn't complain in Lisp) proceeds roughly as follows: f is applied to at least one argument, so f must have type a -> b. Therefore, f's argument (f) must have type a. So, we conclude: f :: a -> b f :: a But f can only have one type, so we set its types equal: a = a -> b This is clearly recursive, right? > so I'm wondering if it could be at all sane for Haskell to allow > such stuff Sure. All it has to do is: 1. Create its own newtype in response to such things as `self-apply' above. 2. Ensure that self-apply f = f f and self-apply' g = g g have the same type. I would *love* to hear ideas on how to do that, but it's difficult. > and if Haskell is somehow keeping us on the straight and narrow by > disallowing the exact counter that was originally requested. Well, it's a more general problem than the `exact counter'. And, yes, Haskell is erring on the side of safety here; there's a reason it's called a ``statically typed language''. > The beauty of his request was that it was so simple and seemed to > make sense; I went ahead and tried to fulfill it before realising I > couldn't do it either. Well, it makes sense, yes. But I think learning to manually `unify' these things (add a newtype) is not hard, and probably useful --- Haskell is not the only statically-typed language out there. Learn how to solve this problem, and you understand the problem, the solution, typing, and life in general :) > -- Mark Jon Cast

3 2

Writing a counter function
by Shlomi Fish 01 Jul '02

01 Jul '02

I'm trying to write a counter function that would return a tuple whose first element is the current value and whose second element is a new counter. The following line: counter n = (n,(counter (n+1))) Generates the following error on Hugs and a similar one with ghci: ERROR "counter.hs":6 - Type error in function binding *** Term : counter *** Type : a -> (a,b) *** Does not match : a -> b *** Because : unification would give infinite type Is there any way to do it? I tried using data, type and newtype and none of them worked. Regards, Shlomi Fish ---------------------------------------------------------------------- Shlomi Fish shlomif(a)vipe.technion.ac.il Home Page: http://t2.technion.ac.il/~shlomif/ Home E-mail: shlomif(a)iglu.org.il He who re-invents the wheel, understands much better how a wheel works.

11 20

Defining instances of superclasses
by Zdenek Dvorak 30 Jun '02

30 Jun '02

Hello. I've got the following problem: I have class Structure, that says that its members have some internal parts that can be read from/written to Strings: >data StrItem a=forall b c.(Show b,Parse c)=>StrItem String (a->b) (a->c->a) > >class Structure a where > strName::a->String > strEmpty::a > strItems::[StrItem a] Now I would like to use it to create instances of Show and Parse (and perhaps others (for input/output from various formats, ...)). The approaches I have tried: 1) >instance Structure a => Show a where > showsPrec _ w = > showsStruct (strName w) (map showsPair strItems) > where > showsPair (StrItem fName fGet _)=(fName,shows $ fGet w) This is nice, but it only works in ghc with allowed overlapping and undecidable instances. It may not seem too important, when I use existential types anyway, but existential types are supported by at least one more implementation (hugs) and they seem to me to be a well defined extension that has good chances to become part of standard Haskell later; handling of overlapping and undecidable instances in ghc seems quite too much ad-hoc and experimental for me. 2) >newtype Str a=Str a >instance Structure a => Show (Str a) where > showsPrec _ (Str w) = > showsStruct (strName w) (map showsPair strItems) > where > showsPair (StrItem fName fGet _)=(fName,shows $ fGet w) This seems to be a standard way how to do it. The problem is, that I don't get Show instance for a, but for isomorphic type Str a; this is a bit annoying when I want to use it; also I may no longer use deriving Show on types containing such structure. Is there some better way how to do this? Zdenek Dvorak _________________________________________________________________ Send and receive Hotmail on your mobile device: http://mobile.msn.com

1 0

Greencard-Canada-Aupair
by Greencard 28 Jun '02

28 Jun '02

1 0

Biz Kurduk Ordan Biliyoruz
by �leti... 22 Jun '02

22 Jun '02

1 0