April 2002 - Haskell-Cafe

convering Fds to Handles
by Abraham Egnor 22 May '02

22 May '02

Is there a way to convert an Fd variable to a Handle variable? Specifically, I'd like to use "pipe" from the Posix library to create a pipe and then pass one end of it to runProcess as its stdin. Abe

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(no subject)
by Ricky Kurniawan 30 Apr '02

30 Apr '02

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some algorithms are hard to code in O(n) in Haskell
by klusacek＠atrey.karlin.mff.cuni.cz 29 Apr '02

29 Apr '02

Hi - does anybody know how to do a term unification in Haskell in O(n), where n is length of the input terms? I know Montanari-Martelli algorithm for this, but I'm unable to code it in Haskell (in O(n)). I would need a destructive update of the directed graph representing terms (to do a substitution) and checking for cycles in directed graphs (these cycles emerge when unifying things like x=f(x) -- their detection is also called occurs-check). I've been trying hard but still have no solution. Maybe I'm not smart enough, maybe I don't know some trick, maybe it's impossible in Haskell. Thanks, _dave_

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Re: Binary search tree
by Rijk J. C. van Haaften 29 Apr '02

29 Apr '02

>Hi everybody, > >I studied haskell this semester at the university and I was required to >implement a binary search tree in haskell. >I would appreciate if anyone could send me an example code of this data >structure. Just read a standard textbook. Some useful course notes by Jeroen Fokker are available online at http://www.cs.uu.nl/people/jeroen/courses/fp-eng.pdf Enough to solve your problem. Rijk-Jan van Haaften

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Binary search tree
by Claudio Marcio de Souza Vicente 29 Apr '02

29 Apr '02

Hi everybody, I studied haskell this semester at the university and I was required to implement a binary search tree in haskell. I would appreciate if anyone could send me an example code of this data structure. thanks in advance, Claudio

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RE: defining (-> Bool) as a set
by Hal Daume III 26 Apr '02

26 Apr '02

Would there be any problems if there were an artificial "<-" type constructor in Haskell which was basically: type a <- b = b -> a but not a type synonym so you could define it to be instances of classes, but then those get applied "backwards" to ->? Okay, that made no sense. I'll try again. Type lambda = bad (undecidable). What about only type lambda on function types and not even type lambda; simply allow partial application to the second argument *only* on ->? This would solve my problem and I believe that of an earlier poster who wanted to define catamorphisms (I think; don't feel like checking), but couldn't because of this restriction. Is this still "too loose" to be made to work? -- Hal Daume III "Computer science is no more about computers | hdaume(a)isi.edu than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume On Mon, 22 Apr 2002, Simon Peyton-Jones wrote: > Hal, > > [I think this sort of question would be better on the haskell-cafe > list.] > > I don't think what you want can be done directly. It's the old > thing about not having lambdas at the type level. You want: > > instance Eq a => Coll (\x. x -> Bool) a where ... > > and you just can't do that. You *can* abstract the second argument > of (->): > > instance Eq a => Coll ((->) Bool) a where ... > > but not the first. It's a well known shortcoming in Haskell, that you > can > partially apply type constructors, but you can't do argument > permutation. > > I know of no good solution. Adding type lambdas in their full glory > makes type inference pretty much impossible. What we'd like is > a compromise. Maybe someone can invent one. But take care. > The ground is littered with corpses. > > Simon > > > | -----Original Message----- > | From: Hal Daume III [mailto:hdaume@ISI.EDU] > | Sent: 23 April 2002 01:30 > | To: Jorge Adriano > | Cc: Haskell Mailing List > | Subject: Re: defining (-> Bool) as a set > | > | > | Yeah, both options suggested are valid, of course. But I > | really don't want to have a constructor and I'm using Edison > | where Coll is defined something like: > | > | class Coll c e where > | empty :: c e > | insert :: c e -> e -> c e > | > | etc., which precludes the fun dep solution. > | > | - Hal > | > | -- > | Hal Daume III > | > | "Computer science is no more about computers | hdaume(a)isi.edu > | than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume > | > | On Tue, 23 Apr 2002, Jorge Adriano wrote: > | > | > > | > > class Collection e ce | ce -> e where > | > > empty :: ce > | > > insert :: e -> ce -> ce > | > > member :: e -> ce -> Bool > | > > > | > > instance Eq a => Collection a (a -> Bool) where > | > > empty = (\x -> False) > | > > insert e f = (\x -> if x == e then True else f x) > | > > member e f = f e > | > > | > This is way better than my solution... > | > > | > I had never used multi-parameter classes before, so I forgot the > | > functional > | > dependency (right name? the "|ce->e"), and there was > | obviously no need for my > | > extra constructor. > | > > | > J.A. > | > > | > | _______________________________________________ > | Haskell mailing list > | Haskell(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell > | >

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RE: lazy comparison for equality ?
by Konst Sushenko 24 Apr '02

24 Apr '02

In addition to what Hal said, I think that even assuming that you want to write a function like that that is only supposed to be applied to finite lists (like, say, prelude's 'and' function, which does not terminate on an infinite list of True values) you still cannot do it because haskell lists are not like C lists. There is no notion, to my understanding, of list circularity. There are finite and infinite lists. Your example of a list that you call a circular list is just a definition of a recursive function that produces an infinite list. So what you are almost asking is how to write a function that checks if a list has repeated sub-sequences, which is not of course precisely the same that you asked (because even a finite list can have repeated sub-sequences), but close. And this is just one of many problems taken care of by various string search algorithms. konst > -----Original Message----- > From: Hal Daume III [mailto:hdaume@ISI.EDU] > Sent: Wednesday, April 24, 2002 12:38 PM > To: klusacek(a)atrey.karlin.mff.cuni.cz > Cc: haskell-cafe(a)haskell.org > Subject: Re: lazy comparison for equality ? > > > I don't think you can write such a function. For instance, > how would you > know whether [1..] is circular or not? In order to know that it's not > you'd need to evaluate it fully. > > -- > Hal Daume III > > "Computer science is no more about computers | hdaume(a)isi.edu > than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume > > On Wed, 24 Apr 2002 klusacek(a)atrey.karlin.mff.cuni.cz wrote: > > > > > Hi - > > I'm a Haskell beginner and I have a problem. > > > > Let's have a list which may be normal list > > list1 = [1,2,3] > > or a circular list > > list2 = 1:2:list2 > > > > Now I'd like to have a function which tells me whether the > > given list is circular or not. This doesn't work: > > > > circ l = l l > > circ2 l [] = False > > circ2 l (_:as) | l==as = True > > | True = (circ2 l as) > > > > > > It seems that comparison l==as really compares element by > element thus > > falling into an infinite loop. I would need to compare > pointers instead of > > values. > > > > Does anybody know how this could be done ? > > > > Thanks. > > > > > > > > > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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lazy comparison for equality ?
by klusacek＠atrey.karlin.mff.cuni.cz 24 Apr '02

24 Apr '02

Hi - I'm a Haskell beginner and I have a problem. Let's have a list which may be normal list list1 = [1,2,3] or a circular list list2 = 1:2:list2 Now I'd like to have a function which tells me whether the given list is circular or not. This doesn't work: circ l = l l circ2 l [] = False circ2 l (_:as) | l==as = True | True = (circ2 l as) It seems that comparison l==as really compares element by element thus falling into an infinite loop. I would need to compare pointers instead of values. Does anybody know how this could be done ? Thanks.

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RE: "do" notation and ">>"
by Simon Marlow 24 Apr '02

24 Apr '02

> I was not suggesting that the Haskell'98 report should > change or even give a warning -- I was giving a warning to > compiler implementors, that this simple change might have > disastrous effects. > > BTW, I remember a similar discussion along these lines on > the Haskell mailing list that happened in 1997 I think, but > I cannot find the archives. > > Another comment one can make here is the following: if > Haskell does not care about sharing, why is the monomorphism > restriction there? One difference is that when the compiler gives you *less* sharing than you were expecting, then it is in some sense worse than getting more sharing. More sharing might result in space leaks, but in practice (in my experience anyway) this isn't as noticeable as the time leaks you get when sharing is lost. GHC has implemented almost-full-laziness for a long time, and I don't recall anyone ever complaining about space leaks introduced as a result. The monomorphism restriction, on the other hand, is concerned with the user getting no less than the amount of sharing he/she was expecting (personally I think this should be a compiler warning rather than a language restriction, but I'm aware there are other reasons for wanting the MR too). Cheers, Simon

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how to call Fortran procedures in Haskell program?
by Zhe Fu 21 Apr '02

21 Apr '02

Does anybody know how to call Fortran procedures in Haskell program? I tried Green Card, but seems it only works with C codes. Thanks.

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