- Haskell - Haskell.org

RE: Exhaustive Pattern-Matching
by Simon Marlow 29 Aug '03

29 Aug '03

> I have a question about pattern-matching. In the Haskell-report it is > not postulated, that > pattern matching has to be exhaustive. Would it be possible at all to > implement an > algorithm, which checks Haskell-style patterns for > exhaustiveness? What > kinds of > complication can be expected? Maybe you have some pointers to other > resources about > this topic. GHC tries to do so, but sometimes gets it wrong. See the -fwarn-incomplete-patterns flag. We'd appreciate it if someone could overhaul this code - it's been on the wish list for a long time. Cheers, Simon

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Haskell-report, chapter 3 - Expressions
by Steffen Mazanek 28 Aug '03

28 Aug '03

Hello, I do not completely understand the first part of chapter 3 of the Haskell-report. Concretely I am stumbling about the notation of nonterminals indexed by their precedence level. This should be a number ranging from 0 to 9. But what about this exp^{10} production rule? I would be very appreciated if someone could explain this part in some more detail. I am trying to introduce a new language construct into this grammar and I want to insert it at the right place :-) Ciao, Steffen

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Need some help please
by Rajiv Patel 28 Aug '03

28 Aug '03

Hi, I need to define a function called safetail; it's like tail except that this one maps the empty list to the empty list. It has to be defined using the following: 1. a conditional expression 2. guarded equation 3. pattern matching It's probably quite simple, but I am struggling on this, so help would be appreciated. Thanks Raj

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Interval arithmetic
by Jonas Ritter 28 Aug '03

28 Aug '03

Hi all, I am interested in interval arithmetic. Does anyone know something about haskell implementations in this domain? It would be nice to have a datatype Interval with the elementary operations {+,-,*,/}, where >+ :: Intervall -> Intervall -> Intervall >(a,b) + (c,d) = (a+c,b+d) >- :: Intervall -> Intervall -> Intervall >(a,b) - (c,d) = (a-d,b-c) etc. Bye Jonas

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RE: Need some help please.
by Hal Daume 27 Aug '03

27 Aug '03

There's a small problem with this solution, namely that it requires you to have an Eq instance for the elements in the list. You can fix this by using 'null' from the Prelude. -- Hal Daume III | hdaume(a)isi.edu "Arrest this man, he talks in maths." | www.isi.edu/~hdaume > -----Original Message----- > From: haskell-admin(a)haskell.org > [mailto:haskell-admin@haskell.org] On Behalf Of b.i.mills(a)massey.ac.nz > Sent: Wednesday, August 27, 2003 1:57 PM > To: haskell(a)haskell.org > Subject: Re: Need some help please. > > > What is Nick Name doing? Trolling? > > I don't approve of people getting homework done on this emailing > list, you learn more from doing, but baiting newbies is worse. > You want to give them the idea that Haskell is complex and abtruse? > > > I need to define a function called safetail; it's like tail > > except that this one maps the empty list to the empty list. > > It has to be defined using the following: > > Ok, so logicaly, we are saying that we want (safeTail x) to be > (tail x) if x is not [], but for x==[] (normally an error) we > want (safeTail []) == []. Just write this down in Haskell ... > > safeCond x = if x==[] then [] > else tail x > > Guards are just another way of producing conditional expressions > > safeGuard x | x==[] = [] > | True = tail x > > And pattern matching likewise, for this simple example > > safePat [] = [] > safePat x = tail x > > If you want to test the result ... > > tester 0 safe = [(safe [])] > tester n safe = (safe [1 .. n]) : (tester (n-1) safe) > > call it as (tester 10 safePat) for example, to get a list > of some of the outputs from the safePat routine. > > Regards, > > Bruce. > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell >

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Re: Need some help please.
by b.i.mills＠massey.ac.nz 27 Aug '03

27 Aug '03

What is Nick Name doing? Trolling? I don't approve of people getting homework done on this emailing list, you learn more from doing, but baiting newbies is worse. You want to give them the idea that Haskell is complex and abtruse? > I need to define a function called safetail; it's like tail > except that this one maps the empty list to the empty list. > It has to be defined using the following: Ok, so logicaly, we are saying that we want (safeTail x) to be (tail x) if x is not [], but for x==[] (normally an error) we want (safeTail []) == []. Just write this down in Haskell ... safeCond x = if x==[] then [] else tail x Guards are just another way of producing conditional expressions safeGuard x | x==[] = [] | True = tail x And pattern matching likewise, for this simple example safePat [] = [] safePat x = tail x If you want to test the result ... tester 0 safe = [(safe [])] tester n safe = (safe [1 .. n]) : (tester (n-1) safe) call it as (tester 10 safePat) for example, to get a list of some of the outputs from the safePat routine. Regards, Bruce.

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RE: Type class problem
by oleg＠pobox.com 27 Aug '03

27 Aug '03

Simon Peyton-Jones wrote: > > instance (Show (f (Mu f))) => Show (Mu f) where > > show (In x) = show x > > > > instance Show (N (Mu N)) where > > show Z = "Z" > > show (S k) = "S "++show k > But again, it's fragile isn't it? You are dicing with non-termination > if you have instance declarations where the stuff before the '=>' is more > complicated than the stuff after the '=>'. A less carefully written > instance for Show (N (Mu N)) would make the type checker loop. That's true that care is required. However, the same care is required when we want to write 'fix' or a set of mutually recursive type aliases. We have to introduce a real constructor somewhere to break the loop. I'm curious if you could post an example of > You are dicing with non-termination > if you have instance declarations where the stuff before the '=>' is more > complicated than the stuff after the '=>'. I'm wondering if the general method of avoiding non-termination can be made to work in these more complex cases. Incidentally, the constraint solver stack overflow problem can be turned to our advantage. The typechecker's exhausting the stack should be considered a failure to match the instance -- and so the typechecker should mark the current instance inappropriate and try another one, if any. I have just implemented this strategy in a logic programming system I'm working on, and it works beautifully. I can find counter-examples of Haskell instances violating functional dependencies no matter in which order the instances are given. Thank you!

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AW: Need some help please
by Markus.Schnell＠infineon.com 27 Aug '03

27 Aug '03

If you post some of your tries, we could give you specific tips. -- Markus Schnell -----Ursprüngliche Nachricht----- Von: Rajiv Patel [mailto:raj_patel@cwcom.net] Gesendet: Mittwoch, 27. August 2003 16:09 An: haskell(a)haskell.org Betreff: Need some help please Hi, I need to define a function called safetail; it's like tail except that this one maps the empty list to the empty list. It has to be defined using the following: 1. a conditional expression 2. guarded equation 3. pattern matching It's probably quite simple, but I am struggling on this, so help would be appreciated. Thanks Raj

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PVM-free GpH
by mgross＠dorsai.org 27 Aug '03

27 Aug '03

I am pleased to be able to announce that the Metis Project at Brooklyn College has been able to modify the gum RTS for ghc-5.02.3 so that it runs without using PVM. We have verified operation under the Mosix clustering system and are currently working on implementing a version that we will be able to use for parallel execution on our Solaris machines. It is to be expected that our code will run without modification on any Linux SMP. While our code is still alpha (among other things, it generates numerous annoying but harmless informational messages we hope will be soon be silenced) and we lack the resources to provide technical support, I will be pleased to provide copies of our CVS repository to those who ask (sorry, no anonymous CVS access at present). We would greatly appreciate assistance in improving the code, and particularly in locating and fixing the bug whose symptoms are either a failed assertion in the garbage collector, or a message about improper packet packing. Anyone who would like a copy of the CVS archive should send e-mail to magross(a)its.brooklyn.cuny.edu. I will reply as soon as possible to arrange for a transfer. Murray Gross Brookly College, City University of New York

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POSIX enviornment standard?
by John Meacham 26 Aug '03

26 Aug '03

I was wondering if perhaps there was a need for a standard (or at least documentation for all the major compilers) on how one can expect the haskell runtime to interact with the OS enviornment? in particular, questions which I can think of which I would like answered (for POSIX systems, I imagine Win32 would have a seperate set of concerns) are: SIGPIPE, does it kill the process or is it ignored and an IO exception is raised? I would prefer to be able to count on the second behavior. is setlocale(LC_ALL,"") called before main to initialize locale? is SIGALRM safe to use or is it reserved for the haskell implementation? is the full path of the program stored in $_ ? is argv[0] modified in place? (ghc appears to do so brokenly on linux at the moment) There are probably more, Perhaps some sort of document listing such concerns for a given enviornment stating that compilers should document their behavior (or specify a specific behavior when appropriate) and perhaps set some #defines based on them? John -- --------------------------------------------------------------------------- John Meacham - California Institute of Technology, Alum. - john(a)foo.net ---------------------------------------------------------------------------

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