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Re: Implict parameters and monomorphism
by Dylan Thurston 05 May '01

05 May '01

On Fri, May 04, 2001 at 07:56:24PM +0000, Marcin 'Qrczak' Kowalczyk wrote: > I would like to make pattern and result type signatures one-way > matching, like in OCaml: a type variable just gives a name to the given > part of the type, without constraining it any way - especially without > "negative constraining", i.e. without yielding an error if it will > be known more than that it's a possibly constrained type variable... I'm not sure I understand here. One thing that occurred to me reading your e-mail was that maybe the implicit universal quantification over type variables is a bad idea, and maybe type variables should, by default, have pattern matching semantics. Whether or not this is a good idea abstractly, the way I imagine it, it would make almost all existing Haskell code invalid, so it can't be what you're proposing. Are you proposing that variables still be implicitly quantified in top-level bindings, but that elsewhere they have pattern-matching semantics? Best, Dylan Thurston

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Re: Haskell-Cafe digest, Vol 1 #122 - 3 msgs
by Alan Bawden 04 May '01

04 May '01

Subject: Re: Interesting: "Lisp as a competitive advantage" Date: Thu, 03 May 2001 10:16:37 -0400 From: Norman Ramsey <nr(a)eecs.harvard.edu> > http://www.paulgraham.com/paulgraham/avg.html > > I wonder how Haskell compares in this regard. I loved Graham's characterization of the hierarchy of power in programming languages: - Languages less powerful than the one you understand look impoverished - Languages more powerful than the one you understand look weird When I compare Lisp and Haskell, the big question in my mind is this: is lazy evaluation sufficient to make up for the lack of macros? I would love to hear from a real Lisp macro hacker who has also done lazy functional progrmaming. Norman --__--__-- _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe End of Haskell-Cafe Digest

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Re: Interesting: "Lisp as a competitive advantage"
by andrew＠andrewcooke.free-online.co.uk 03 May '01

03 May '01

Two things in that interested me. First, the comment that "runtime typing" is becoming more popular - did he mean strong rather than weak typing, or dynamic rather than static? Second, more interestingly, I was surprised at his emphasis on macros. Having read his (excellent) On Lisp maybe I shouldn't have been (since that is largely about macros), but anyway, I think it's interesting because it's one of the big differences between Lisp and the statically typed languages (STLs). I have used neither Lisp nor Haskell (or ML) long enough to make a sound judgementon this, so I'd like to hear other views. My first thought is that higher order functions are easier to manage in STLs and might provide some compensation. Also, I'm aware that (limited?) code manipulation is possible in some STLs (there's an ML library whose name I've forgotten). How do these compare? I realise I could get more response by a usenet post to c.l.f and c.l.l, but I'm hoping there might be more light here. Apologies if it's too off-topic. Thanks, Andrew On Tue, May 01, 2001 at 08:58:07PM -0400, Sengan wrote: > http://www.paulgraham.com/paulgraham/avg.html > > I wonder how Haskell compares in this regard. > Any comment from Haskell startups? (eg Galois Connections) > > Sengan > > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell > -- http://www.andrewcooke.free-online.co.uk/index.html

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tutorial and other random bits
by Shae Erisson 29 Apr '01

29 Apr '01

Hi, I've started learning Haskell, but I'm coming from a Python/Java/SQL/etc background. I've learned a bit of elisp and scheme in my travels, but Haskell is very very different even from those two. The gentle introduction seems more like a brickbat to me, the best things I've found so far are homework assignments from various CS classes around the world. I've looked for a good irc channel dedicated to haskell, and not found one. I've also searched for a tutorial that works along the same lines as the Python tutorial, and not found one. I have lots of complete beginner questions... Also, it seems the Haskell Wiki has lost many pages again. Would it be possible to use MoinMoin instead of pyWiki? I looked at the "applications written in Haskell" links, but many of the links are dead. I have found some sample code by searching google, but I'd really like to find a library of small pieces of demo code showing the various idioms and useful basics. Does such a thing exist? Thanks, ---- Shae Matijs Erisson - http://www.webwitches.com/~shae/

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Novice question
by Mark Carroll 24 Apr '01

24 Apr '01

Is this a good place for novice questions? I have just started to try to learn Haskell, and am using Hugs to start with. When I write the not-very-useful function, greaterthan 0 _ _ = False greaterthan _ (x:xs) (y:ys) = x > y and test it, Main> greaterthan 2 [3] [4] False Main> greaterthan 3 [4] [3] True Main> greaterthan 0 [4] [3] False Main> greaterthan 0 [] [] ERROR: Unresolved overloading *** Type : Ord a => Bool *** Expression : greaterthan 0 [] [] Main> ...I don't understand what the problem is. I'm guessing that the problem is with the use of > and it not knowing which 'version' of > to use, but it doesn't need to know because we're not matching with that line anyway. I hope that didn't sound quite as confused as I am. (-: Is there anything easy someone can say that will enlighten me as to why things are as they are? (I understand that Ord is a set of types and that you can probably only apply > to things that are in it, and that > is overloaded somehow.) Thanks. -- Mark

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Re: Dimensional analysis with fundeps
by Ashley Yakeley 19 Apr '01

19 Apr '01

At 2001-04-09 10:29, Dylan Thurston wrote: >I hadn't thought about doing rational arithmetic with GCDs. Might be >nice. Herewith my attempt. I can't test it though, it runs afoul of one of the many bugs in Hugs' type-extension handling, and I haven't been able to run GHC on my MacOS X box. -- Ashley Yakeley, Seattle WA

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Re: Dimensional analysis with fundeps (some physics comments)
by Ashley Yakeley 11 Apr '01

11 Apr '01

I'm cross-posting this to the Libraries list... At 2001-04-10 18:02, Fergus Henderson wrote: >Still, the need to insert explicit `toUnits' is >annoying, and it would be nice to have a system where every number was >already a dimensionless unit. That's easy: -- type Unit rep = Dimensioned Zero Zero Zero rep; instance (Real a) => Real (Unit a) where { -- put instances here }; -- Of course you'll have to appropriately declare superclasses of Real, such as Num, Ord, Show, Eq etc. -- Ashley Yakeley, Seattle WA

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Re: Dimensional analysis with fundeps (some physics comments)
by Ashley Yakeley 11 Apr '01

11 Apr '01

At 2001-04-10 18:16, I wrote: >That's easy: > >-- >type Unit rep = Dimensioned Zero Zero Zero rep; > >instance (Real a) => Real (Unit a) where > { > -- put instances here > }; >-- > >Of course you'll have to appropriately declare superclasses of Real, such >as Num, Ord, Show, Eq etc. I've put this together in the enclosed files (same as previously posted to the libraries list), which derive from anatoli's work. -- Ashley Yakeley, Seattle WA

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Re: Dimensional analysis with fundeps (some physics comments)
by Ashley Yakeley 11 Apr '01

11 Apr '01

At 2001-05-01 18:28, Terrence Brannon wrote: >"has Ashley >shown in her example a means of automating a toUnits imperative in >Haskell that must be done explicitly in Mercury?" ...his... -- Ashley Yakeley, Seattle WA

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Re: Dimensional analysis with fundeps (some physics comments)
by Jerzy Karczmarczuk 11 Apr '01

11 Apr '01

[[MOVED FROM haskell TO haskell-cafe]] Nothing to do with Haskell. Pure computer scientists, not interested in physics might not find anything in this posting. Tom Pledger wrote: //about dimensions, units...// > If you clearly make the type system deal with dimensions rather than > units, there's no problem with plugging in multiple unit systems. You > just have to pick a scale for the representation. > > newtype Dimensioned mass length time rep = Dimensioned rep > > type Mass rep = Dimensioned One Zero Zero rep > > kg, lb :: Num a => Mass a > kg = dm 1 > lb = dm 0.4535924 > > Angles are dimensionless. (Think of the Taylor series for trig > functions.) > > radian, degree :: Unit > radian = dm 1 > degree = dm (pi/180) The distinction between dimensions and units may seem for some of you utterly trivial. Mass is a mass, kg -> lb, etc. are just conversion factors. Angles are dimensionless. It was not so trivial for the authors of SI, and for some speculative physicists, who are not all dead yet. The problem is: what is a *fundamental*, *universal* constant? You see, those constants establish our unit systems! Many, really many among my friends don't hesitate to work in a frame where the speed of light is equal to 1. Yes, ONE, dimensionless. Then, passing from length to time is just a unit conversion, and all velocities are dimensionless. (Like, say, steradians in SI?...) Who needs candelas? Who needs Amperes What is the dimension of a mole? (In SI it is an independent unit) Who needs Kelvins? Everybody (within a particular mafia I happen to know) knows very well that the temperature is/should be measured in energy units, i.e., the Boltzmann constant k is equal to 1, dimensionless. Now, even if you won't (probably) find people who would claim that the status of k or c is the same as PI/180, the question "what is a unit" has no unambiguous and non-conventional answer. You may put the universal gravitational constant to 1. Or simply put to 1 the Plankeon mass, and get rid of all the mass "dimensional units". == And if you leave the world of classical physics, and start to analyze the structure of quantum theories you may get some nasty surprises. //Now I have to trivialize a bit// (First, most people who do some calculations in QM put the Planck constant equal to 1, so the energy is inverse time...) Some quantities which should - for geometric reasons - have fixed dimensions, such as the "force" of a field respecting Laplace equation: ~1/r^2 (Newton, Coulomb), get what is called the "anomalous dimension", instead of 2 one has to play with r^(2+x), with x fractional (some people, I suppose that Andrew Kennedy is one of them, don't like such things...). Now, if something which should be - classically - dimensionless, say p^0 acquires this anomaloud dimension p^x, then the only computationally sane way of dealing with it is to introduce a new "universal" (?) dimensional constant q, such that the expression above may be written as (p/q)^x in appropriate units of p and q. Then, no x may do any harm. This phenomenon, the "dimensional transmutation" suggests that it is not clear which is the set of universal dimensional quantities in quantum world. == I like all papers about dimensions, and it is very refreshing to see how people squeeze them into the type systems. (Or play with geometrical "navigation" in a dimensional world: http://mail.collegestudent.com/gda01/GDA01.HTM http://physics.nist.gov/cuu/Units/SIdiagram.html ) I believe, however, that one should not forget that all that is *conventional*. Jerzy Karczmarczuk Caen, France

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