April 2001 - Haskell-Cafe

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 23 Apr '01

23 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 10 Apr '01

10 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 10 Apr '01

10 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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What do you think about Mercury?
by Terrence Brannon 09 Apr '01

09 Apr '01

I have decided to escape the world of Perl into something more definitional. I gave up on CAML and Erlang because they contain imperative elements and I am tired of that form of coding. So, I have narrowed it down to Haskell and Mercury. The attractive things about Haskell are: - automatic generation of concurrent programs from normal source... instead of sending msgs to processes like in Erlang. - large user base - possible real job in industry (with Galois Connections). - several books based on it - binaries available (even for my mac os x box already!). So, with that said, http://www.cs.mu.oz.au/research/mercury/information/comparison_with_haskell… lists a number of advantages of Mercury over Haskell. And both Haskell and Mercury did well in the ICFP. Does anyone have anything to say about these criticisms? There were two particular things in the list that caught my attention: 1- Haskell is a pure functional language, but I don't see any support for backtracking or other logic features... but my guess is you have some way of handling this? How? 2- Mercury's mode system provides support for subtypes. For example, you can declare that a function whose argument is a discriminated union type accepts only a subset of the possible constructors for that type, and the mode system will enforce this.

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Re: Question about typing
by Yoann Padioleau 09 Apr '01

09 Apr '01

Matt Harden <matth(a)mindspring.com> writes: > > zip :: ZipFunctor f => f a -> f b -> f (a,b) > zip = zipWith (,) > zip3 :: ZipFunctor f => f a -> f b -> f c -> f (a,b,c) > zip3 = zipWith3 (,,) > > One can easily create ZipFunctor instances for trees and other data > structures. I can provide examples if you like. With multiple > parameter type classes (MPTCs, they are not in Haskell 98) as well as > functional dependencies (also not in h98), one can also create a > "Zippable" class to generalize the zip function over multiple tuple > types and eliminate zip3, zip4, etc. can you explain how you do that ?? (i dont know what is functional dependencies, but i think i understand multiple parameter type classes and i dont see how do make something that avoid to define a zip3 zip4 ....

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Question about typing
by Toby Watson 09 Apr '01

09 Apr '01

Intuitively the following scenarios seem to be related, can anyone point my in the direction of formal work on this, or give me the formal terms I need to search around? 1. Adding two integers together: Int -> Int -> Int 2. Adding two lists of Integers together: [Int] -> [Int] -> [Int] 3. Adding two lazy streams of integers together, possibly in seperate (parallel) processes for example. cheers, Toby

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