March 2002 - Haskell-Cafe

Ex 9.9 in Paul Hudak's book
by Ludovic Kuty 04 Apr '02

04 Apr '02

The exercise is short. First, he defines a "fix" function: fix f = f (fix f) Which has the type (a -> a) -> a Then the function "remainder": remainder :: Integer -> Integer -> Integer remainder a b = if a < b then a else remainder (a - b) b The function fix, has far as i understand the matter, has no base case. So if someone wants to evaluate it, it will indefinitely apply the function f to itself, leading the hugs interpreter to its end. That's ok. But next he asks the reader to rewrite the function remainder using fix and i cannot find out how. I tried something like: remainder2 a b = fix (\x -> x - b) but there is no base case too. As soon as the lambda expression wants to evaluate its argument, the interpreter crashes. Can someone help me to solve the problem ? TIA Ludovic Kuty

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newbie:: getting random Ints
by Peter Rooney 04 Apr '02

04 Apr '02

hello all, total newbie to haskell, armed with "the craft of functional programming". i can't seem to generate random numbers. i have read this: http://www.haskell.org/onlinelibrary/random.html and searched the web and archives, and read thru Random.hs (mostly over my head), but have been unable to get any combination of getStdRandom randomR, etc. to work. even the example in the 98 report, import Random rollDice :: IO Int rollDice = getStdRandom (randomR (1,6)) gets me: Main> rollDice Main> after loading the file, which makes me think i'm missing something! TCFP has an example of how to do it yourself, but i can see that my needs are more than met by Random.hs. my various attempts all look a lot like this: Main> getStdGen (random 5) ERROR - Type error in application *** Expression : getStdGen (random 5) *** Term : getStdGen *** Type : IO StdGen *** Does not match : a -> b could someone post an example of how to generate random integers within a range? tia, peter

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guards
by Cagdas Ozgenc 29 Mar '02

29 Mar '02

Greetings. Is it requried that the type used in guards always Bool? It seems to me that the Haskell language is dependent to Bool type, and Bool type is depending to the Haskell language causing a circular dependency problem. Of course this is not a practical issue. Is there a theoretical explanation? Thanks for taking time.

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Re: Little problem
by Jorge Adriano 27 Mar '02

27 Mar '02

[moving to haskell-cafe] > Hello > I have little problem. I want to solve this equation > > f(n-1) = f(n)+n > f(6)=6 This is a simple recursive definition.. By the way, are you sure you didn't meant: f(n-1)=f(n)+(n-1) > => f(2)=? Not exactly sure what you're after here. You could simply do: f(2) =f(3)+3 =f(4)+4+3 =f(5)+5+4+3 =f(6)+6+5+4+3 =6+6+5+4+3 =24 anyway it's simple to see, or even prove by induction, that f(n) = (\sum_{i=n+1}^{7} i) -1 = (n+8)(7-n)/2 - 1 If I'm rigth and you're formula is suposed to be f(n-1) = f(n)+n it is even easier. J.A.

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Re: Is there a name for this structure?
by dominic.j.steinitz＠britishairways.com 27 Mar '02

27 Mar '02

The name escapes me but any category theory book should supply the answer. Dominic. Tom Pledger <Tom.Pledger(a)peace.com>@haskell.org on 27/03/2002 04:22:29 Sent by: haskell-cafe-admin(a)haskell.org To: haskell-cafe cc: bcc: Subject: Is there a name for this structure? Joe English writes: : | Suppose you have two morphisms f : A -> B and g : B -> A | such that neither (f . g) nor (g . f) is the identity, | but satisfying (f . g . f) = f. Is there a conventional name | for this? Is it equivalent to saying that (f . g) is the identity on the range of f? That's shorter, though still not a snappy single word term. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ------------------------------------------------------------------------------------------------- Save time by using an eTicket and our Self-Service Check-in Kiosks. For more information go to http://www.britishairways.com/eservice1

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Is there a name for this structure?
by Joe English 27 Mar '02

27 Mar '02

Not really a Haskell question, but someone here might know the answer... Suppose you have two morphisms f : A -> B and g : B -> A such that neither (f . g) nor (g . f) is the identity, but satisfying (f . g . f) = f. Is there a conventional name for this? Alternately, same question, but f and g are functors and A and B categories. In some cases (g . f . g) is also equal to g; is there a name for this as well? I find myself running into pairs of functions with this property over and over again, and am looking for a short way to describe the property... Thanks, --Joe English jenglish(a)flightlab.com

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Re: I need some help
by Jorge Adriano 26 Mar '02

26 Mar '02

> > However the user of the program give some argument then program is > > starting. And I want to use this arguments for example at the end of the > > program. What will I do. > > Good question. I've had a bad time with this, some in this list proably got > tired of me saying the same thing over and over again... those of you might > want to skip the rest of my mail :-) huh? what is this line doing here.. I thought I had cut/pasted it below, but instead I copy/pasted it (then changed it). Most of the is pretty readble, I just complain about stuff in the last part of the e-mail :-) J.A.

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Re: I need some help
by Jorge Adriano 26 Mar '02

26 Mar '02

[Obs: moving to haskell cafe] [beware, huge answer follows] > Hello > I am writing a whole program which is analyz a elegtromagnetic wave. This > is ok but in the program I use a lot of constant for example pi,epsilon > etc. I want to ask that > how I use a constant in Haskell ? Defining constants it's easy, examples. -- with no type signature epsilon = 3.1 -- with explicit type signature epsilon2 :: Double epsilon2 = 3.1 -- specifying the type but no type signature epsilon3 = 3.1 :: Double (by the way, wouldn't it make sense not to get a warning about missing type signature in this last case. I know there is no signature there but still...) > However the user of the program give some argument then program is > starting. And I want to use this arguments for example at the end of the > program. What will I do. Good question. I've had a bad time with this, some in this list proably got tired of me saying the same thing over and over again... those of you might want to skip the rest of my mail :-) If you want the user to be able to chose those values, then they are NOT constants, period. Yes their values won't chnage during the rest of the algorithm, but that doesn't makes it constants, so you can't declare them that way. What *can* you do. 1. One *awfull* option is to pass thos values as arguments to all your functions. Yes signatures will get monstruous, it will complicate the developing process etc, 2. Instead of passing all those variables, group them in a record - data type with named fields - and pass that record arround. (http://www.haskell.org/onlinereport/exps.html#sect3.15) Signatures will get better. It's kind of weird though, to always have to pass around that record. Things can get quite messy, IMO. -- foo f :: Double -> Double -> Double f x y = eta*x - epsilon^2*y -- foo2 f :: Options -> Double -> Double -> Double f opt x y = (eta opt)* (epsilon opt)^2*y And this is just a very simple example, it can get much worst. 3. Implicit parameters. (At least in Ghc, what about others?) Now this is the closest to an elegant solution. *Read both*: http://citeseer.nj.nec.com/246042.html http://www.cs.chalmers.se/~rjmh/Globals.ps f :: (?eta::Double, ?epsilon::Double)=> Double -> Double -> Double f x y = ?eta*x - ?epsilon^2*y Now you don't have to specify eta and epsilon in the type signature itself, but in the context - no passing around extra (not natural) parameters. It makes an huge diference IMO when developing your algorithms. Still signatures wil get even bigger than if you just passed them as parameters, you can't even type the context like: (?eta, ?epsilon::Double) So that is just awfull! Once again records will help. f :: (?opt::Options)=> Double -> Double -> Double f x y = (eta ?opt) *x - (epsilon ?opt)^2*y It still lokes quite bad. I *hate* the field projection functions, and I hate the '?'!! So you could simply rename you data opt fields and declare, eta :: (?opt:: Options) => Double eta = etaF ?opt epsilon :: (?opt:: Options) => Double epsilon = epsilonF ?opt Now f could be written like: f :: (?opt::Options)=> Double -> Double -> Double f x y = eta*x - epsilon^2*y Which differs from the first f function only in the context. It is *almost* good enough for me :-) "Why the almost?" you ask... Well, implicit parameters are not only non-standard features, but also experimental features (right?), so you they can change anytime. -------------------------------------------------------------------------------------- NOTE: some of you might wanna quit reading here, I keep complaining about this stuff, you are probably tired of it :-) -------------------------------------------------------------------------------------- In a couple of days you will also want to use more than one record, because grouping certain constants together doesn't feel right, and contexts will get bigger. Also, you will want to use global variables and instead of a record you will use an STRef to a record, and very small signatures will have in they're context something like: (?opt :: STRef s Opt). And then you will need to update some option fields, and you will want to use modifySTRef, and you will need to create an applyField function to every damn field of the record... So you will start asking 1. (when) will we have implicit contexts available? 2. wouldn't it be good to have, besides the data projection functions, some update and apply field functions, automaticly derived when declaring the data type? [yes am actually *asking* if *anyone agrees* with me? haven't had any feedback on this one, so I don't no wether it is a good or just plain dumb idea] IMVHO, it is of extreme importance and *urgent* to get implicit parameters right. This question about how to use this 'user difined constants' is a *simple* question and should have a *very simple* and standard answer. The same goes to global variables. We need them in certain situations and it is supposed to be, and it can be from my experience, *simple*. J.A.

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(no subject)
by nulkinulki 25 Mar '02

25 Mar '02

subscribe Haskell Michelle Yu <nulkinulki(a)sina.com> ______________________________________ =================================================================== ÐÂÀËÃâ·Ñµç×ÓÓÊÏä (http://mail.sina.com.cn) ÐÂÀË·ÖÀàÐÅÏ¢£º¶þÊÖÊÐ³¡×ßÒ»×ß£¬žÃ³öÊÖÊ±ŸÍ³öÊÖ£¡ (http://classad.sina.com.cn/2shou/)

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typeclasses
by Cagdas Ozgenc 24 Mar '02

24 Mar '02

Greetings. How can I make all types that belong to class A and instance of class B, if the implementations of functions in class B can be realized by only using the functions in class A? Thanks for taking time.

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