October 2003 - Haskell-Cafe

Why no findM ? simple Cat revisited
by Ahn Ki-yung 11 Nov '03

11 Nov '03

Simple Cat (revisitied) \begin{code} import IO findM f [] = return Nothing findM f (x:xs) = do { v <- x; if f v then return (Just v) else findM f xs } isLeft (Left _) = True isLeft _ = False main = findM (isLeft) (hCat stdin) where hCat h = try (hGetLine h) : hCat h \end{code} This is my answer for the question of my own, which is posted a couple There are mapM, filterM in the Haskell 98 Standard Library. But why no findM there ? As you can see from simple cat, it seems quite useful. I think fildM should be added to the module Monad. -- Ahn Ki-yung

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Modeling multiple inheritance
by Brandon Michael Moore 04 Nov '03

04 Nov '03

I'm trying to build a nicer interface over the one generated by jvm-bridge. I'm using fancy type classes to remove the need to mangle method names. I would like methods to be automatcially inherited, following an inheritance hierarcy defined with another set of type classes. My basic classes look like this class HasFooMethod cls args result | cls args -> result where foo :: cls -> args -> result If I have classes A and B with foo methods like foo_JA_Jint :: ClassA -> Jint -> Bool foo_JB_Jboolean :: ClassB -> Bool -> Jint then I can make instances instance HasFooMethod ClassA Jint Bool instance HasFooMethod ClassB Bool Jint Now I can just use foo everywhere. I would like to avoid declaring an instance for every class though. In java methods are inherited from a superclass, and I would like to inherit methods automatically as well. In the bindings jvm-bridge generates a method is invoked with a function mangled after the highest ancestor that defined that particular overloading, so the implementation of HasFooMethod at a particular overloading is the same for any descendant. So I defined a class to model the inheritance relationships class SubType super sub | sub -> super where upCast :: sub -> super Now I can define a default instance of HasFooMethod: instance (HasFooMethod super args result, SubClass super sub) => HasFooMethod sub args result where foo sub args = foo (upCast sub) args This will propagate foo methods down the inheritance hierarcy. If a new class C is derived from A, I just need to say instance SubClass ClassA ClassC and ClassC gets a foo method. (In the actually code I piggy-back on a transitive subclass relation jvm-bridge defines that already includes an upcast method, so upCast has a default that should always be acceptable). The problem comes when interfaces are added to the mix. Interfaces are treated just like classes by jvm-bridge, and even though no implementation is inherited from instances in Java, the method accessors generated by jvm-bridge should be inherited. One problem is that the subclass relationship needs the functional dependency so that the default instance of HasFooMethod will respects the functional dependencies of HasFooMethod, so I can't declare subclass instances for multiple inheritance. On the other hand, if I don't use the functional dependency on HasFooMethod I end up needing to annotate most of the return values in a program. I run into similar problems trying to use numeric literals as arguments, because they are also overloaded. Does anyone know of clever solutions that would model multiple inheritance while preserving the functional dependencies (unsafe compiler flags are fine too), or ways to reduce the pain of overloading resolution without the functional dependency? One alternative is generating seperate HasFooMethod instances for every class in the system. The problem is that this would require alterating the bit of jvm-bridge that uses JNI to find information on classes, which currently only reports newly defined methods. JNI is black magic to me. Thanks Brandon

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Data representation, maybe reflection, laziness
by Mark Carroll 04 Nov '03

04 Nov '03

People have been talking about transmitting Haskell values on the GHC users' list, and I wanted to ask some more general stuff, partly out of mild ignorance. Ralf Hinze and Simon Peyton-Jones wrote an interesting paper on generic programming and derivable type classes. It looked like maybe programmers would be able to write their own "deriving xml" stuff and whatever, which looked great because, if there's not already one out there, I'd love to derive some read/show analogue automatically for data in some encoding that's very efficient to write and parse (i.e. not XML (-:). I was also wondering how the ability to write "deriving" stuff related to what one might think of as being reflection - that is, for example, could a definition of deepSeq be derived that automatically knows how to recurse into and traverse data structures and make sure everything's evaluated? This leads me to the point that little of the code I write needs laziness, but many of my unpleasant performance surprises come from laziness. I do find it hard to figure out where to put strictness annotations to actually make things work - for instance, I think it's laziness things that are causing my uses of Control.Exception.evaluate to actually work more by trial and error. No doubt it'll all grow clearer in time. Maybe I need "laziness annotations" instead. (-: Still, I was wondering what current thinking and directions were with regard to any of the above! -- Mark

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newbie question regarding type declarations
by gangadhar npk 31 Oct '03

31 Oct '03

hi, I am a newbie to Haskell. I am reading the tutorial by Hal Daume III. Regarding the function types which are an extension of lambda calculus, I was wondering if at all it is possible to write a type that would return two values as the output - something like a square function which would take a pair and return a pair. I tried this, but there are errors square :: (Num a, Num b) => (a ,b) square (x , y) = (x*x , y*y) How can I specify that the square functions output would be a pair ? thank you gangadhar

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Monad @ Microsoft
by Josef Svenningsson 31 Oct '03

31 Oct '03

OK, this one is just too good not to post about it. Microsoft has developed a now command line interface. Guess what they call it? Read it here: http://slashdot.org/articles/03/10/31/1346201.shtml?tid=185&tid=190&tid=201 Well, it seems Simoj PJ is doing a good job introducting functional programming at MS although his methods are subtle in the extreme :-) /Josef

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RE: fixed point
by Harris, Andrew 31 Oct '03

31 Oct '03

Hi - I am trying to work out how the following function using "fix" is evaluated. I am hoping someone could look at my step-by-step breakdown of how I think evaluation works and see if I'm correct. My main question is how the order of operation (fixity?) is understood in going from step [3] to [4], if this is indeed how the evaluation would take place. Any help is appreciated, -andrew Here's the Haskell function: --- fix f = f (fix f) wierdFunc2 x y z = if y - z > z then x (y-z) z else y - z myRemainder = fix wierdFunc2 --- Here's my "evaluation": myRemainder 12 5 -- [1] = fix wierdFunc2 12 5 -- [2] by substitution = wierdFunc2 fix wierdFunc2 12 5 -- [3] apply "fix" = wierdFunc2 myRemainder 12 5 -- [4] by substitution (?) = myRemainder 7 5 -- [5] apply "wierdFunc2" = fix wierdFunc2 7 5 -- [6] by substitution = wierdFunc2 fix wierdFunc2 7 5 -- [7] apply "fix" = wierdFunc2 myRemainder 7 5 -- [8] by substitution (?) = 2 -- [9] apply "wierdFunc2"

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newbie question regarding type declarations
by gangadhar npk 31 Oct '03

31 Oct '03

hi, I am a newbie to Haskell. I am reading the tutorial by Hal Daume III. Regarding the function types which are an extension of lambda calculus, I was wondering if at all it is possible to write a type that would return two values as the output - something like a square function which would take a pair and return a pair. I tried this, but there are errors square :: (Num a, Num b) => (a ,b) square (x , y) = (x*x , y*y) How can I specify that the square functions output would be a pair ? thank you gangadhar

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Heap profiling in GHC broken?
by Markus.Schnell＠infineon.com 30 Oct '03

30 Oct '03

Hi. When I tried to profile my program in search for space leaks, it core dumped on me (actually on Win2000/Cygwin). I'm using ghc 6.0.1. Does anybody know what could be the problem? (To be more precise: any +RTS -h_ options make problems, +RTS -p works fine. I get a message meaning the equivalent of "statement in '0x0058db43' points to memory in '0xfffffffc'. Could not read.") Cheers, Markus -- Markus Schnell

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Haskell Asignement not working
by Rachel Reid 29 Oct '03

29 Oct '03

My code is --1)Define a function bLine to give a line of horizontal #s import Pictures sb, sw :: Picture bLine :: Int -> Picture bLine n ) |n==0 = ) |n>0 = `sideBySide` sb (bLine (n-1)) and when running Hugs, it says that there is a <unexpected symbol "#"> in line 9 which is bLine n ). There was a hash sign previously, but I have reloaded Hugs since then. can you help Thanks Rachel

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fixed point
by Harris, Andrew 29 Oct '03

29 Oct '03

Hi - I am trying to do Exercise 9.9 in HSOE; and I've come up with an answer that works but I'm not sure if it answers the question properly. The problem is: The Question: ------------- Suppose we define a function "fix" as: fix f = f (fix f) Suppose further we have a recursive function: remainder :: Integer -> Integer -> Integer remainder a b = if a < b then a else remainder (a - b) b Rewrite this function using "fix" so that it's not recursive. My Answer: ---------- wierdFunc x y z = if y - z > z then ((\a b c -> a b c) x (y-z) z) else ((\a b c -> a b c) (\d e -> d) (y-z) z) myRemainder = fix wierdFunc My Question: ------------ Does the ((\a b c -> a b c) x (y-z) z) mean that I've not removed the recursion? I was assuming that I was returning a function that is to be evaluated and not actually doing any recursion. That's why I thought I answered the question. However, I have a headache now and would like another opinion. thanks, -andrew

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