December 2003 - Haskell-Cafe

RE: no continuations
by Kevin S. Millikin 12 Jan '04

12 Jan '04

On Tuesday, December 30, 2003 5:04 PM, Kevin S. Millikin [SMTP:kmillikin@atcorp.com] wrote: > Oh, sure. I didn't mean to quibble with the idea that continuations > are computational effects. Just wanted to point out that (I think) > you can't macro express mutation with call/cc, unless you've already > got mutation anyway. [snip] > Yup. If you do that, you can use d as your setter and c as your > getter: > > > (define c (make-cell)) > > (define d c) > > ((d 'set) 9) > > (c 'get) > 9 > > ((d 'set) 17) > > (c 'get) > 17 It sure looks like the example contradicts the assertion, but I happen to know that there is a set! (or some other assignment) in the macro expansion of define. I'm just using call/cc to get at that, rather than getting at the one in the expansion of letrec. Moved to Haskell Cafe.

4 9

Type checking
by Lee Dixon 09 Jan '04

09 Jan '04

Hi, Can anyone explain to me how hugs manages to derive that f x y z = y (y z) x is of type f :: a -> ((a -> b) -> a -> b) -> (a -> b) -> b Many thanks and a happy new year to all! Lee _________________________________________________________________ Stay in touch with absent friends - get MSN Messenger http://www.msn.co.uk/messenger

3 2

Re: Perspectives on learning and using Haskell
by Derek Elkins 05 Jan '04

05 Jan '04

On Tue, 23 Dec 2003 17:26:20 +0000 Graham Klyne <gk(a)ninebynine.org> wrote: (moved to Haskell-Cafe as this reply might generate several more) > I've spent part of the past few months learning Haskell and developing > a moderately sized application. I came to this from a long background > (20 years or so) of "conventional" programming in a variety of > languages (from Fortran and Algol W to Java and Python). For me, > learning Haskell has been one of the steepest learning curves of any > new language that I have ever learned. Before this project, I was > aware of some aspects of functional programming, but had never > previously done any "in anger" (i.e. for real). Well, the obvious question is: after climbing partway up that curve, what do you think of the view? Was it worth learning? Is it worth continuing to use? Does the code seem 'better' than what you might produce in other languages you've used? > Throughout this period, I've been accumulating some notes about some > things that I found challenging along the way. The notes are not > organized in any way, and they're certainly not complete. I've > published them on my web site [1] in case the perspective might be > useful to any "old hands" here. > > [1] http://www.ninebynine.org/Software/Learning-Haskell-Notes.html When I saw this page earlier, I was thinking about suggesting to you to add it and any new thoughts to the wiki so that it's more readily accessible to other Haskell newbies and can be annotated with pointers to resources and comments; kind of like an annotated unidirectional version of "A Newbie's on-going tutorial" on the Squeak wiki (http://minnow.cc.gatech.edu/squeak/1928) > Also on the topic of perspectives: > > In recent conversation with a colleague, he mentioned to me that the > term "functional programming" has an image problem. He suggested that > the term conveys an impression of an approach that is staid, > non-progressive or lacking novelty, and is prone to elicit a response > of "been there, done that" from programmers who don't realize the full > significance of the term "functional". I've also noticed that when I > talk about "functional programming", some people tend to think I'm > talking about using techniques like functions in C or Pascal (which is > course is very desirable, but old hat and not worthy of great > excitement). Yes. People often use the term 'procedural' to describe the C style break down of a problem. Of course, some people use procedural to refer to functional and others functional to refer to procedural. Then, of course, there are also dynamic typers who equate static typing with C/Pascal. On the OO side, there are the people who equate OO with C++/Java much to the chagrin of Smalltalkers and CLOS users. At any rate, functional programming is a pretty well established term so there isn't much Haskell can do about it. Perhaps throwing in 'higher-order' will help, and certainly laziness/purity can't be said to be lacking novelty.

8 13

Stack usage with a state monad
by Joe Thornber 02 Jan '04

02 Jan '04

Hi, I was wondering if anyone could give me some help with this problem ? I'm trying to hold some state in a StateMonad whilst I iterate over a large tree, and finding that I'm running out of stack space very quickly. The simplified program below exhibits the same problem. This is the first time I've hit space problems in Haskell, I hope judicial use of 'seq' or '$!' would be enough to fix it, but I don't know where to start. Any ideas as to what I'm doing wrong would be much appreciated. Thanks, - Joe module Main (main) where -- Program to count the leaf nodes in a rose tree. Written to try and -- reproduce a stack space leak present in a larger program. -- How can I use a state monad to count the leaves without eating all -- the stack ? import Control.Monad.State data Tree = Tree [Tree] | Leaf buildTree :: Int -> Int -> Tree buildTree order = buildTree' where buildTree' 0 = Leaf buildTree' depth = Tree $ map (buildTree') $ take order $ repeat (depth - 1) countLeaves1 :: Tree -> Int countLeaves1 (Tree xs) = sum $ map (countLeaves1) xs countLeaves1 (Leaf) = 1 incCount :: State Int () incCount = do {c <- get; put (c + 1); return (); } countLeaves2 :: Tree -> Int countLeaves2 t = execState (aux t) 0 where aux :: Tree -> State Int () aux (Tree xs) = foldr1 (>>) $ map (aux) xs aux (Leaf) = incCount main :: IO ()B main = print $ countLeaves2 $ buildTree 15 6

5 10

Module creation
by J & M van Berchem 01 Jan '04

01 Jan '04

New to Haskell, I got a version, through Fink, for my Mac OS X. I like the interpreter very much, but I do not see how I may get out of "Prelude" in order to write a program. Could you advise me ? Thanks in advance Jacques van Berchem

2 1

Type checking/inference
by Lee Dixon 28 Dec '03

28 Dec '03

Hi, I've run into a small problem whilst doing some manual type checking, to see if I could match the results given by hugs prelude> :t foldr filter foldr filter :: [a] -> [a -> Bool] -> [a] -- This was fine and was the same as my answer, so I tested it with prelude> foldr filter [1,2,3,4] [even,odd] -- and the answer was indeed an empty list -- However I got stuck when attempting to derive the result of map (foldr filter) -- Hugs says that: prelude> :t map (foldr filter) map (foldr filter) :: [[a]] -> [[a -> Bool] -> [a]] Two main questions: 1/ How does hugs derive this answer? 2/ What input can I give so that it yields a correct result? I've tried giving it a list of lists but it fails... Any pointers would be greatly appreciated! Many Thanks, Lee _________________________________________________________________ Stay in touch with absent friends - get MSN Messenger http://www.msn.co.uk/messenger

4 3

Re: Perspectives on learning and using Haskell
by Graham Klyne 24 Dec '03

24 Dec '03

[switching to Haskell-cafe] At 19:37 23/12/03 +0100, Tomasz Zielonka wrote: >On Tue, Dec 23, 2003 at 05:26:20PM +0000, Graham Klyne wrote: > > > [1] http://www.ninebynine.org/Software/Learning-Haskell-Notes.html > >Thanks, that was a nice reading :) Thanks! (If by any chance there's anything here that might be useful for the Wiki, anyone may feel free to plunder it.) >I have some comments: > >8. Your explanation of Functor excludes many useful Functors which are > rather not collections. For example, every monad (like IO) can > be a Functor if you take fmap = Monad.liftM. > > For [] and Maybe this would give the same operation as in their > normal instances. That's an interesting perspective that I wasn't aware of... I need to think about that. Meanwhile, I've added your observation to my notes. [later] It now seems to me that (some?) Monads are kinds of Functors, generalized to handle the "no value" case, and also composition. This also had me thinking about sequence: is there a generalization to arbitrary monads that rearranges the monadic structure? >11 and 18. > If you define an instance of Monad for ((->) e) then > > return (putStrLn "Hello!") 'x' > > is a proper IO () value. Probably still not sensible ;) Ah, I think I see your point. It would apply where monads are "nested", right? > Special treatment of 'return' could be helpful, but I am afraid that > it could also make it look special, like a return keyword in C. I certainly wouldn't argue for special treatment _in the language_, but OTOH, I think it might be helpful if compiler diagnostics hinted at the possibility when a type error is detected in a form like return x y. #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

2 1

Re: looking for data structure advice
by Jon Cast 24 Dec '03

24 Dec '03

Conor McBride <c.t.mcbride(a)durham.ac.uk> wrote: <snip> > I've always been a little bothered by the MonadPlus class: zero and > plus are associated (no pun intended) in my mind with monoidal > structure. Is there more to MonadPlus than a clumsy workaround for > the lack of quantified constraints? Yes. For every functor m with a MonadPlus instance, m () has /two/ monoids, (mzero, mplus) and (return (), (>>)). MonadPlus is thus no more a workaround than the category Field; MonadPlus is simply a way to specify that a functor has two monoid-like structures, one more exact than the other, together with the expectation that the two monoids are related in a manner similar to the relationship of the two (more-or-less-) groups that make up a field. > If we could have quantified constraints, e.g. > > (Monad m, forall x. Monoid (m x)) > > wouldn't that be better than having Monad-specific monoids? No. Consider the monad Writer (IO ()). The Writer monad wants a Monoid instance, and the most natural such instance is often (return (), (>>)). OTOH, this monad is always wrong in the cases where, currently, you would use mplus. So you can't just replace MonadPlus with Monoid; you need to be more specific about your choice of monoid than can be accounted for by a single class. <snip> Jon Cast

2 1

Hugs Nov-2003 release: traceShow goes missing
by Graham Klyne 19 Dec '03

19 Dec '03

I've just upgraded my Hugs installation to the Nov-2003 release, and I notice that function traceShow is missing from library module Debug.Trace. I assume this is unintentional, as it's still mentioned in the Hugs manual. (In my installation, I've copied the 2-line function implementation from the previous Hugs release into this module, and added it to the module export list.) #g -- ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

1 0

Re: To show or not to show french accents
by MR K P SCHUPKE 19 Dec '03

19 Dec '03

Lets assume I get a unicode terminal, and I am reading and writing from unicode files, how does Haskell (or more specifically GHC cope with this)? (PS: I am not a member of haskell-cafe mailing list, so please CC me in to any replies in this thread)... Keean.

1 0