- Haskell-Cafe - Haskell.org

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

Re: To show or not to show french accents
by Jon Fairbairn 19 Dec '03

19 Dec '03

(note change of forum from glasgow-haskell-users) On 2003-12-19 at 12:28GMT MR K P SCHUPKE wrote: [ > I wrote: ] > >The problem is that if you are reading single bytes, 233 is > >not necessarily é. > > Erm, Internationalisation is not my thin as such... but I can't help > commenting that from a systems point of view this is an utterly bad > sitiation to be in... It certainly is. > I though Haskell used unicode? It's supposed to. Note that I said "/if/ you are reading a single byte", meaning that I was talking about the (not uncommon) curcumstance where the underlying operating system presents you with bytes each of which represents a single character (ie not something like utf-8). Haskell is supposed to deal with Unicode internally, but the I/O system's treatment of it is not currently in a happy state, at least partly because of what lies beneath. > I thought in unicode the id of a character was fixed > irrespective of language. It is. > Where is unicode support lacking? All over the place. There's this monstrous notion of "locale" that means that if you move your computer across the border from Germany into Poland and change the system settings appropriately, all character files change their meanings. It stems from files being untyped, and as all good Haskell programmers know, untyped is a Bad Thing. Jón -- Jón Fairbairn Jon.Fairbairn(a)cl.cam.ac.uk

1 0

Trace and loop examples?
by Vincenzo aka Nick Name 16 Dec '03

16 Dec '03

Hi all, I am trying to lear more about arrows in haskell, so I am reading the paper at: http://www.soi.city.ac.uk/~ross/papers/fop.html However, I can't understand how to produce a working and meaningful example of the trace function, or the loop arrow (I am using the "automata" example). The trace function is defined like: trace f b = let (c,d) = f (b,d) in c Can someone show an example function f to pass to trace and what the purpose is? Thanks Vincenzo

2 1

looking for data structure advice
by David Roundy 16 Dec '03

16 Dec '03

Hello everyone, I am trying to restructure some of my code to eliminate some O(n^2) operations, and have realized that this will require a change in how my data is represented, but I don't see any standard data types (in Data) that will really suit my needs. Currently I have a sequence of patches is just a list, so when two sequences or patches are reordered, I have an O(n^2) operation (where n is the length of each sequence). I would like to reduce this, since most patches are orthogonal, since they modify a single file, but this isn't possible as long as the data is represented in lists. What I'd like would be to have a data structure that holds an ordered list, but has a mapping from keys to its members, a sort of ordered FiniteMap. Each member would have to match several keys. I don't think this multiplicity of keys can be hidden in a single key, since I want to be able to search by any key in O(log n). Then I'd want to be able to extract its contents matching some keys, extractKeys :: [key] -> OrderedFiniteMap key elem -> [elem] with the above preserving the order of the elements. And to be able to modify only those patches matching a given key modifyOFM :: [key] -> a -> ((elem, a) -> ([key],elem,a)) -> OrderedFiniteMap key elem -> (OrderedFiniteMap key elem, a) where the second argument indicates how the first element to be modified changes, and possibly changes the keys when matching the second element... this is looking pretty complicated... maybe what I want is some sort of a monad. Of course I'd need "normal" access functions too, but those are more obvious. Is there an existing set of algorithms that might serve my needs (or some existing data structure, that would adequately suit my needs)? I really would rather not right my own data structure module for this purpose. Creating multiple references to a given data element to describe its list and all its keys is a bit intimidating... In case I haven't been clear enough, the algorithm I want to speed up looks like (when simplified a tad) commute :: ([Patch], [Patch]) -> Maybe ([Patch], [Patch]) commute (a:as, bs) = case commuteOneMany a bs of Nothing -> Nothing Just (bs', a') -> case commute (as, bs') of Nothing -> Nothing Just (bs'', as') -> Just (bs'', a':as') commuteOneMany a (b:bs) = case commuteOne a b of Nothing -> Nothing Just (b', a') -> case commuteOneMany a' bs of Nothing -> Nothing Just (bs', a'') -> Just (b':bs', a'') commuteOne :: Patch -> Patch -> (Patch, Patch) where there exists a function patchModifies :: Patch -> [Key] such that commuteOne a b | null (patchModifies a `intersect` patchModifies b) = (b, a) Any suggestions would be appreciated. At this point I've gotten myself pretty confused as to what exactly I want to do. :( -- David Roundy http://www.abridgegame.org

7 11

Importing strategies
by Henk-Jan van Tuyl 15 Dec '03

15 Dec '03

L.S., I am trying to compile the following program: > import Concurrent.Strategies > main = putStrLn "" Using the following command: ghc -v -package concurrent -package data -o "TryStrategies" "TryStrategies.lhs" -iC:/Programs/ghc-6.0.1/hslibs-imports > compile.log I receive the error message: TryStrategies.o(.text+0x22):fake: undefined reference to `__stginit_Concurrentzi Strategies_' What am I doing wrong here? -- Best regards, Henk-Jan van Tuyl

2 1

homework help: upn parser
by Max Hoffmann 15 Dec '03

15 Dec '03

Dear Haskell cafe members, I am somebody one could well call a bloody beginner but the elegance of functional programming and am therefore truly interested to learn more about Haskell. Now I have an assignment to write a parsing function to convert a string of a mathematical expression into a tree in UPN style to evaluate it afterwards. So my evaluating function looks like this: data Op = Plus|Minus|Times|Div|Fac deriving(Show,Eq) data OpTree = Term OpTree Op OpTree |Number Float |E deriving(Show,Eq) eval(Number z) = z eval(Term li op re) |op == Plus= (eval li) + (eval re) |op == Minus= (eval li) - (eval re) |op == Times= (eval li) * (eval re) |op == Div= (eval li) / (eval re) And I tried to write a parsing function but I got stuck in the very beginning. So far I wrote: --parsing parsing :: String->[OpTree] parsing (a:b:c:xs) t |isDigit b = ((parsing xs)) ((Term (read a) c (read b)):t) And the Hugs compiler gives me so many error messages, that I have no clue how to get rid of this mess. Could anybody give me a hint where my error is and whether I am right in the general direction. Thanks a lot for any kind of help, Max Hoffmann

2 1

Re: homework help: upn parser
by Leon Smith 15 Dec '03

15 Dec '03

Hi Max! This is a type definiton, which says that parsing takes one argument, a String, and gives you back a list of OpTree: > parsing :: String->[OpTree] > This is a function definition. The part before the = is called the left hand side, while the part after the = is called the right hand side. > parsing (a:b:c:xs) t > |isDigit b = ((parsing xs)) ((Term (read a) c (read b)):t) > > The (a:b:c:xs) is an example of a pattern. You are using the list constructor (:) to break down the list into individual items. According to your type definition, this argument is of type String, which is the same thing as [Char]. Thus, a, b, and c are bound to single characters, while xs is the bound to the rest of the string, assuming your string has at least three characters. If your string has less than three characters, then this pattern fails and the next clause in your function definiton will be tried. Then, according to your function definition, you then bind the second argument of the function to the variable "t". However, your type definition says you only have one argument. Since they disagree, this is an error. The right hand side of your function definition contains lots of errors, as you suggest. You probably want to rethink your type definition. I'd suggest the following: parsing :: String -> [OpTree] -> OpTree This says that parsing is a function that takes two arguments, a list of characters and a list of OpTrees, and returns a single OpTree. You are going to treat the second list argument like a stack. When you run across a number in your string, you are going to take the number off the string and push it onto the stack. When you run across an operation in your string, you are going to take two elements off the stack, combine the elements from the stack using the operation, and push the result back on the stack. Thus, your solution would contain this kind of structure: parsing ( string_with_number_in_front ) ( stack ) = parsing ( string_without_number ) ( Number (the_number) : stack ) parsing ( string_with_operation_in_front ) (last_element : next_element : rest_stack) = parsing ( string_without_operation ) ( Term last_element (the_operation) next_element : rest_stack ) Now, you must consider the boundary cases of the function "parsing". What I mean is, what kinds of clauses do you need for when you are done with recursion, and what should your first call to this function look like? Of course, you need to try out your code and carefully think about what it's doing. When asking for homework help on haskell-cafe, Haskellers have been known to introduce small intentional errors into their advice. Also, it's very impressive that Paul Natorp high school is using Haskell in it's coursework! In the United States, I've never heard of a high school that takes functional programming seriously. Sadly, many American universities don't take functional programming seriously either. Americans are really behind the game in this regard. Hope this helps. best, leon

1 0

Preventing/handling space leaks
by Henk-Jan.van.Tuyl 12 Dec '03

12 Dec '03

L.S., Does anyone know about documentation (preferably on the Web) on how to prevent/find/remove space leaks? Are there any differences between Hugs and GHC or any other Haskell platform, regarding space leaks? I read some discussions about why not everybody uses Haskell; it looks to me, that the problem of space leaks is a very good reason to not use Haskell for commercial applications. Java, for example, does not have this problem. -- Best regards, Henk-Jan van Tuyl

10 17

Re: import modules from a different directory
by Georg Martius 11 Dec '03

11 Dec '03

<moved to haskell-cafe> Hi, You can give GHC with -iPath a directory where to look for modules. With -v you see where GHC actually looks for libraries. Cheers, Georg On Thu, 11 Dec 2003 15:59:04 +0800 (GMT-8), Wang Meng <wangmeng(a)comp.nus.edu.sg> wrote: > Hi All, > > Anybody has the experience of importing a module from a different > directory?In my code, i need to import a module that is not in the > current > directory. How can I do that in GHC? > > > -W-M- > @ @ > | > \_/ > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell >

1 0

type of (/) ?
by Yaroslav Korchevsky 10 Dec '03

10 Dec '03

I have been trying Hugs98 This is the average of floats list calculation program: ================================ accum :: [Float] -> Float accum list = foldr (+) 0 list my_avg list = (accum list) / (length list) ================================ While loading it I got the compiler message: ERROR "U:\slav\FP\Avg.hs":11 - Type error in application *** Expression : accum list / length list *** Term : accum list *** Type : Float *** Does not match : Int Why "accum list" should match to "Int"? When I try to replace (length list) with number - it works. ================================ my_avg list = (accum list) / 5 --works fine ================================ xx = 5 my_avg list = (accum list) / xx --doesn't work -- same message as above ================================ Possibly, I don't know something important, or Hugs is not quite mature? Thank you. Slav.

4 3

Re: Multiple functions applied to a single value
by Graham Klyne 09 Dec '03

09 Dec '03

A little while ago, I sent a long rambling message [1] containing a key question in response to an earlier helpful response. I guess my ramblings were quite reasonably passed over as too much waffle, and the question was lost. Here, I isolate the question. From the earlier response, I understand that: ((->) r) is an instance of Monad, but I haven't found a definition of the Monad instance functions. I think these do the job, and they satisfy the monad laws [2]: [[ instance Monad ((->) r) where return a = const a g1 >>= f = \e -> f (g1 e) e ]] Can anyone confirm, or point me at an actual definition? #g -- [1] http://haskell.org/pipermail/haskell-cafe/2003-November/005566.html [2] Checking the Monad laws for the above definitions (cf. Haskell report p90): (A) return a >>= k = [g1/const a][f/k] \e -> f (g1 e) e = \e -> k (const a e) e = \e -> k a e = k a -- as required. (B) m >>= return = [g1/m][f/const] \e -> f (g1 e) e = \e -> const (m e) e = \e -> (m e) = m -- as required. (C) m >>= (\x -> k x >>= h) = [g1/m][f/(\x -> k x >>= h)] \e -> f (g1 e) e = \e -> (\x -> k x >>= h) (m e) e = \e -> (k (m e) >>= h) e = \e -> ([g1/k (m e)][f/h] \e1 -> f (g1 e1) e1) e = \e -> (\e1 -> h (k (m e) e1) e1) e = \e -> h (k (m e) e) e (m >>= k) >>= h = ([g1/m][f/k] \e -> f (g1 e) e) >>= h = (\e -> k (m e) e) >>= h = [g1/(\e -> k (m e) e)][f/h] \e1 -> f (g1 e1) e1 = \e1 -> h ((\e -> k (m e) e) e1) e1 = \e1 -> h (k (m e1) e1) e1 = \e -> h (k (m e) e) e So both expressions are equivalent, as required. ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

3 2

RE: Common subexpressions are not optimized to single one?
by Simon Marlow 09 Dec '03

09 Dec '03

> I'm wondering about the optimization of ghc. I thought that since > functions are pure in haskell, compiler can/do factorize common > subexpressions when it optimizes a code. But the result of > the following > experiment looks negative; "g 10 100" is caluculated twice. > > Am I missing something? If not, > What prevents ghc from performing that optimization? > Should I always factorize common subexpressions > manually(using let or where)? GHC *does* do common sub-expression elimination. However it only considers simple applicative subexpressions as candidates for commoning up, and in your example it turns out that by the time the CSE pass runs, various inlinings and transformations have happened which means that the candidate expressions are quite large. Perhaps we could run the CSE pass at an earlier stage, or perhaps it should run twice - that's one of those tradeoffs between compilation time and runtime. Perhaps the extra pass should be done for -O2 only. We'll look into it. Thanks for the example, anyhow. Cheers, Simon

1 0

Infinite types
by Jeffrey A. Scofield 09 Dec '03

09 Dec '03

Greetings. Say I have the following function, adapted from Pierce, Types and Programming languages: f n () = (n, f (n + 1)) Now, this function fails to typecheck in ghc 6.0.1 because the subexpression f (n + 1) has the infinite type t = () -> (t1, t) And so I assume (perhaps wrongly?) that f is ill-typed in Haskell 98. I have two questions: 1. How can I tell from the Haskell 98 Revised Report that this function isn't allowed? The discussions of typing in the Report generally defer to the ``standard Hindley-Milner analysis.'' Is this really enough to tell that the function isn't well-typed in Haskell? I concede that I haven't read the cited Hindley and Damas/Milner papers, but it seems like there might be more to say about what is and isn't allowed. 2. Is there by chance a compiler option for ghc that causes the restriction to be dropped? I don't need this kind of function for anything practical (and I realize I can get an equivalent effect with an algebraic datatype). I'm just trying to figure out what typing restrictions (if any) are implicitly assumed in the Report. (Generally, this is the kind of stuff I don't know, as a relative newcomer to fp.) Regards, Jeff Scofield (PhD) Seattle

6 9

Re: Infinite types
by jeff＠dhitechnologies.com 07 Dec '03

07 Dec '03

Alastair Reid wrote: > > 1. How can I tell from the Haskell 98 Revised Report that [a function > > with an infinite type] > > isn't allowed? The discussions of typing in the Report > > generally defer to the ``standard Hindley-Milner analysis.'' > 1) The only way to write the recursive type you want > in Haskell is using type synonyms. > 2) The section describing type synonyms (4.2.2) explicitly rules it out. This really makes sense, but I had been interpreting 4.2.2 as a limitation on type synonym declarations rather than on types themselves. It says: Although recursive and mutually recursive datatypes are allowed, this is not so for type synonyms, unless an algebraic datatype intervenes. To my (untutored) eye, this looks like a limitation on what can appear in a type synonym declaration, not a limit on what types can appear in a program. Is it a reliable mental model to assume that if there's no way to declare a type in Haskell, the type itself isn't allowed in a program? (Note that this can't be true in every case; according to 4.5.4, the monomorphism restriction introduces types that can't be declared in Haskell.) Thanks very much for the response. Regards, Jeff Scofield Seattle

1 0

Common subexpressions are not optimized to single one?
by Koji Nakahara 03 Dec '03

03 Dec '03

Hi, I'm wondering about the optimization of ghc. I thought that since functions are pure in haskell, compiler can/do factorize common subexpressions when it optimizes a code. But the result of the following experiment looks negative; "g 10 100" is caluculated twice. Am I missing something? If not, What prevents ghc from performing that optimization? Should I always factorize common subexpressions manually(using let or where)? --- $ cat op.hs module Main where import System.IO.Unsafe foreign import ccall "count" io_count:: IO Int f :: Int -> IO Int f x = do {y <- io_count; return $ x + y} g :: Int -> Int -> Int g x y = unsafePerformIO $ do {z <- f x; return $ z + y} main = do print (g 10 100, g 10 100) $ $ cat ffi_test_c.c static int counter = 0; int count() {return counter++;} $ $ gcc -c ffi_test_c.c;ghc -O2 -ffi op.hs;ghc -o op_test op.hs ffi_test_c.o $ ./op_test (110,111) --- I want to use some C functions from haskell each of which is not pure but the result of their sequential combination is pure. I'm planning to write some functions like g above(but more complex and actually pure) and considering the optimization of the code using them. Thanks in advance. Koji Nakahara

3 3

Re: show function
by Hamilton Richards 03 Dec '03

03 Dec '03

At 10:05 PM -0500 12/2/03, rui yang wrote: >I want to print a function which itself have some functions as it's parameters >and will return some functions as the results, and I want to print out the >result, does anyone knows how to define the instance declaration of show class >to this function type? [moving to Haskell Cafe] The mechanics of such an instance definition are simple enough. Letting F be your function type, this would do it: > instance Show F where > showsPrec f = ... some String ... The hard question is what the result should be. Unlike the types already in Show (such as Bool, Int, Char, etc.), there's no canonical representation for functions (other than a complete list of argument-result pairs, which would not be very useful). Regards, --Ham -- ------------------------------------------------------------------ Hamilton Richards, PhD Department of Computer Sciences Senior Lecturer The University of Texas at Austin 512-471-9525 1 University Station C0500 Taylor Hall 5.138 Austin, Texas 78712-0233 ham(a)cs.utexas.edu hrichrds(a)swbell.net ------------------------------------------------------------------

1 0

Mistake in Haskell wiki MonadError?
by Graham Klyne 01 Dec '03

01 Dec '03

At: http://www.haskell.org/hawiki/MonadError I see: [[ instance Monad (Either e) where return = Right (Left e) >>= _ = e (Right a) >>= f = f a ]] Shouldn't that be: [[ (Left e) >>= _ = Left e ]] ? #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

2 1

Re: Why are strings linked lists?
by Mark Carroll 01 Dec '03

01 Dec '03

(shifting to Haskell-Cafe) On Fri, 28 Nov 2003, Donald Bruce Stewart wrote: > ajb: (snip) > > As a matter of pure speculation, how big an impact would it have if, in > > the next "version" of Haskell, Strings were represented as opaque types > > with appropriate functions to convert to and from [Char]? Would there be > > rioting in the streets? I'd be sad to lose some convenient list-based string type because I make a lot of use of the fact that strings are lists in processing them. > You could look at GHC's FastString representation (used internally). > It is in $fptools/ghc/compiler/utils/FastString.lhs It does make sense to have a rather faster form of string conveniently available in /some/ form. -- Mark

4 3

Multiple functions applied to a single value
by Graham Klyne 28 Nov '03

28 Nov '03

There's a possible programming idiom that I repeatedly find myself thinking about as I write my code. It feels like a kind of dual of fold, except that it's not necessarily confined to lists. I'm wondering if anything like this is discussed in the literature, or implemented in the libraries... Scene-setting: broadly speaking, fold takes a function and applies it to several values, and combines the results in some way. What I'm thinking of is a combinator or suchlike that takes two or more functions, applies them (separately) to a single value, then combines the results of those combinations. One function that I have defined and use in my code is called 'flist': flist :: [a->b] -> a -> [b] flist fs a = map ($ a) fs (which is similar to Monad.ap, except that the 'a' parameter is not a list/monad). I could imagine combining this with a fold in some cases. A more generalized form of this that works with arbitrary monads was suggested by Derek Elkin: fmonad :: Monad m => m (a->b) -> a -> m b fmonad fm a = do { f <- fm ; return $ f a } But not all cases I encounter involve lists or monads. A different case might look like this: > eval :: (b->c->d) -> (a->b) -> (a->c) -> (a->d) > eval f g1 g2 a = f (g1 a) (g2 a) So, for example, a function to test of the two elements of a pair are the same might be: > pairSame = eval (==) fst snd giving: > pairSame (1,2) -- false > pairSame (3,3) -- true Or a function to subtract the second and subsequent elements of a list from the first: > firstDiffRest = eval (-) head (sum . tail) > firstDiffRest [10,4,3,2,1] -- 0 That's about as far as I've taken this, but it feels as if it might be part of a more general pattern. Hence my question: is there anything like this in the libraries? #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

2 2

Re: ANNOUNCE: Hugs98 November 2003 release
by Ross Paterson 26 Nov '03

26 Nov '03

[moving to haskell-cafe] On Tue, Nov 25, 2003 at 08:31:12PM +0100, Vincenzo aka Nick Name wrote: > Alle 17:51, martedì 25 novembre 2003, Ross Paterson ha scritto: > > - Integrated .NET support (on Windows). > > What does this exactly mean? See the dotnet/doc subdirectory of the bundle, or http://galois.com/~sof/hugs98.net/ > And, besides, is there any hope of supporting mono? I would guess it's possible, if someone volunteers to do the work. Can't see the point myself.

1 0

RE: Hypothetical reasoning in type classes
by Simon Peyton-Jones 24 Nov '03

24 Nov '03

Yes, absolutely. See http://research.microsoft.com/~simonpj/Papers/derive.htm Section 7, and Trifanov's paper at the Haskell Workshop 2003 Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Ken | Shan | Sent: 13 November 2003 05:40 | To: haskell-cafe(a)haskell.org | Subject: Hypothetical reasoning in type classes | | Hello, | | Has anyone thought about adding hereditary Harrop formulas, in other | words hypothetical reasoning and universal quantification, to the | instance contexts in the Hsakell type class system? Just today (and not | only today) I wanted to write instance definitions like | | instance (forall a. C a => D a) => E [a] where ... | | This is analogous to wanting to write a rank-2 dictionary constructor | | (forall a. C a -> D a) -> E [a] | | at the term level, but with type classes, this dictionary constructor | should be applied automatically, in a type-directed fashion, at compile | time. The theory behind such instance contexts doesn't seem so | intractable; indeed it looks decidable to my cursory examination. The | opreational intuition is that we would like the type checker to generate | an eigenvariable "a" and perform hypothetical reasoning. | | I would also like to quantify universally over type classes; in other | words, if "?" is the kind of a type class constraint (aka a dictionary | type; perhaps "o" would be a better choice of notation), then I would | like to define not just types of kind "*->*->?" (aka type classes) or | kind "(*->*)->?" (aka constructor classes), but also types of kind | "(*->?)->(*->?)". But that's for another day... | | Ken | | -- | Edit this signature at http://www.digitas.harvard.edu/cgi-bin/ken/sig | "Hi, my name is Kent, and I let people change my .sig on the internet." | "Hi, Ken!" | "Put midgets back in midget porn!" -- Not authorized by Ken Shan. Supported by | the association of midget porn workers, Local 9823.

2 1

Haskell Server Pages
by "Grigory Makeev 23 Nov '03

23 Nov '03

Hello, I'm interested in writing a personal home-page in a functional way, and i'm in need of some clues. Is there any resource, devoted exactly to the functional approaches to the web programming and web design? I've seen a WASH project, but i'm not really saticefied with the way pages look there. What i trully need is something like Haskell Server Pages system, from Erik Meijer's "Haskell Server Pages. Functional programming and the Battle for the Middle Tier" paper, but i haven't found any implementation of that. I'm not very good at either web programming, or Haskell, i'm just a fan of it, and there's no way i can implement that myself. Is anyone writing currently something like table = <table border="1"> <% mkRows cells%></table> Can anyone provide me with some links? Best regards, Grigory

2 1

collection library in Haskell
by Alexandru D. Salcianu 21 Nov '03

21 Nov '03

Hello! Can somebody recommend me a good collection library in Haskell? I'm interested in more than one Set implementation, one FiniteMap implementation etc. (we already have this in the standard Haskell'98 library). What I'm searching for is a library where I have the Set, FiniteMap etc. specifications (i.e., typeclasses), and several implementations (i.e., instances) for them. E.g., we can have a list-based implementation of Set (for small sets), but also a tree-based implementation (for larger sets). So, I would like a library with something like this -- sa is a type corresponding to sets of a's -- requires multi-parameter typeclasses class (Eq a) => Set a sa where addSetElem :: sa -> a -> sa -- add an element to a set searchElem :: sa -> a -> Bool -- search an element in a set ... -- list based Set implementation instance (Eq a) => Set a [a] where ... -- ordered-tree Set implementation instance (Ord a) => Set a (OrdTree a) where ... and so on. Furthermore, I'm interested in a library that can be compiled / interpreted by a mainstream tool, e.g., ghc, hugs etc. I've already started writting such a library (mainly for didactic purposes), but I would like to know if an enstablished collection library already exists. Thanks, Alex

4 4

Resolving Ambiguous Types
by Jared Warren 20 Nov '03

20 Nov '03

Thanks to Tom Pledger, Brian Boutel, and Keith Wansbrough for helping me with my last type problem, but now I have another one: :) ----- I understand why it is impossible to infer the intermediate type in `show . read`, but I was hoping someone could help me grasp why this (and only this) is problematic: > class Fooable a where foo :: a > instance Fooable Char where foo = '0' > class Barable a where bar :: a -> String > instance Barable Char where bar = const "Zero" `bar foo` generates an ambiguous type error; but anyone can deterministically see that there is only one intermediate type that satisfies the classes. Why can't the compiler figure it out? Are there any compiler directives, language extensions, or Template Haskell tricks I can use to make the compiler smarten up? (The requirements of my project rule out explicit type signatures.) If not, does anyone know if Chameleon's type system can avoid this problem? Thanks for your help, ~ Jared Warren <warren(a)cs.queensu.ca> Computing Science, Queen's University

2 1

Re: Partially ordered collections (revisited)
by Tom Pledger 20 Nov '03

20 Nov '03

Graham Klyne writes: : | Ah, yes. I've grown a little wary of foldl, but I see it's | appropriate in this case. As it's part of the standard prelude, I | personally have no qualms about depending upon it ... or should I? (Hope you don't mind my returning this to haskell-cafe.) Most of foldl's bad reputation comes from stack overflows, because it builds deeply nested suspensions such as foldl f z [1..99] --> f (... (f (f z 1) 2) ...) 99 *regardless* of the strictness of f. Contrast that with foldr foldr f z [1..99] --> f 1 (foldr f z [2..99]) which enters f straight away. Your particular f, i.e. add, is strict in both its parameters, so I don't think the choice between foldl and foldr will affect whether you get stack overflows. However, if you do get stack overflows, try a strict folding function, such as a copy of Hugs's Prelude.foldl' (not exported). - Tom

1 0

Partially ordered collections (revisited, again)
by Graham Klyne 20 Nov '03

20 Nov '03

Further to my previous message: [[ I've taken the suggestions made by Tom and Robert and combined them into a new module with some supporting functions and test cases. A copy is attached, in case it's of use to anyone. ]] I've tightened the maxima function code in response to a comment from Tom, and added some more partial-ordering comparison primitives. Updated code is attached: -- $Id: PartOrderedCollection.hs,v 1.4 2003/11/20 18:35:59 graham Exp $ (I just had a session trying to use the sourceforge CVS, but I can't find out how to get my combination of WinCVS and SSH tunnelling working.) #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

1 0

AW: GHC and Panther (Mac OS X 10.3)
by Markus.Schnell＠infineon.com 20 Nov '03

20 Nov '03

try using (sudo) gcc-select 2 Cheers, Markus > -----Ursprüngliche Nachricht----- > Von: haskell-cafe-bounces(a)haskell.org > [mailto:haskell-cafe-bounces@haskell.org] Im Auftrag von Thomas Davie > Gesendet: Donnerstag, 20. November 2003 02:07 > An: haskell-cafe(a)haskell.org > Betreff: GHC and Panther (Mac OS X 10.3) > > > Hi, > I've just upgraded to OS X 10.3 and have been having some > problems > with ghc. I'm using ghc 5.04.3 because it is compatible with > hat - I'm > doing a uni project on hat at the moment, so it's critical that it > works. When I attempt to build hmake using ghc I get a whole > load of > errors like these: > > ghc -DUSE_READLINE=1 -L/sw/lib/ -lreadline -I/sw/include -package > lang -c -o > /Users/tatd100/Documents/Work/Project/Tools/hmake-3.08/targets > /powerpc- > Darwin7/obj/hmake/QSort.o QSort.hs > slow-fast dropthrough not mangled > _sAi_entry: > addi r0,r22,12 > cmplw cr7,r0,r23 > ble- cr7,L11 > lis r2,ha16(L_MainCapability$non_lazy_ptr) > lwz r2,lo16(L_MainCapability$non_lazy_ptr)(r2) > lwz r29,12(r2) > b L12 > L11: > lwz r15,0(r22) > lwz r16,4(r22) > lwz r17,8(r22) > addi r22,r22,12 > lis r2,ha16(L_sAi_fast3$non_lazy_ptr) > lwz r29,lo16(L_sAi_fast3$non_lazy_ptr)(r2) > L12: > mtctr r29 > bctr > .align 2 > .section __TEXT,__text,regular,pure_instructions > > Which eventually drop down to one error that kills the whole process: > > ghc -DUSE_READLINE=1 -L/sw/lib/ -lreadline -I/sw/include -package > lang -c -cpp -o > /Users/tatd100/Documents/Work/Project/Tools/hmake-3.08/targets > /powerpc- > Darwin7/obj/hmake/Compat.o Compat.hs > Compat.hs:1: parse error on input `#' > make[1]: *** > [/Users/tatd100/Documents/Work/Project/Tools/hmake-3.08/targets/ > powerpc-Darwin7/obj/hmake/Compat.o] Error 1 > make: *** [targets/powerpc-Darwin7/hmake-ghc] Error 2 > > I'm guessing that there's some binary compatibility issues here... > Could it be panther using gcc3.3? > > Thanks > > Bob > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

3 2

RE: Aggressive inlining
by Simon Peyton-Jones 20 Nov '03

20 Nov '03

Sounds as if you might need Template Haskell http://research.microsoft.com/~simonpj/papers/meta-haskell/ Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Blue | Steel | Sent: 19 November 2003 22:43 | To: haskell-cafe(a)haskell.org | Subject: Aggressive inlining | | I'm working on a parlor trick. | | I would like to solve a 3-SAT problem at compile time, | so that the executable is super fast. | | Consider | --------------------------------------------------- | formula (x1,x2,x3) = (x1 || x2) && x3 | | tf = [True, False] | | assignments = [(x1,x2,x3)|x1<-tf,x2<-tf,x3<-tf] | | solutions = filter formula assignments | | main = do print solutions | --------------------------------------------------- | | Theoretically, a compiler could optimize this to | --------------------------------------------------- | main = do print [(True,True,True),...] | --------------------------------------------------- | | Is this actually possible in GHC? I've played with | inline pragmas and looked at -ddump-..., but I haven't | made much progress. | | Of course, the goal is to replace "formula" with a | formula that takes a 30-tuple instead of a triple and | really let GHC crank. | | Thanks for any help. | | | __________________________________ | Do you Yahoo!? | Protect your identity with Yahoo! Mail AddressGuard | http://antispam.yahoo.com/whatsnewfree | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

1 0

GHC and Panther (Mac OS X 10.3)
by Thomas Davie 20 Nov '03

20 Nov '03

Hi, I've just upgraded to OS X 10.3 and have been having some problems with ghc. I'm using ghc 5.04.3 because it is compatible with hat - I'm doing a uni project on hat at the moment, so it's critical that it works. When I attempt to build hmake using ghc I get a whole load of errors like these: ghc -DUSE_READLINE=1 -L/sw/lib/ -lreadline -I/sw/include -package lang -c -o /Users/tatd100/Documents/Work/Project/Tools/hmake-3.08/targets/powerpc- Darwin7/obj/hmake/QSort.o QSort.hs slow-fast dropthrough not mangled _sAi_entry: addi r0,r22,12 cmplw cr7,r0,r23 ble- cr7,L11 lis r2,ha16(L_MainCapability$non_lazy_ptr) lwz r2,lo16(L_MainCapability$non_lazy_ptr)(r2) lwz r29,12(r2) b L12 L11: lwz r15,0(r22) lwz r16,4(r22) lwz r17,8(r22) addi r22,r22,12 lis r2,ha16(L_sAi_fast3$non_lazy_ptr) lwz r29,lo16(L_sAi_fast3$non_lazy_ptr)(r2) L12: mtctr r29 bctr .align 2 .section __TEXT,__text,regular,pure_instructions Which eventually drop down to one error that kills the whole process: ghc -DUSE_READLINE=1 -L/sw/lib/ -lreadline -I/sw/include -package lang -c -cpp -o /Users/tatd100/Documents/Work/Project/Tools/hmake-3.08/targets/powerpc- Darwin7/obj/hmake/Compat.o Compat.hs Compat.hs:1: parse error on input `#' make[1]: *** [/Users/tatd100/Documents/Work/Project/Tools/hmake-3.08/targets/ powerpc-Darwin7/obj/hmake/Compat.o] Error 1 make: *** [targets/powerpc-Darwin7/hmake-ghc] Error 2 I'm guessing that there's some binary compatibility issues here... Could it be panther using gcc3.3? Thanks Bob

2 1

Functional Code Generation
by Brandon Michael Moore 20 Nov '03

20 Nov '03

I think I remeber seeing a paper about techniques for writing code generators in Haskell targeting stack based bytecodes that encoded bytecode sequences in a way that the typechecker could give you some sanity guarantees, and then talked a bit about using dynamic programming for optimizing bytecode sequences. I haven't turned up anything searching Google and CiteSeer (though I don't really know good words to search for), so I'm hoping someone here remembers. A friend of mine is implementing a Scheme -> OCaml bytecode compiler for a class and I think some of the ideas from that paper would help, whatever they were. Thanks Brandon

3 2

Aggressive inlining
by Blue Steel 20 Nov '03

20 Nov '03

I'm working on a parlor trick. I would like to solve a 3-SAT problem at compile time, so that the executable is super fast. Consider --------------------------------------------------- formula (x1,x2,x3) = (x1 || x2) && x3 tf = [True, False] assignments = [(x1,x2,x3)|x1<-tf,x2<-tf,x3<-tf] solutions = filter formula assignments main = do print solutions --------------------------------------------------- Theoretically, a compiler could optimize this to --------------------------------------------------- main = do print [(True,True,True),...] --------------------------------------------------- Is this actually possible in GHC? I've played with inline pragmas and looked at -ddump-..., but I haven't made much progress. Of course, the goal is to replace "formula" with a formula that takes a 30-tuple instead of a triple and really let GHC crank. Thanks for any help. __________________________________ Do you Yahoo!? Protect your identity with Yahoo! Mail AddressGuard http://antispam.yahoo.com/whatsnewfree

1 0

Re: Haskell Server Pages
by Niklas Broberg 19 Nov '03

19 Nov '03

I am part of a project working on an implementation of an HSP system, very much inspired by the paper you mention by Eric Meijer and Daan Leijen. It is our goal to make a complete HSP system with a model for HTML in Haskell, a pre-processor, and a runtime system/plugins for various web servers. The ETA is set to May -04, until then you can follow our progress at http://www.dtek.chalmers.se/~d00nibro/hsp/ There is a simple version of a pre-processor available from our Download page, but use it at your own risk ;) /Niklas _________________________________________________________________ MSN 8 with e-mail virus protection service: 2 months FREE* http://join.msn.com/?page=features/virus

1 0

Partially ordered collections (revisited)
by Graham Klyne 19 Nov '03

19 Nov '03

I've taken the suggestions made by Tom and Robert and combined them into a new module with some supporting functions and test cases. A copy is attached, in case it's of use to anyone. #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

1 0

Haskell Server Pages
by "Grigory Makeev 19 Nov '03

19 Nov '03

Hello, I'm interested in writing a personal home-page in a functional way, and i'm in need of some clues. Is there any resource, devoted exactly to the functional approaches to the web programming and web design? I've seen a WASH project, but i'm not really saticefied with the way pages look there. What i trully need is something like Haskell Server Pages system, from Erik Meijer's "Haskell Server Pages. Functional programming and the Battle for the Middle Tier" paper, but i haven't found any implementation of that. I'm not very good at either web programming, or Haskell, i'm just a fan of it, and there's no way i can implement that myself. Is anyone writing currently something like table = <table border="1"> <% mkRows cells%></table> Can anyone provide me with some links? Best regards, Grigory

1 0

Haskell Server Pages
by "Grigory Makeev 19 Nov '03

19 Nov '03

Hello, I'm interested in writing a personal home-page in a functional way, and i'm in need of some clues. Is there any resource, devoted exactly to the functional approaches to the web programming and web design? I've seen a WASH project, but i'm not really saticefied with the way pages look there. What i trully need is something like Haskell Server Pages system, from Erik Meijer's "Haskell Server Pages. Functional programming and the Battle for the Middle Tier" paper, but i haven't found any implementation of that. I'm not very good at either web programming, or Haskell, i'm just a fan of it, and there's no way i can implement that myself. Is anyone writing currently something like table = <table border="1"> <% mkRows cells%></table> Can anyone provide me with some links? Best regards, Grigory

1 0

oh, i'm sorry for doubling a message
by "Grigory Makeev 19 Nov '03

19 Nov '03

1 0

There is flexible exception handling in Haskell?
by 7253146＠informatica.alumnos.uma.es 18 Nov '03

18 Nov '03

Hi all. I am porting to Haskell a small zlib-based library for .zip files (I have not seen any released package for it, although it should very useful). The matters come when I try to address exceptional conditions: all the library functions return a integer code with OK/SOMEERROR meaning. The most natural way to carry the exceptional situations should be raise IO exceptions, but here comes the problem: how can I define new Exception codes, instead of raising userError all the time? I think it makes sense for a library to raise specialized exceptions, instead of userErrors. There is such a mechanism? Can someone help? Thanks. --------------------------------------------- Este mensaje lo ha enviado un Alumno de la Universidad de Malaga. http://www.alumnos.uma.es/

4 3

Subsumption in partially ordered sets
by Graham Klyne 18 Nov '03

18 Nov '03

I have a need for an algorithm to perform "subsumption" on partially ordered sets of values. That is, given a selection of values from a partially ordered set, remove all values from the collection that are less than some other member of the collection. Below is some code I have written, which works, but I'm not sure that it's especially efficient or elegant. Are there any published Haskell libraries that contain something like this? #g -- (The implementation here is based on values of type (Eq a) => [Maybe a], where the partial ordering is defined by function 'pcompare'. Function dropSubsumed (and helpers) is the subsumption calculation. testds1, testds2, testds3, testds4, testds5 are test cases, and all should be True.) [[ -- Type for result of partial order comparison (PNR is no-relationship) data PartOrdering = PLT | PEQ | PGT | PNR deriving (Eq, Show) -- Drop tuples from the supplied list that are subsumed by -- more specific ones. -- dropSubsumed :: (Eq a) => [[Maybe a]] -> [[Maybe a]] dropSubsumed [] = [] dropSubsumed [a] = [a] dropSubsumed (a1:as) = dropSubsumed1 a1 as dropSubsumed1 a1 [] = [a1] dropSubsumed1 a1 (a2:a2s) = case pcompare a1 a2 of PEQ -> dropSubsumed1 a1 a2s PGT -> dropSubsumed1 a1 a2s PLT -> dropSubsumed1 a2 a2s PNR -> dropSubsumed2 [] a1 $ dropSubsumed1 a2 a2s -- Merge new head element into list from which subsumed elements -- have already been removed. The extra (first) parameter is used -- to construct a result in which the order of remaining elements -- is preserved with respect to the original list. dropSubsumed2 a1s a [] = a : revConcat a1s [] dropSubsumed2 a1s a ar@(a2:a2s) = case pcompare a a2 of PEQ -> a : revConcat a1s a2s PGT -> a : revConcat a1s a2s PLT -> revConcat a1s ar PNR -> dropSubsumed2 (a2:a1s) a a2s revConcat :: [a] -> [a] -> [a] revConcat [] a2s = a2s revConcat (a1:a1s) a2s = revConcat a1s (a1:a2s) -- Perform subsumption calculation between a pair of tuples -- A tuple with more information subsumes a one with less but -- consistent information. -- pcompare :: (Eq a) => [Maybe a] -> [Maybe a] -> PartOrdering pcompare a1s a2s = pcompare1 a1s a2s PEQ pcompare1 [] [] po = po pcompare1 (Just _:a1s) (Nothing:a2s) po = if (po == PEQ) || (po==PGT) then pcompare1 a1s a2s PGT else PNR pcompare1 (Nothing:a1s) (Just _:a2s) po = if (po == PEQ) || (po==PLT) then pcompare1 a1s a2s PLT else PNR pcompare1 (a1:a1s) (a2:a2s) po = if a1 == a2 then pcompare1 a1s a2s po else PNR pcompare1 _ _ _ = PNR testds1 = ds1a == ds1b ds1a = dropSubsumed [ [Just 'a',Just 'b',Just 'c'] , [Just 'a',Just 'b',Nothing ] , [Just 'a',Nothing ,Just 'c'] , [Just 'a',Nothing ,Nothing ] , [Nothing ,Just 'b',Just 'c'] , [Nothing ,Just 'b',Nothing ] , [Nothing ,Nothing ,Just 'c'] , [Nothing ,Nothing ,Nothing ] ] ds1b = [ [Just 'a',Just 'b',Just 'c'] ] testds2 = ds2a == ds2b ds2a = dropSubsumed [ [Just 'a',Just 'b',Nothing ] , [Just 'a',Nothing ,Just 'c'] , [Just 'a',Nothing ,Nothing ] , [Nothing ,Just 'b',Just 'c'] , [Nothing ,Just 'b',Nothing ] , [Nothing ,Nothing ,Just 'c'] , [Nothing ,Nothing ,Nothing ] ] ds2b = [ [Just 'a',Just 'b',Nothing ] , [Just 'a',Nothing ,Just 'c'] , [Nothing ,Just 'b',Just 'c'] ] testds3 = ds3a == ds3b ds3a = dropSubsumed [ [Just "a1",Just "b1",Just "c1"] , [Just "a2",Just "b2",Nothing ] , [Just "a3",Nothing ,Just "c3"] , [Just "a4",Nothing ,Nothing ] , [Nothing ,Just "b5",Just "c5"] , [Nothing ,Just "b6",Nothing ] , [Nothing ,Nothing ,Just "c7"] , [Nothing ,Nothing ,Nothing ] ] ds3b = [ [Just "a1",Just "b1",Just "c1"] , [Just "a2",Just "b2",Nothing ] , [Just "a3",Nothing ,Just "c3"] , [Just "a4",Nothing ,Nothing ] , [Nothing ,Just "b5",Just "c5"] , [Nothing ,Just "b6",Nothing ] , [Nothing ,Nothing ,Just "c7"] ] testds4 = ds4a == ds4b ds4a = dropSubsumed [ [Just 1, Just 1 ] , [Just 2, Nothing] , [Nothing,Just 3 ] , [Nothing,Nothing] ] ds4b = [ [Just 1, Just 1 ] , [Just 2, Nothing] , [Nothing,Just 3 ] ] -- Check handling of equal values testds5 = ds5a == ds5b ds5a = dropSubsumed [ [Just 1, Just 1 ] , [Just 2, Nothing] , [Nothing,Just 3 ] , [Nothing,Nothing] , [Just 1, Just 1 ] , [Just 2, Nothing] , [Nothing,Just 3 ] ] ds5b = [ [Just 1, Just 1 ] , [Just 2, Nothing] , [Nothing,Just 3 ] ] ]]

4 4

There is flexible exception handling in Haskell?
by 7253146＠informatica.alumnos.uma.es 18 Nov '03

18 Nov '03

Hi all. I am porting to Haskell a small zlib-based library for .zip files (I have not seen any released package for it, although it should very useful). The matters come when I try to address exceptional conditions: all the library functions return a integer code with OK/SOMEERROR meaning. The most natural way to carry the exceptional situations should be raise IO exceptions, but here comes the problem: how can I define new Exception codes, instead of raising userError all the time? I think it makes sense for a library to raise specialized exceptions, instead of userErrors. There is such a mechanism? Can someone help? Thanks. --------------------------------------------- Este mensaje lo ha enviado un Alumno de la Universidad de Malaga. http://www.alumnos.uma.es/

1 0

Hypothetical reasoning in type classes
by Ken Shan 16 Nov '03

16 Nov '03

Hello, Has anyone thought about adding hereditary Harrop formulas, in other words hypothetical reasoning and universal quantification, to the instance contexts in the Hsakell type class system? Just today (and not only today) I wanted to write instance definitions like instance (forall a. C a => D a) => E [a] where ... This is analogous to wanting to write a rank-2 dictionary constructor (forall a. C a -> D a) -> E [a] at the term level, but with type classes, this dictionary constructor should be applied automatically, in a type-directed fashion, at compile time. The theory behind such instance contexts doesn't seem so intractable; indeed it looks decidable to my cursory examination. The opreational intuition is that we would like the type checker to generate an eigenvariable "a" and perform hypothetical reasoning. I would also like to quantify universally over type classes; in other words, if "?" is the kind of a type class constraint (aka a dictionary type; perhaps "o" would be a better choice of notation), then I would like to define not just types of kind "*->*->?" (aka type classes) or kind "(*->*)->?" (aka constructor classes), but also types of kind "(*->?)->(*->?)". But that's for another day... Ken -- Edit this signature at http://www.digitas.harvard.edu/cgi-bin/ken/sig "Hi, my name is Kent, and I let people change my .sig on the internet." "Hi, Ken!" "Put midgets back in midget porn!" -- Not authorized by Ken Shan. Supported by the association of midget porn workers, Local 9823.

2 1

what's the deal with "user error" on fail?
by David Roundy 15 Nov '03

15 Nov '03

When one triggers an exception with something like fail "Error opening file" The user gets a message like Fail: user error Reason: Error opening file which is confusing to the user, because it the user's fault. Is there some other way that it is recommended one fail? Or should I be catching userErrors at the top level and failing with my own error message? -- David Roundy http://www.abridgegame.org/darcs

3 3

RE: Type tree traversals [Re: Modeling multiple inheritance]
by Simon Peyton-Jones 15 Nov '03

15 Nov '03

| More overlapping: | Allow any overlapping rules, and apply the most specific rule that | matches our target. Only complain if there is a pair of matching | rules neither of which is more specific than the other. | This follow the spirit of the treatment of duplicate imports... Happy days. I've already implemented this change in the HEAD. If you can build from source, you can try it. | Backtracking search: | If several rules matched your target, and the one you picked didn't | work, go back and try another. | | This isn't as well through out: you probably want to backtrack through all | the matching rules even if some are unordered by being more specific. It | would probably be godd enough to respect specificity, and make other | choices arbitrarilily (line number, filename, etc. maybe Prolog has a | solution?). This probably isn't too hard if you can just add | nondeterminism to the monad the code already lives in. I didn't follow the details of this paragraph. But it looks feasible. | Overloading resolution: | This one is really half-baked, but sometimes it would be nice if there was | some way to look at | class MyNumber a where | one::a | instance MyNumber Int where | one = 1 | | then see (one+1) and deduce that the 1 must have type Int, rather than | complaining about being unable to deduce MyNumber a from Num a. This is | really nice for some cases, like a lifting class I wrote for an Unlambda | interpreter, with instances for LiftsToComb Comb and (LiftsToComb a => | LiftsToComb (a -> Comb)). With some closed world reasoning lift id and | lift const might give you I and K rather than a type error. Also, for | this work with modelling inheritance you almost always have to give type | signatures on numbers so you find the method that takes an Int, rather | than not finding anything that takes any a with Num a. This obviously | breaks down if you have instances for Int and Integer, and I don't yet | know if it is worth the trouble for the benefits in the cases where it | would help. Implementation is also a bit tricky. I think it requires | unifying from both sides when deciding if a rule matches a goal. I'm much less sure about this stuff. Mark Shields and I did something about closed classes in our OO paper http://research.microsoft.com/~simonpj/Papers/oo-haskell/index.htm, and Martin Sulzmann and colleagues have done lots of foundational work -- but the dust is still swirling I think. Simon

4 3

RE: haskell httpd
by Simon Marlow 14 Nov '03

14 Nov '03

> <rant> > Frustrating. > > Its been 4 years since I seriously looked at > Haskell. I would have expected that over this > time, someone here would have consolidated it into > a language useful for real world applications. > > The Haskell in Practice page is shockingly short > and many of the examples are not actually Haskell > in practice. > > Does anyone here eat the Haskell dog food or is > this all just fooling around (basic reasearch)? > > Simon, why did you write the Haskell web server? 1. To show that it could be done 2. Because it was fun, and 3. So I could write a paper about it Note that this list does *not* include 4. So I could use it However, several people have found the web server useful and are using it, either as the basis of further research, or in some cases as the basis for a real production web server. It isn't a production web server, as you noticed, but it's a start. Cheers, Simon

2 1

Re: what's the deal with "user error" on fail?
by Jon Cast 14 Nov '03

14 Nov '03

David Roundy <droundy(a)abridgegame.org> wrote: > hello, > well that's because haskell is not as pure as it claims to be :-) Nonsense. Haskell is perfectly pure; the IO monad is just nondeterministic :) > if every monad supports errors depends on what you mean by "supporting > errors". I mean `supporting errors' in the only possible sense that `supporting errors' can be implied by the existence of `fail': we have a throw function. > i would argue that supporting errors and divergence is not the same > thing. I never said it was. > the difference is that one should be able to handle an error thrown by > a computation, Although this doesn't imply the existence of a `catch' HOF; see the MonadPlus instance for Maybe. > while clearly we cannot detect nonterminating computations. Of course. But we can detect exceptions thrown by `error'. Even though we can't distinguish them from non-termination /in pure Haskell/. We can of course distinguish them in the IO monad (which perhaps ought to be re-named the SinBin monad, to be more honest...). So we can detect erroneous computations in any monad. Jon Cast

1 0

record matching syntax forensics
by John Meacham 14 Nov '03

14 Nov '03

so I noticed DrIFT was generating code like f Foo{bar} = ... as seemingly a shorthand for f Foo{bar = bar} = ... which does not seem to work with current compilers. I fixed this in 2.0.2 but was curious why it was generating code like that in the first place. it appears to be illegal looking at the haskell report and I was uncertain who wrote the derive rules in the first place so thought I would ask here. John -- --------------------------------------------------------------------------- John Meacham - California Institute of Technology, Alum. - john(a)foo.net ---------------------------------------------------------------------------

2 1

Re: what's the deal with "user error" on fail?
by Jon Cast 14 Nov '03

14 Nov '03

David Roundy <droundy(a)abridgegame.org> wrote: > i would say that if you are wanting to report errors to users, you > should not use "fail" or "error". you should instead explicitly > report the error. when i program i typically use "error" in > situations that i think are imposible, but if there is a bug one gets > a better error messeage. i don't use fail (well except sometimes > implicitly in list comprahensions). the reason i avoid "fail" is that > it seems hackish to me -- it implies that every monad supports errors ^^^^^^^^^^^^^^^^^^^^^^^^^^^ In Haskell, this is indeed the case---every monad is after all a monad transformer applied to Haskell's identity monad, which supports errors (hence the existence of Haskell's `error' function :). So even if a monad designer doesn't explicitly add error support, the monad still supports errors. Jon Cast

3 2

Re: ANNOUNCE: Release of Vital, an interactive visual programming environment for Haskell
by Graham Klyne 13 Nov '03

13 Nov '03

[Switching to haskell-cafe] For "serious programming", I entirely agree. But my view is that we are seeing some degree of programmability entering all sorts of everyday objects -- video recorders spring to mind as an early example -- and there's lots of work going on in the field of "ubiquitous computing". Many of these pervasive devices may be fire-and-forget, but I suspect many will not be. Graphical displays may be more common than full keyboards. So how is the user to be presented with options to enter programming information? I don't have any final answers here, but I do have an intuition that for many users, where the "programming" requirement is a simple but flexible composition of existing functions, that a graphical, self-documenting interface may be an appropriate response to the "video recorder programming hell" syndrome. Some of my thoughts about this came from considering issues faced by a friend of mine who has recently wired his new home for "total data" (several kilometres of Cat5A cable in the loft!) -- it's all very well having all these intelligent devices around the home, but how to actually tell them what to do? Opening a door may signal that a light should turned on, or an alarm should be set off -- how to describe the distinction? (Assuming the owner is not an experienced programmer.) Finally, as evidence for this view of user interfaces, I note that for tasks like computer system administration, graphical interfaces have pretty much taken over from the old command-line-and-text-file approach. Even Linux systems have graphical front-ends for most of the common configuration, even though, for an experienced sysadmin, the text-based versions are generally quicker to set up and understand what's happenning. In short, it's the occasional user, not the full-time expert, who may be better served by a non-textual approach. #g -- At 23:56 12/11/03 +0100, Marcin 'Qrczak' Kowalczyk wrote: >W li¶cie z ¶ro, 12-11-2003, godz. 11:06, Graham Klyne pisze: > > > I've sometimes thought that a functional language would be the ideal > > platform to usher in a purely graphical style of programming; > >I don't understand why so many people talk about graphical programming, >i.e. putting together functions, arguments, definitins etc. with the >mouse instead of the keyboard, drawing arrows instead of naming etc. > >No wonder it didn't succeed. It would be much less convenient than >typing text and less readable too. > >-- > __("< Marcin Kowalczyk > \__/ qrczak(a)knm.org.pl > ^^ http://qrnik.knm.org.pl/~qrczak/ > >_______________________________________________ >Haskell mailing list >Haskell(a)haskell.org >http://www.haskell.org/mailman/listinfo/haskell ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

2 1

Graphical Programming Environments (was: ANNOUNCE: Release of Vit al, an interactive visual programming environment for Haskell)
by Markus.Schnell＠infineon.com 13 Nov '03

13 Nov '03

> > I've sometimes thought that a functional language would be > the ideal > > platform to usher in a purely graphical style of programming; > > I don't understand why so many people talk about graphical > programming, > i.e. putting together functions, arguments, definitins etc. with the > mouse instead of the keyboard, drawing arrows instead of naming etc. > > No wonder it didn't succeed. It would be much less convenient than > typing text and less readable too. That's not necessarily correct. If 'graphical' isn't taken too literally, you can think of a dialog per function with the possibility to specify pre and post conditions, tests, comments, etc. Then it would be possible to view only things your are interested in: default code, exception handling, interfaces, ... without cluttering the screen with 'code of no interest'. I agree that putting together programs just by click and point would be tedious. But _viewing_ programs this way could be advantagous. What do you think? Best wishes, Markus

3 2