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List comprehensions
by Rijk J. C. van Haaften 31 Jan '03

31 Jan '03

Hello, Recently, I came accross this expression: [ x + y | x <- xs | y <- ys ] As far as I can see (Haskell Report), this is not allowed by the haskell 98 standard. So I assume it to be an ex- tension. Where can I find information about this? Thanks, Rijk

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Problem with backtracking monad transformer
by Guest, Simon 31 Jan '03

31 Jan '03

I'm trying to make a backtracking state monad using Ralf Hinze's backtracking monad transformer. My problem is that it won't backtrack very far. Suppose I try ( a >> b ) `mplus` c. If b fails, it should try c, but it doesn't rewind past a. My sample code is below. GHCI> c [0,1] match_1 -- (1 or 0) then 1, OK GHCI> c [1,0] match_2 -- (1 then 0) or (1,1), OK GHCI> c [1,1] match_2 -- (1 then 0) or (1,1), fails What have I misunderstood? cheers, Simon (A disclaimer in an attachment? - it wasn't my idea.) -- backtracking state monad -- requires -fglasgow-exts import qualified Monad as M import qualified Control.Monad.Trans as MT -- turn tracing on and off by uncommenting just one of the following lines import Debug.Trace( trace ) --trace s x = x -- -- Ralf Hinze's efficient backtracking monad transformer -- newtype BACKTR m a = BACKTR { mkBACKTR :: (forall b. (a -> m b -> m b) -> m b -> m b) } instance (Monad m) => Monad (BACKTR m) where return a = BACKTR (\c -> c a) m >>= k = BACKTR (\c -> mkBACKTR m (\a -> mkBACKTR (k a) c)) -- We don't use a Backtr class, but do it with the MonadPlus class, -- mzero is false (fail), -- mplus is ¦ (orelse) instance (Monad m) => M.MonadPlus (BACKTR m) where mzero = BACKTR (\c -> id) m1 `mplus` m2 = BACKTR (\c -> mkBACKTR m1 c . mkBACKTR m2 c) -- standard MonadTrans class has lift for promote, and doesn't have observe instance MT.MonadTrans BACKTR where lift m = BACKTR (\c f -> m >>= \a -> c a f) observe :: (Monad m) => BACKTR m a -> m a observe m = mkBACKTR m (\a f -> return a) (fail "false") -- -- State Monad -- data SM st a = SM (st -> (a,st)) -- The monadic type instance Monad (SM st) where -- defines state propagation SM c1 >>= fc2 = SM (\s0 -> let (r,s1) = c1 s0 SM c2 = fc2 r in c2 s1) return k = SM (\s -> (k,s)) -- extracts the state from the monad readSM :: SM st st readSM = SM (\s -> (s,s)) -- updates the state of the monad updateSM :: (st -> st) -> SM st () -- alters the state updateSM f = SM (\s -> ((), f s)) -- run a computation in the SM monad runSM :: st -> SM st a -> (a,st) runSM s0 (SM c) = c s0 -- backtracking state monad -- type NDSM st a = BACKTR (SM st) a readNDSM :: NDSM st st readNDSM = MT.lift readSM updateNDSM :: (st -> st) -> NDSM st () updateNDSM f = MT.lift (updateSM f) --run a computation in the NDSM monad runNDSM :: st -> NDSM st a -> (a,st) runNDSM s0 m = runSM s0 (observe m) -- -- the state -- type Bit = Int data CState = CState { ok :: Bool, remaining_data :: [Bit], history :: [String] -- log, kept in reverse } deriving Show initState xs = CState True xs [] -- prepend a message in the log logit :: CState -> String -> CState logit s logmsg = s { history = logmsg : (history s) } -- -- matching action -- match_bits :: [Bit] -> NDSM CState () match_bits xs = do s <- readNDSM let s' = logit s ("attempt match_bits " ++ show xs ++ " remaining: " ++ show (remaining_data s)) s'' = if xs == take (length xs) (remaining_data s') then s' { remaining_data = drop (length xs) (remaining_data s') } else s' { ok = False } if ok s'' then updateNDSM (\s -> s'') else trace (unlines $ "MATCH FAILED":(reverse $ history s'')) M.mzero -- -- test routines -- -- just fine match_1 = (match_bits [1] `M.mplus` match_bits [0]) >> match_bits [1] -- this one only rewinds past the [0] attempt, not the [1] attempt match_2 = ( (match_bits [1] >> match_bits [0]) `M.mplus` match_bits [1, 1] ) c :: [Bit] -> NDSM CState () -> ([Bit], [String]) c h hspec = let (v, s) = runNDSM (initState h) hspec in case (ok s) of True -> ([], "ok":(reverse $ history s)) _ -> ([(negate)1], ["fail"])

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RE: List comprehensions
by Jan de Wit 30 Jan '03

30 Jan '03

> Hello, > > Recently, I came accross this > expression: > [ x + y | x <- xs | y <- ys ] > > As far as I can see (Haskell Report), > this is not allowed by the haskell 98 > standard. So I assume it to be an ex- > tension. Where can I find information > about this? This is a parallel list comprehension, a GHC extension. See http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#PA RALLEL-LIST-COMPREHENSIONS for more info. The code above does the same as zipWith (+) xs ys, basically. Cheers, Jan ########################################### This message has been scanned by F-Secure Anti-Virus for Microsoft Exchange. For more information, connect to http://www.F-Secure.com/ ###########################################

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Haskell and professional aspirations.
by guilhermelopes＠netcabo.pt 28 Jan '03

28 Jan '03

Hello there I'm a computer science undergraduate student, and would like to continue (or rather start) working with Haskell after graduation. Which universities welcome postgraduate work using functional programming languages? That is, which groups/departments of which universities should I keep an eye on? Which companies use functional programming for medium-large scale projects? What should be in my curriculum vitae if I wanted to have a good array (hmmm... list!) of offers to choose from? If Haskell is indeed being used professionally, it is responsible for which part of the job? I am aware of some of the issues concerned with "Haskell in the real-world", through haskell.org, and search for an account on the subject more shifted towards personal experience. If you do have experience on working with Haskell, I would love to hear about it. Although this question is essentially personal in nature, I consider it to be (and tried to make it) broad enough to justify its presence here, propped up by Haskell being used around the world for teaching purposes. If you think otherwise, reply directly to my address: tretalhada(a)netcabo.pt . Guilherme Lopes

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RE: Exception handling in GHC - ThrowCatch.hs (1/1)
by Simon Marlow 27 Jan '03

27 Jan '03

> You might be interested in this program to investigate what catches > what. Pinched for GHC's testsuite - thanks very much :) Cheers, Simon

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Creating COM objects and passing out pointers to them via a COM interface
by Sarah Thompson 25 Jan '03

25 Jan '03

Hi again, A slightly harder problem this time. I want to implement something that looks something like the Microsoft XML parser. This is a very COM heavy contraption that provides an interface that looks to client applications like a tree of COM objects that reflects the internal structure of an XML document. The idea is, you start with the document object, make a method call requesting the root node, and have a COM object that wraps that node returned to you. You can then request the children of that node, and their children and siblings, all which involves being passed back interface pointers to COM objects. Effectively, there is one COM object per syntactic item in the XML document. I already have my document object working nicely. This is where the actual database resides. I want to be able to spawn views of this database as COM objects. When I've done this in the past (using ATL mostly, euuuugh), I've used an IDL definition something like HRESULT CreateView([in,string] BSTR query, [out,retval] IDbView **); What's the correct way to implement the necessary Haskell code to create the coclass and return a pointer to it? I fiddled around for ages, but either couldn't get GHC to like the types I was using, or when I finally got it to compile, I found that my test harness was being passed an invalid COM object. This should be simple stuff, so I'm sure someone will have done it! Secondly, it is a nasty, though fairly common trick, if you know your objects are going to be created in the same process space, to allow them some 'behind the scenes' communication. In my case, I'd like to avoid needing to serialise and deserialse the data in a view during creation of these child objects (of which there may be a great many, and which may be created on the fy rather often). Ideally, I'd like to be able to do something like: newView :: String -> State -> IO State newView db (State st) = do db <- readIORef st r <- newIORef db -- Insert some kind of COM magic here return (State r) so the database content gets copied within the Haskell environment (lazily, I'd hope), without needing to be serialised and deserialised. Is this feasible? One (messy) approach might be to create a bunch of extra interfaces on the main database object, then create 'thin wrapper' child objects that simply make calls back into the parent object. I'm not so keen on that for various reasons, however. Sarah

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Exeption handling in GHC
by Sarah Thompson 25 Jan '03

25 Jan '03

Hi all, I'm trying to get exception handling working in GHC, but don't seem to be able to make it work. Am I likely to be missing a compiler switch, or something? Cutting things down to basics, I'd have thought that the following expression (typed at the ghci command prompt) should work: try (error "x") but the result is *** Exception: x I seem to get the same problem with compiled code. Any ideas anyone? TIA, Sarah

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Re: Lazy evaluation alternative
by Kevin S. Millikin 24 Jan '03

24 Jan '03

>>>>> "Chris" == Chris Clearwater <chris(a)sharkzone.com> writes: Chris> Sarcasm, Kevin, sarcasm. :) Why is it that those who most need to point out that they were joking are least able to see it in others? Chris> The difference is with the CPS way it is implicit. For Chris> example take the Y combinator written using both methods Chris> (again this is pseduo code, not intended to be real haskell Chris> code or run in hugs) That's a funny definition of "implicit". All you need to delay evaluation is lambda. You're performing a CPS transformation as a way to insert a lambda in the proper place. BUT THAT'S NOT NECESSARY. Just write the lambda where you want it. You don't need to abstract over a continuation to delay evaluation, you can abstract over anything at all (it doesn't matter what). And you only ever apply the most recent continuation (notice how you could use the identifier "c" for every one of your continuations?). You don't have to apply c to pass a value to the current continuation, there's alread a way to do that: just return the value. Chris> Using the lambda way: Y f = f (lazy Y f) Then writing ones Chris> would be something like: ones_fix f = 1:(force f) Chris> Now the CPS way: Y f c = c (f (Y f)) Now writing ones: Chris> ones_fix f c = c 1:f Your translation is wrong. Here's the correct "lambda way" (in the same pseudo functional language): Y f = \ () -> f (Y f) where I didn't presume any "delay" or "lazy" macro. Here's ones_fix: ones_fix f = \ () -> 1 : f Compare to: Y f = \ c -> c (f (Y f)) ones_fix f = \ c -> c (1 : f) Which one seems to be more "implicit"? Where you abstract over a continuation to delay evaluation, I abstract over anything I feel like. Where you apply a continuation to a value, I just return a value. Where you would pass in a continuation to a CPSed function, I would just apply the result to some suitable token (unless I had zero argument functions) (that is, I would "force" an expression wherever you would pass a continuation to a CPSed function). You've performed a CPS transformation, but you never do anything *interesting* with the continuation. That might suggest that, whatever you seem to have achieved, it didn't come from the CPS transformation. Chris> Notice how the CPS way behaves as if it were actually Chris> passed itself, while the lambda way behaves as if it were Chris> passed itself quoted with lambda. The lambda way seems Chris> like a hack while the CPS way feels consistant. I guess I don't understand. Change "\ c ->" to "\ () ->". Remove the application of every "c". Change occurrences of "c" in argument position to "()". -- Kevin S. Millikin Architecture Technology Corporation Research Scientist Specialists in Computer Architecture (952)829-5864 x. 162 http://www.atcorp.com

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Handling error in production code
by Sarah Thompson 24 Jan '03

24 Jan '03

What is the best way to handle error in production code? I have successfully built a COM object (many thanks Sigbjorn!!!), which works fine. The only hassle is that an error being thrown causes the program using the COM object to hard terminate. This is clearly not desirable, especially since parse errors can cause this. What I'd *ideally* like would be for a call to return an error code as an HRESULT, but leave the internal state of the object unchanged. Is this feasible? Do I need to think carefully about using something other than 'error'? If so, how do I handle library code's use of error here and there? TIA, Sarah

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Re: A question about dynamic typing
by Jon Cast 23 Jan '03

23 Jan '03

Nick Name <nick.name(a)inwind.it> wrote: > On Thu, 23 Jan 2003 15:38:31 -0600 > Jon Cast <jcast(a)ou.edu> wrote: > > > Another idea: if I allow the list of objects in the panel to be > > > infinite, > > Sorry, but I'm having difficulty figuring out where you'd get an > > infinite list of objects to put in the panel. I suspect any > > solution is going to depend on how the list of objects is put > > together. > It could be any lazy list, potentially infinite (and I would not like > to evaluate it). So we're not talking about an actually infinite list, just an I'd-rather-not-evaluate-it list. That makes things clearer (in particular, it is /possible/ to evaluate the whole thing, if necessary). The reason I think this a good thing is it occurs to me that if you want to map an IO action run for its side-effects (e.g. ``play every file'') to every element in the list, you're going to have to compute out the whole thing anyway, so there's no great harm done in pre-computing every element so you can get the list of elements. OTOH, if you want to say ``give me a new panel with every element derived from the elements in this panel by this transformation'' (either pure or IO), you want the result to still be lazy. So, you have to compute the list of allowed transformations /without/ touching every element. As you've predicted, this requires that you be conservative about what operations are offered. I would store a list of types, or a filter for such a list, with every panel. Panels generated lazily would have to be conservative (see below), but perhaps once the last panel element was computed you could re-create the type list from the actual values. Of course, this leads to the FAQ entry: Q. Why does the list of operations on a panel sometimes grow 3 times as long once I've looked at the last item? But that may be an acceptable price to pay (assuming somebody figures out an answer that'll be understandable by the average user). In any case, once you've got the list of possible types, you can take the transformers/actions for each type and compute the intersection. > An example is a "find" function, wich returns a lazy list with the use > of unsafeInterleaveIO. > It would be useful to see the actual first N files as objects into the > "panel" or desktop area, or list, anything visual, and then be able to > obtain, for example, the filtered list of all the "sound files", which > would still be a lazy list. In this case, I would attach a (possibly empty) list of new constraints to each filter. For example, if you had a class SoundObject, and a function isSoundObject, I would represent the filter as (isSoundObject, [HasClass "SoundObject"]. Then, when the filter was run, I would add the filter's constraints to the argument's constraints. That way, the system would know that any action available across all sound objects was now available. Maps seem harder, but I'm sure the transformations on constraints could be encoded and used to find the new constraints. > Should the user scroll the panel down with some arrow-like control, > the list would get evaluated and the find function would run under > control of the user interface. > I am just thinking about what a "lazy functional desktop environment" > could mean, trying to figure out some use case. That sounds like an excellent idea; please don't take anything I say as a discouragement to that project. > Vincenzo Jon Cast

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Re: A question about dynamic typing
by Jon Cast 23 Jan '03

23 Jan '03

Nick Name <nick.name(a)inwind.it> wrote: > On Sun, 19 Jan 2003 16:02:41 -0600 > Jon Cast <jcast(a)ou.edu> wrote: > > > But, to fully exploit the power of a "functionally-programmed" > > > desktop, the interface should allow the user to map an operation > > > onto all the objects of the panel; in this case the allowed > > > operations should be those that all the object share. > > This is fairly easy if you name the operations; then you can use > > that to compute the intersection of the operations for all the > > types. If you don't name the operations, then of course the > > problem becomes undecidable. > Another idea: if I allow the list of objects in the panel to be > infinite, Sorry, but I'm having difficulty figuring out where you'd get an infinite list of objects to put in the panel. I suspect any solution is going to depend on how the list of objects is put together. > obviously the set of allowed operations in a map is the set of > operations on "objects". But if I filter an infinite list of objects > to all the objects of a certain type, I wish to see all the operations > on that type in the possibilities for a map. If you have a finite list of the types of your objects, then you can list the operations available at each type and use that to get the intersection, I suppose. > How would you deal with such a case? > Vincenzo

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Re: A question about dynamic typing
by Jon Cast 23 Jan '03

23 Jan '03

Nick Name <nick.name(a)inwind.it> wrote: <snip> > Ok, this is clear. I am still not sure of how to decide the type to > give to the "extracted" object. This is fairly easy if you have a finite number of possible types; you can use typeOf to determine which type you have, and then do a case analysis to use the particular type typeOf says you have. Another alternative is to write your operations to take Dynamic. If all of the functions you want to apply have some ``no action'' return value nop (say, they all return IO () with nop = return ()), and they all take monomorphic arguments, (maybe nop f . fromDynamic) will perform f on Dynamics of appropriate type. > In the graphical interface, any object should have a menu saying on > its top row: "Text file" or "Applet" (depending from the type of the > object) and then a list of operations. > This should happen even when the object is inside the panel-list, (so > maybe a panel would be a list of untyped "objects" with an attribute > of type [Operation]). I wouldn't attach a list of operations to each /object/; I should think the list of operations wouldn't change across the type, and in any case you would want to allow the user to dynamically load new operations. So, I would attach a list of named operations of type (Dynamic -> Result) to each type, and offer the operations for a given type to the user. > But, to fully exploit the power of a "functionally-programmed" > desktop, the interface should allow the user to map an operation onto > all the objects of the panel; in this case the allowed operations > should be those that all the object share. This is fairly easy if you name the operations; then you can use that to compute the intersection of the operations for all the types. If you don't name the operations, then of course the problem becomes undecidable. > This is what I think, however I can't realize the connection between > the haskell type system and what I wish to have. Can someone lead me > to a clearer vision of the topic? > Vincenzo Jon Cast

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Stupid questions...
by Sarah Thompson 23 Jan '03

23 Jan '03

Sorry about such lame questions. I'm currently using Haskell 'in anger' for the first time, so differences with other functional languages are biting me occasionally. I need to convert Ints to Strings and vice-versa. What's the best way to do this? I've not found library functions for this. Second question: Using division over integers. I appreciate that a division operator with integers as arguments can not be guaranteed to return an exact integer result. However, if I *do* want to do integer division, how should I do it? Is floor(a/b) appropriate? TIA Sarah Thompson

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RE: Arrays and arrays
by Simon Marlow 23 Jan '03

23 Jan '03

> Since you're on the line, could you confirm or deny that the (//) > operator is something equivalent to (roughly): > > a // ps = runST (thaw a >>= \u -> update ps >> unsafeFreeze u) > > I.e. that the list of updates are treated as an atomic operation? > Because doing it that way explicitly sure didn't seem to help. Yes, it is. > OTOH, doing lots of small (//) seems to be faster than doing a few > large ones (containing the same updates). Go figure. That's bizarre. Perhaps the results are obscured by some other optimisations which are happening. Cheers, Simon

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RE: Arrays and arrays
by Simon Marlow 22 Jan '03

22 Jan '03

> Hal Daume III <hdaume(a)ISI.EDU> writes: > > [snip my functions that use the ST monad to do (//)] > > > You shouldn't try to write these functions. You should do all array > > modifications within the ST monad, rather than looking for a pure > > solution. > > All right, but why? It seems an obvious trick, take some pieces that > benefit from imperative processing, and wrap them in ST, leaving the > rest of the program as it were. > > Can you (or anybody else) explain the memory behaviour I see (using > hundreds of megabytes, but only tens of K visible in the profiling > output)? That's because currently large objects aren't included in the profile. There's no really good reason for this, except that threads (including their stacks) count as large objects, and seeing them in the profile might be confusing. On the other hand, it might also be enlightening if you have a program that uses a lot of stack space. I'll look into fixing this. Cheers, Simon

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Arrays and arrays
by ketil＠ii.uib.no 22 Jan '03

22 Jan '03

Hi Having written a suffix-array based program using lists, I thought I'd speed it up a bit, and perhaps more importantly, save space, using arrays. Basically, the problem is to sort all suffixes of a string, represented as an array of offsets, alphabetically. (Does anybody have such code? Efficient?) The good news is that space consumption is reduced -- except for a few weird spikes (which makes me suspect there are many of them, living for very short durations), memory use is low. But speed is another issue -- the list based version is a lot faster. Okay, I use UArrays and permute them by (//), and this operation is totally dominant in the profile. Is it correct that GHC is very naive about updating them, and even small updates cost O(n)? Is it better to (//) over few large lists than over many small ones? I'll try a IOUArray next; however, is there anything else I could/should try? I know there has been array sorts implemented previously, are anybody aware of recent results? -kzm -- If I haven't seen further, it is by standing in the footprints of giants

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RE: Trying to get a copy of HaskellDirect urgently...
by Simon Peyton-Jones 21 Jan '03

21 Jan '03

I've just grabbed it ok, so I'll send it to you in a separate message. It says that it's built for GHC 4.08.1 which is pretty old. I wonder if you've made a more recent release somewhere, Sigbjorn? Sarah: you can always get the sources from the CVS tree Simon | -----Original Message----- | From: sarah(a)telergy.com [mailto:sarah@telergy.com] | Sent: 21 January 2003 13:26 | To: haskell-cafe(a)haskell.org | Subject: Trying to get a copy of HaskellDirect urgently... | | Hi Folks, | | I'm currently trying to urgently get a copy of the HaskellDirect release, but I'm | finding it impossible to get to the ftp server at Glasgow (it seems to be | refusing connections - it's not a firewall problem). | | If anyone has a copy they could email me, or knows of somewhere else I can get | the release, I would be extremely grateful. | | TIA, | Sarah Thompson | | PS: Are there any known issues using HaskellDirect with the latest GHC release | (GHC 5.04.2)? | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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IORefs, MVars und CVars
by Martin Huschenbett 21 Jan '03

21 Jan '03

I just started some multithreaded programming and as I am a newbie, I've gat a question. 1.) What are the differences between IORefs and MVars? 2.) What are CVars for? Thanks, Martin.

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Trying to get a copy of HaskellDirect urgently...
by sarah＠telergy.com 21 Jan '03

21 Jan '03

Hi Folks, I'm currently trying to urgently get a copy of the HaskellDirect release, but I'm finding it impossible to get to the ftp server at Glasgow (it seems to be refusing connections - it's not a firewall problem). If anyone has a copy they could email me, or knows of somewhere else I can get the release, I would be extremely grateful. TIA, Sarah Thompson PS: Are there any known issues using HaskellDirect with the latest GHC release (GHC 5.04.2)?

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RE: Evaluation Question
by Simon Marlow 20 Jan '03

20 Jan '03

[ moved to haskell-café ] > Simon Marlow wrote: > > The original version should also evaluate the expression > 'cis wn' only > > once: [...] > > Nice theory, but GHC's interpreter and compiler behave differently: > > -- Main.hs ----------------------------------------------------------- > module Main where > > import Data.Complex ( Complex ) > import qualified Data.Complex ( cis ) > import Debug.Trace > > cis :: (RealFloat a) => a -> Complex a > cis x = trace "cis" $ Data.Complex.cis x > > nco1, nco2 :: RealFloat a => a -> [ Complex a ] > nco1 wn = 1 : map ((*) (cis wn)) (nco1 wn) > nco2 wn = iterate (cis wn *) 1 > > main :: IO () > main = do > let test f = print . take 5 . f $ (pi/2 :: Double) > test nco1 > test nco2 > ---------------------------------------------------------------------- Oops, I didn't notice that nco1 was recursive. I should have said the expression (cis wn) is evaluated once for each call to nco1. It is not evaluated once for each element of the list in map's second argument, which is what I thought was being asked. > panne@jeanluc:~> ghci Main.hs > ___ ___ _ > / _ \ /\ /\/ __(_) > / /_\// /_/ / / | | GHC Interactive, version 5.05, > for Haskell 98. > / /_\\/ __ / /___| | http://www.haskell.org/ghc/ > \____/\/ /_/\____/|_| Type :? for help. > > Loading package base ... linking ... done. > Compiling Main ( Main.hs, interpreted ) > Ok, modules loaded: Main. > *Main> main > [1.0 :+ 0.0,cis > 6.123031769111886e-17 :+ 1.0,cis > (-1.0) :+ 1.2246063538223773e-16,cis > (-1.836909530733566e-16) :+ (-1.0),cis > 1.0 :+ (-2.4492127076447545e-16)] > [1.0 :+ 0.0,cis > 6.123031769111886e-17 :+ 1.0,(-1.0) :+ > 1.2246063538223773e-16,(-1.836909530733566e-16) :+ (-1.0),1.0 > :+ (-2.4492127076447545e-16)] > *Main> :quit > Leaving GHCi. > panne@jeanluc:~> ghc -Wall -O Main.hs && ./a.out > cis > cis > cis > cis > [1.0 :+ 0.0,6.123031769111886e-17 :+ 1.0,(-1.0) :+ > 1.2246063538223773e-16,(-1.836909530733566e-16) :+ (-1.0),1.0 > :+ (-2.4492127076447545e-16)] > cis > [1.0 :+ 0.0,6.123031769111886e-17 :+ 1.0,(-1.0) :+ > 1.2246063538223773e-16,(-1.836909530733566e-16) :+ (-1.0),1.0 > :+ (-2.4492127076447545e-16)] > ---------------------------------------------------------------------- This difference is due to the different buffering behaviour on stdout/stderr between compiled and interpreted programs. GHCi sets these handles to unbuffered by default, which tends to be more useful for an interpreter. Cheers, Simon

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Re: A question about dynamic typing
by Jon Cast 20 Jan '03

20 Jan '03

Nick Name <nick.name(a)inwind.it> wrote: > On Sun, 19 Jan 2003 16:02:41 -0600 > Jon Cast <jcast(a)ou.edu> wrote: > > So, I would attach a list of named operations of type (Dynamic -> > > Result) to each type, and offer the operations for a given type to > > the user. > Thanks for your answer, it's interesting. > What do you mean with "named operations"? Something like operations > labeled with string, to be able to compare it? Yes: (String, Dynamic -> Result). > Vincenzo Jon Cast

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Question about lists
by cris cris 20 Jan '03

20 Jan '03

hello there, To be honest I'm not very familiar with haskell and stuff so don't laugh with my question... here it goes.. We can represent a matrix as a list of lists... which means that 1 2 3 4 7 6 2 5 1 could be written as follows: [[1,2,3],[4,7,6],[2,5,1]] The question is how can I reverse the matrix (each row becomes a column) Thanx a lot ______________________________________________________________________________________ http://mobile.pathfinder.gr - Pathfinder Mobile logos & Ringtones! http://www.pathfinder.gr - Δωρεάν mail από τον Pathfinder!

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Re: A question about dynamic typing
by Jon Cast 19 Jan '03

19 Jan '03

Nick Name <nick.name(a)inwind.it> wrote: > Or maybe the subject has nothing to do with my question :) It does, I think. <snip subject="class Visible"/> > My problem is: an user putting someway different "visible" elements in > a list (wich could be represented in a side panel) will want to > retrieve an object with its full type when clicking on it (to be able > to perform specific applications), but this isn't allowed by the type > system. This does require dynamic, run-time typing (to make the cast safe). GHC has a dynamic typing library (for monomorphic types only, but I doubt that'll be an issue for you); it's in Data.Dynamic. Short version: if Typeable is a super-class of Visible or mentioned in your existential type (i.e., forall a. (Visible a, Typeable a) => Con) you can use (fromDynamic . toDyn) to safely (attempt to) convert the abstract type into the concrete one. If that last paragraph wasn't coherent, I can try to explain it more. > It's not a problem of functional programming: even in object oriented > programming I see the same problem if one doesn't use some form of > typecast. > Where am I wrong? Are there alternatives? > Vincenzo Jon Cast

2 1

A question about dynamic typing
by Nick Name 18 Jan '03

18 Jan '03

Or maybe the subject has nothing to do with my question :) I was thinking of an "haskell desktop interface", i.e. just a desktop interface, but written in haskell. I thought this: one can define a type class with the "view" method: data Draw -- the drawing monad class Visible t where view :: t -> Time -> Draw And so one can have a list of viewable objects and by mapping some "runDrawIntoIO" function wich spawns a thread for each element of a list, obtain animated icons and applets where one expects just icons. If I use the "Visible" class, I can use existential types to create a non-homogeneous list and "view" all the objects. The same thing could be achieved, of course, with record types: data Visible = { view :: Time -> Draw } and I can achieve the same result I get with existential types. My problem is: an user putting someway different "visible" elements in a list (wich could be represented in a side panel) will want to retrieve an object with its full type when clicking on it (to be able to perform specific applications), but this isn't allowed by the type system. It's not a problem of functional programming: even in object oriented programming I see the same problem if one doesn't use some form of typecast. Where am I wrong? Are there alternatives? Vincenzo

1 0

non-question, instance decl
by ketil＠ii.uib.no 14 Jan '03

14 Jan '03

Hi, experimenting with QuickCheck, I write > instance (Arbitrary e) => Arbitrary (Array Int e) where > arbitrary = undefined and I get: | Illegal instance declaration for `Arbitrary (Array Int e)' | (The instance type must be of form (T a b c) | where T is not a synonym, and a,b,c are distinct type variables) | In the instance declaration for `Arbitrary (Array Int e)' |Failed, modules loaded: QuickCheck. Okay, after experimenting a bit (more fun than reading documentation) I see what's happening: I can't instantiate a halfway specialized type, but 'Ix i => Arbitrary (Array i e)' is Okay. Unfortunately, I really depend on i being Int in my (real) definition of 'arbitrary'. I got around it by liberal doses of 'fromInteger' and a 'Num' context -- ugly, but it seems to work. I suppose there's a solid reason why my previous attempt is illegal? -kzm -- If I haven't seen further, it is by standing in the footprints of giants

1 0

A newbie question about Arrows.
by Nicolas Oury 14 Jan '03

14 Jan '03

Hello, I read some of the material about Arrows on www.haskell.org/arrows and I have some questions : * Why is made the choice to use (,) as Cartesian in first? Can't we write something like : class Cartesian p where pair :: a -> b -> (p a b) projLeft :: (p a b) -> a projRight :: (p a b) -> b class Cartesian pair => Arrow ar where arr ::( ar pair a b) -> (ar pair a b) (>>>) :: (ar pair a b) -> (ar pair b c) -> (ar pair a c) first :: (ar pair a b) -> (ar (pair a d) (pair b d)) (The same could be said for ArrowChoice) I think this could be more powerful and could allow a smarter management of inputs and outputs of arrows than tuples. I think for example to event-driven arrows : we could make a pair of inputs without mixing the "event happened" information. * Why is made the choice of first as being a base function? We have first f = f *** (arr id) second f = (arr id) *** f So they could be define from (***). Moreover, the definition of f *** with first and second creates an order in the use of the f and g (first f >>> second g) whereas it seems that (***) is a "parallel" operator. In an event-driven arrow we have to give the same events to f and g even if they are independent. Best regards, Nicolas Oury

4 4

Re: Web Application Development
by Dominic Steinitz 11 Jan '03

11 Jan '03

I have summarised the responses I have received and cross posted this to Haskell-Cafe in case anyone wishes to carry on the conversation. > In a moment of weakness, I offered to help a friend develop a website. It's > a simple database application which carries out very little calculation and > then displays the results via HTML. > > The original code was in PHP and MySQL so rather than re-invent the wheel I > > So my question is: what should I do to re-write this in Haskell? > 1. SQL 1.a. MySQL 1.a.i Use MySQL-HS which requires the use of H/Direct. Available via www.haskell.org. 1.a.ii Use http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/hbase/Source/Foreign/. This is pre-alpha. 1.b Oracle 1.b.i Use C2HS and libsqlora8 (a simple C library for interfacing to Oracle http://www.poitschke.de/libsqlora8/) There is no publicly available code. 1.c Database Independent 1.c.i Use the DBI model as found in Perl and the libpq binding as listed on Haskell.org (http://www.astercity.net/~khaliff/haskell/dbi/dbi.tgz) This currently has a PostgreSQL binding but is not listed on SourceForge. 1.c.ii Use the code at http://sourceforge.net/projects/htoolkit/. This is planning (that is pre pre-alpha). There appear to be bindings for MySQL, ODBC and PostgreSQL. 2. HTML I have tried out Wash/CGI and this seems satisfactory for my purposes at the moment. Many thanks to everyone who replied, Dominic.

1 0

Re: Interpret haskell within haskell.
by cwitty＠newtonlabs.com 10 Jan '03

10 Jan '03

Matt Hellige <matt(a)immute.net> writes: > So it seems natural, especially to new users, to hope that Haskell and > the ML variants, sharing many other features common to these > languages, would offer this facility as well. The answer, in general, > is that they do not. (Sorry for the belated reply.) Evidently, some ML variants do include eval or eval-like facilities. When reading through the sources of the Isabelle 2002 proof verification system, I noticed the "use_text" eval-like function which is implemented using the following system-specific functions: smlnj-0.93: System.Compile.use_stream polyml: PolyML.Compiler mlworks, mosml: use Carl Witty

1 0

newbie questions
by Max Ischenko 09 Jan '03

09 Jan '03

Hello! I'm started to learn Haskell and have some questions. After reading a "Gentle introduction..." I've decided to write a simple program -- a complement to Literate.hs from hugs demos which reads a plain haskell source file and converts it to literate haskell source. The source code of this program follows. Now the questions: - To be able to run it both in Hugs and by GHC I need an `import Data.Char(isSpace) for GHC`. Can this be conditionally included in a source for GHC only? - Am I reinventing the wheel with my `pad` function? Could I use some standard function(s) instead? - What if I'd want to do a more complex processing, like handling {--} comments? The `classify` function analize one line at a time and I'd want too see a surronding context? Should I use another pass, just like for `addblanks`? module Lit where import System(getArgs) -- classification of the haskell source lines data Classified = Program String | Blank | Comment String deriving (Show) classify :: String -> Classified classify ('-':'-':xs) = Comment (dropWhile isSpace xs) classify s | all isSpace s = Blank | otherwise = Program s unclassify :: Classified -> String unclassify (Program s) = "> " ++ s unclassify (Comment s) = s unclassify Blank = [] process :: String -> String process hs = unlines (map unclassify (addblanks (clines hs))) clines hs = map classify (lines hs) -- inserts a blank between a program line and a comment line where needed addblanks :: [Classified] -> [Classified] addblanks = pad needBlank Blank needBlank :: Classified -> Classified -> Bool needBlank (Program p) (Comment c) = True needBlank (Comment c) (Program p) = True needBlank _ _ = False -- inserts a value between those adjanced elements for which -- function p returns True pad :: (a -> a -> Bool) -> a -> [a] -> [a] pad p value [] = [] pad p value (x:[]) = [x] pad p value (x:xs) | p x (head xs) = x:value:rest | otherwise = x:rest where rest = pad p value xs usage err = err ++ "\n" ++ "USAGE: lit <filename>" main :: IO () main = do strs <- getArgs case strs of [] -> ioError (userError (usage "Not enough arguments.")) [str] -> mklit str _ -> ioError (userError (usage "Too many arguments.")) -- takes a filename without suffix, reads a plain haskell source -- and creates a literate haskell source. mklit :: String -> IO () mklit f = do hs <- readFile (f ++ ".hs") writeFile (f ++ ".lhs") (process hs) -- Bst rgrds, M.A.X.: Mechanical Artificial Xenomorph.

2 2

Re: A newbie question about Arrows.
by Ashley Yakeley 09 Jan '03

09 Jan '03

At 2003-01-06 03:14, Ross Paterson wrote: >class PreArrow ar where > arr :: (a -> b) -> ar a b > (>>>) :: ar a b -> ar b c -> ar a c ... > class (PreArrow ar, Monoidal p u) => GenArrow ar p u where > first :: ar a b -> ar (p a c) (p b c) My own preference is something like this: class (PreArrow ar) => GenArrow' ar where arrApply :: ar p (q -> r) -> ar p q -> ar p r My GenArrow' is actually equivalent to Hughes' Arrow and I assume your GenArrow. In an ideal world, I could leverage my FunctorApply class to have something like this: class (Functor f) => FunctorApply f where -- first arg (fab) "executed" first fApply :: f (a -> b) -> (f a -> f b) -- first arg (fa) "executed" first fPassTo :: f a -> f (a -> b) -> f b fPassTo fa fab = fApply (fmap (\a ab -> ab a) fa) fab class (PreArrow ar,forall p. FunctorApply (ar p)) => GenArrow'' ar Unfortunately, GHC does not yet allow this kind of superclassing. <http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/*checkout*/hbase/Source/HBa se/Category/Functor.hs?rev=HEAD&content-type=text/plain> <http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/*checkout*/hbase/Source/HBa se/Category/Arrow.hs?rev=HEAD&content-type=text/plain> -- Ashley Yakeley, Seattle WA

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OT: Project Idea: Aristo - A Framework for Embedding and Integrating High-Level Languages
by Shlomi Fish 09 Jan '03

09 Jan '03

I just returned from a dinner a few members of the Linux Israeli scene had with Richard Stallman (of GNU fame), as he his now visiting Israel. I got the opportunity to talk with him, and asked him the GNU project insistance of using guile for extensibility. He said the reason for that was that converting any other language to Scheme was a good a choice out of the fact Scheme was so simple, powerful and generic. I said converting Perl 5 code to Scheme would not be too trivial at all, and he said that was the case for converting Perl to almost everything else. :-) (despite this joke, I'm sure converting many languages to Scheme would be incredibly problematic) Then I noted that they should try the GIMP approach instead: and use a procedural database to integrate several languages back-ends. Then, he said that the type system would not be flexible enough for that. After a while that it was a challenge: create a bridge in ANSI C, that will be flexible enough to interoperate code between many virtual machines: perl 5, python, Ruby, Java, Haskell, O'Caml, etc. The codename is dubbed "Aristo", which is the Hebrew pronouncation and spelling of "Aristotle". (it's simpler and without the "totel" at the end). Now what I need is some advice on designing a type system: OK, I can have arrays of ints, and associative arrays from strings to ints, etc. But an array of records whose one field is an array of ints, the other a linked list of strings, the other an associative array from floats to this record itself, is something I'd rather not have to do type interference with. (usually you don't need such strict typing requirements). Here are some of my ideas, so far. Let me know what you think: 1. Aristo can only be written in ANSI C. Any other language will be unacceptable, as all implementations of the back-ends are written in ANSI C. 2. Aristo should have a nice nested namespace organization, with import and export features, package-scope globals etc. The gimp flat list (in which namespace collision is maintained by prefixes) is not scalable. 3. There should be a union type which is a type identifier followed by the type value, which could assume any value. It is possible some types will not be unions and will be plain types without the type value. 4. A standard enum can be used to store the object meta-type, and a standard pointer/long union can be used to store the data. Almost anything will be done using indirection. 5. Types should be declared using a machine-readable type definition, that can be understood by humans. 6. Maybe I can take a look at GIMP PDB for code and ideas, but probably not much. From my impression it's more of an ad-hoc system for the GIMP. 7. A procedure can restrict the types that pass to it, but I'm not sure to what extent. 8. I'll have to translate between conventions of different languages inside each language back-end. 9. It's first implementation will probably not have all the functionality I'd like it to have, but better to release early and release often. Your ideas and comments are most welcome. At the moment Aristo is at the pre-coding stage. (I'm just thinking about its design). But I'd rather have something well-designed and well-scrutinized in mind before I start actual coding. Regards, Shlomi Fish ---------------------------------------------------------------------- Shlomi Fish shlomif(a)vipe.technion.ac.il Home Page: http://t2.technion.ac.il/~shlomif/ He who re-invents the wheel, understands much better how a wheel works.

1 0

Ambiguous defaults
by Ferenc Wagner 08 Jan '03

08 Jan '03

Hello, What's the way to express the following: a compound object is generally made up of two components with identical type. For this common case I'd like to provide some default methods, which take the object apart, operate on the parts and put the results back together. In other cases this default method should be overridden, eg. when the compound object isn't made of two identical components. My first try (below) doesn't work, can you recommend a solution? Thanks: Feri. \begin{code} module Test where class Component b where property :: b -> Int class Compound a where decompose :: Component b => a -> (b,b) additive :: a -> Int additive x = property l + property r where (l,r) = decompose x \end{code} ghci: test.hs:9: Ambiguous type variable(s) `b' in the constraint `Component b' arising from use of `property' at test.hs:9 hugs: ERROR test.hs:9 - Cannot justify constraints in default member binding *** Expression : additive *** Type : Compound a => a -> Int *** Given context : Compound a *** Constraints : Component b

3 2

RE: Space explosion
by Simon Marlow 06 Jan '03

06 Jan '03

> > -- Specs contains the options for the simulation; > rollBounds simply > finds the lowest and highest possible rolls > > -- type RollValue = Int > > -- type RollValueDist = Array RollValue Int > > probDist :: Specs -> [RollValue] -> RollValueDist > > probDist specs rolls = > > accumArray (+) 0 (rollBounds specs) [(r, 1) | r <- rolls] > > It should be doing an in-place array update as it evaluates the > comprehension, but apparently something's being kept in memory longer > than it has an excuse for. It does do a sequence of in-place updates, but this is another instance of the foldl space leak. The additions aren't being performed strictly, so the array is filling up with suspensions. For example: accumArray (+) 0 (1,10) [(3,1), (3,1), (3,1), ...] would produce this sequence of arrays as accumArray processed its input list: { (3,1), ... } { (3,1+1), ... } { (3,1+1+1), ... } we should really have a strict version of accumArray (the lazy version *is* useful sometimes: you can refer recursively to the final array, for example). A quick workaround in your case (GHC only, I'm afraid) is to use UArray instead, available from Data.Array.Unboxed. These arrays contain unboxed values and are therefore strict by definition; and you can use all the usual array operations including accumArray. This workaround only applies if you're using one of the types supported by UArray though (Int, Char, and other base types). Cheers, Simon

1 0

Using SGML/XML in literate code
by Peter Simons 06 Jan '03

06 Jan '03

I understand that I can mark-up code segments in a literate source module using the syntax: \begin{code} ... \end{code} But is it somehow possible to re-define this to be <haskellcode> ... </haskellcode> instead? I am asking because the latter style would enable me to write literate programs in DocBook, HTML, or any other SGML-based mark-up format. And _that_ would be very useful indeed. :-) -peter

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Re: Web Application Development
by Christopher Milton 06 Jan '03

06 Jan '03

--- Dominic Steinitz <dominic.steinitz(a)blueyonder.co.uk> wrote: [snip] > The original code was in PHP and MySQL so rather than re-invent the wheel I [snip] > So my question is: what should I do to re-write this in Haskell? I haven't tried it out, but I found this project on SourceForge: http://sourceforge.net/projects/functional-php/ on to the Haskell stuff... > From the discussion on this mailing list in June and reading through the > libraries section of haskell.org, I think there are the following options: > > HTML > > 1. Use Wash/CGI but maybe somehow move to mod_haskell if the webserver > performance isn't adquate. HaXml could also be quite useful for generating HTML or XML: http://www.cs.york.ac.uk/fp/HaXml/ > SQL > > 2. Use MySQL-HS which requires HaskellDirect or use HSQL. > > I'd be very grateful for any advice and any progress as a result of the > discussion in June. Ashley Yakeley has an interface to MySQL: http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/hbase/Source/Foreign/ and I was thinking of extending the DBI model as found in Perl and the libpq binding as listed on Haskell.org: http://www.astercity.net/~khaliff/haskell/dbi/dbi.tgz Searching Freshmeat and SourceForge for Haskell turns up a few things. Actually, there's quite a bit of alpha and beta Haskell code out on the net, but we don't seem to get very far with much of it... ===== Christopher Milton cmiltonperl(a)yahoo.com __________________________________________________ Do you Yahoo!? Yahoo! Mail Plus - Powerful. Affordable. Sign up now. http://mailplus.yahoo.com

1 0

Editor Tab Expansion
by Jim Greer 03 Jan '03

03 Jan '03

Greets - [ Better late than never. ] Ran across this thread while perusing list trying to figure out whether to join or not. In the interest of sharing info with the great collective unconscious (i.e. Internet), thought I'd include part of my .vimrc so that Vim users (and others considering the same) can see how it handles the embedded tab/whitespace layout problems of various languages: set background=dark syntax on filetype on set textwidth=72 autocmd Filetype haskell set ai autocmd Filetype haskell set expandtab autocmd Filetype haskell set tabstop=8 autocmd Filetype haskell set shiftwidth=8 autocmd Filetype mail set ai autocmd Filetype mail set tabstop=4 autocmd Filetype mail set shiftwidth=4 autocmd Filetype ruby set ai autocmd Filetype ruby set tabstop=2 autocmd Filetype ruby set shiftwidth=2 A complete ('enhanced') install of Vim includes syntax recognition for a great number of file formats, including Haskell (apparently, haven't done much yet). Setting syntax on manages the syntax recognition part, so that you get colorized syntax. Setting filetype on recognizes files (by extension?), so that you can use the autocmd FileType haskell .... commands to tailor settings for particular filetypes. In this case, I'm using expandtab and setting the tab settings to 8, as mentioned previously. Cordially, Jim G p.s. http://www.vim.org/ p.p.s. I'm suprised that the *macs crowd hasn't chimed in with ways to configure their editos^Hr to set tabs differing by day of week, phase of moon, etc. -- James B. Greer <jgreer(a)midsouth.rr.com> [expires: 2004-12-27] 7F49 3D20 03C6 E859 E4F9 0735 E1F9 B259 21FA 1494

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Firmalar İçin En Verimli ve Karlı TİCARET PORTALI Açıldı...
by Neslihan ÇETİN 03 Jan '03

03 Jan '03

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RE: Parsing date and time specifications
by Simon Marlow 03 Jan '03

03 Jan '03

> I think it is widely agreed that the time and date structures in the > standard libraries are brok^H^H^H^Hslightly less than useful. > > I suggest you feel free to rewrite it as you see fit, and then lobby > for its inclusion in the hierarchical libraries. > > (My preju^H^Hference would be to store a date-time internally in a > posix-like manner (seconds,microsecond since the epoch).) But a ClockTime *is* implemented as (seconds,picoseconds) since the epoch. CalendarTime is just a human-readable interpretation of the ClockTime, or a way to synthesise a ClockTime from a known date/time. (BTW Peter: the weekday/yearday in a CalendarTime are ignored by the toClockTime in GHC's Time library, so you can set them to anything). I'm still not sure I understand why the Time library is considered to be broken; sure I can see that our implementation of addToClockTime is wrong (adding one minute shouldn't always be the same as adding 60 seconds, for example), but is the design fundamentally wrong? GHC's TimeExts library (package lang) is allegedly a better implementation of addToClockTime/diffClockTime, if anyone is looking for one. Cheers, Simon

5 4

Sets
by Mark T.B. Carroll 02 Jan '03

02 Jan '03

Am I right in thinking that sets of sets don't really work with GHC's Data.Set because "Set a" things aren't instances of Ord? How should I do sets of sets with it? (e.g. Set.mkSet [Set.unitSet 1] gives "No instance for (Ord (Set a))") -- Mark

3 2

Lazy streams and unsafeInterleaveIO
by Jyrinx 02 Jan '03

02 Jan '03

As an experiment for a bigger project, I cooked up a simple program: It asks for integers interactively, and after each input, it spits out the running total. The wrinkle is that the function for calculating the total should be a non-monadic stream function (that is, type [Integer] -> [Integer] so that runningTotals [1,2,3,4,5] == [1,3,6,10,15]). The task is then to write a function to return a stream of integers, grabbing them from IO-land lazily (a la getContents). My first attempts had it not displaying a running total until all input (terminated by an input of 0) had finished, at which point it spit out all the totals (i.e. it wasn't an interactive program anymore). I poked around in the docs and on the Web for a while, and found out about unsafeInterleaveIO, which solved the problem neatly (after I modified runningTotals to be less eager, as it was reading ahead by an extra integer each time). I ended up with the attached code (for GHC 5.04.2). My question is this: Is there a more elegant (i.e. non-"unsafe") way to do this? I vaguely recall from the Hudak book (which I unfortunately don't have convenient at the moment) that he used a channel for something like this (the interactive graphics stuff), but IIRC his system would be overkill for my application (including the bigger project). It doesn't seem like it should need any black magic, and concurrency (which channels need, right?) doesn't appear worth the hassle. Really, my desire comes down to a simple, safe, single-threaded way to write a function to generate a lazy stream. Is there such? Luke Maurer jyrinx_list(a)mindspring.com -- running-total -- Haskell program that takes integers as input, outputting a running total -- after each input -- Demonstrates use of lazy streams module Main where import IO import System.IO.Unsafe import Monad runningTotals :: [Integer] -> [Integer] runningTotals [] = [] runningTotals (x:xs) = rt' 0 (x:xs) where rt' tot (x:xs) = (tot+x) `seq` (tot+x):(rt' (tot+x) xs) rt' _ [] = [] -- Note that runningTotals does what appears to be a stateful calculation when -- numbers are read one at a time; however, lazy streams allow this to be a -- pure function. Haskell is cool. inputNumbers :: IO [Integer] inputNumbers = do x <- putStr "? " >> readLn if x == 0 then return [] else do xs <- (unsafeInterleaveIO inputNumbers) return (x:xs) main = do numbers <- inputNumbers mapM_ (putStrLn . (flip shows) "") (runningTotals numbers)

6 11

call china 6.2¢ www.callez.com
by promotion＠callez.com 31 Dec '02

31 Dec '02

1 0

RE: AW: slide: useful function?
by Mark Carroll 27 Dec '02

27 Dec '02

On Mon, 2 Dec 2002, David Bergman wrote: (snip) > Till then, we "Haskellers" will probably continue expressing our > patterns either directly in Haskell or using highly formal language, > with terms such as "catamorphisms". > > The virtue, and weakness, of traditional design patterns is their > vagueness and informal character, making them (1) comprehensible to the > 90% of the developer community not familiar with category theory but (2) (snip) If there are any good ways in which non-mathematicians can get to grips with these terms from category theory, they would be well worth promoting. For example, despite having a good computer science degree (in which I was at least introduced to FP, proof, etc. and even learned to draw the dual graph of hypercubes) I'm really not equipped to understand catamorphisms in terms of algebras and homomorphisms, and don't currently have time to take the math degree I fear I'd need in order to do so. Last time I was looking at category theory books I think I came to the conclusion that Lawvere and Schanuel cover things kindly but Pierce seemed to get the syllabus right, so the "right" book wasn't quite out there. My understanding of monads is already a matter of record. Does anyone know of a friendly text that might help new Haskellers to understand functors, etc. and what they mean for program design? I'm not averse to the formal language per se if it can be easily acquired; right now, I worry that I'm using Haskell suboptimally because, not only do I not know the terminology well, but I fear that I'm not even cognisant of the concepts that these terms represent. In a nutshell: if these category theory concepts indeed have an important impact in Haskell land, how to introduce them to working Haskell programmers well enough that they can use them in engineering software that's at least half as good as it could be? (I'm making the assumption here that it would be good for Haskell to be much more widely used - it shouldn't solely be for researchers.) -- Mark

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RE: Record of STRefs better than STRef to a Record?
by Simon Marlow 27 Dec '02

27 Dec '02

> If I use an STRef to a record, will a new record be created > each time I want > to update a single field? Or can I expect GHC to optimize it > and have the field of the record updated in place? You'll get a new record for each update. This might not be so bad though, depending on the number of fields in your record. > Right now I'm using a record of STRefs, like: > data E s = E{ > refi :: STRef s Int, > refc :: STRef s Char > } > > but it can get a little messy, since thread s propagates to > every datatype that uses the record type in it's definition. Here's another trick if you use this route: data E s = E{ refi :: !STRef s Int, refc :: !STRef s Char } and compile with -funbox-strict-fields. This will eliminate the boxing of the STRefs. Cheers, Simon

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by G��MENL�K/ABD 27 Dec '02

27 Dec '02

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RE: Parsing date and time specifications
by Simon Marlow 26 Dec '02

26 Dec '02

> I was probably a bit quick on the trigger there. Sorry! > > It't been a while since I tried using CalendarTime and friends; I did > have some difficulty making things fit, and eventually gave up the > whole thing. My impression (which may well be a wrong one) was that > others also had trouble adapting the library to their uses, too, and > that the Time library was one of the less successful ones. That may well be true, and I've heard others suggest that the library is inconvenient. I'm trying to get at whether that is due to current bugs in the implementation, or whether the design is fundamentally broken. > Looking at it now, possible criticisms could arise from the data types > containing redundancy and non-uniqueness: e.g. one can easily > construct non-consistent CalendarTimes, Ok, I don't consider that to be a problem. The toClockTime operation should fail if the CalendarTime is inconsistent. Non-uniqueness isn't a problem: you can convert to a ClockTime which is unique. I think of the CalendarTime as a slightly more tractable representation of a date/time string; lots of date strings don't make sense either (eg. "31 February"). > and there are multiple ways > to represent the same interval as TimeDiff -- in fact, two TimeDiffs > may or may not be equal, depending on what CalendarTime they apply > to. (The 'deriving' of Eq makes this a moot point, perhaps, but > seems to make equality rather non-intuitive) I think this is the wrong way to look at it: two TimeDiffs should be equal if and only if all their components are respectively equal (i.e. the deriving Eq meaning). This is different from saying that "the effect of adding this TimeDiff is the same as adding this other TimeDiff", which depends entirely on what you're adding it to. So this cannot lead to a sensible meaning for Eq on TimeDiff. Ah - I've just realised one problem with the design. If you have a TimeDiff with multiple non-zero components (eg. one hour and one second), then the order in which you add them makes a difference, and this order isn't specified by the spec. That's probably why TimeExts has only single-unit differences. And another thing: it should be specified that diffClockTimes gives a TimeDiff in units of seconds & picoseconds, because these are the only forms of TimeDiff that have the same meaning independent of what ClockTime they are added to. > It seems that TimeDiffs perform two functions; deltas for ClockTimes > (in the functions) and intervals of calendar times (specification of > the data type), and I'm not sure it's a good idea to mix those. I think TimeDiffs are deltas for ClockTimes only. Why do you say they are intervals of calendar times too? If we were to have an operation which adds a TimeDiff to a CalendarTime, then its meaning should be the same as converting the CalendarTime to a ClockTime, adding the TimeDiff, and converting back to a CalendarTime again. > Oh well. I'm not using Time at the moment, so its not an issue that > will ruin my holiday for me :-) me either :) Cheers, Simon

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Re: Interpret haskell within haskell.
by Christopher Milton 26 Dec '02

26 Dec '02

--- David Sankel <camio(a)yahoo.com> wrote: > I was wondering if there is any project that aims to > interpret haskell within haskell. http://www.haskell.org/implementations.html <quote type="partial"> GHC, the Glasgow Haskell Compiler The Glasgow Haskell compiler is a full implementation of Haskell. It is itself written in Haskell and is designed to act as a substrate for the research work of others. The source code is freely available. It produces fast code. </quote> The GHC interpreter is ghci. (It's not as slow anymore.) http://www.haskell.org/ghc/ The other Haskell interpreters also load and interpret users' Haskell source code, as well. If you have defined functions in "myprog.hs": :load myprog.hs then the functions defined in the file are available, or else you'll get error message(s) about problems found parsing "myprog.hs". __________________________________________________ Do you Yahoo!? Yahoo! Mail Plus - Powerful. Affordable. Sign up now. http://mailplus.yahoo.com

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Space explosion
by Jyrinx 26 Dec '02

26 Dec '02

I'm tinkering with a little program I wrote a while back to generate probability distributions and averages for rolling various kinds and quanitities of dice and applying various rules (for instance, roll four six-sided dice, drop the lowest, and add the other three). I'm trying to optimize the thing, and I've run into a space leak. I've found that, if I tell it to roll four 20-sided dice and add, it finishes in under a second and heap usage peaks at under 2MB (not sterling, but not catastrophic); but if I make it five 20-sided dice, it runs out of stack space after thrashing for a while (and getting partway through the final output); for six dice, it thrashes into oblivion. (Running WinXP with 256MB memory.) What puzzles me is that, according to GHC's heap-usage-by-call-center graph, the hog isn't the code that generates every single outcome of rolling the dice (and we're talking about four 20-sided dice here ...), whose heap usage is constant (go GHC! :-) ), but the one that turns the list of outcomes (for instance, [2,3,3,4,4,4 ... 7,7,7,7,7,7 ... 11,11,12] for adding two 6-sided dice) into a probability distribution (like [(2,1),(3,2),(4,3) ... (7,6) ... (11,2),(12,1)]). It is this: > -- Specs contains the options for the simulation; rollBounds simply finds the lowest and highest possible rolls > -- type RollValue = Int > -- type RollValueDist = Array RollValue Int > probDist :: Specs -> [RollValue] -> RollValueDist > probDist specs rolls = > accumArray (+) 0 (rollBounds specs) [(r, 1) | r <- rolls] It should be doing an in-place array update as it evaluates the comprehension, but apparently something's being kept in memory longer than it has an excuse for. I tried another, more hand-coded version, to get rid of the mid-calculation comprehension and some possible overhead: > probDist specs = > foldl update orig > where orig = array bounds [(r, 0) | r <- range bounds] > update dist r = dist // [(r, 1 + dist!r)] > bounds = rollBounds specs But this one was even worse. I've sprinkled $! around quite liberally, to no avail; also, I'm compiling with -O, and I've tried various RTS options with no radical change in behavior. What's going on here? The array constructed has to be big, but not *that* big (77 Ints for four 20-sided) ... Is there an inherent inefficiency with GHC's arrays when used like this? I can't come up with a better algorithm - making a zeroed array with an element for each outcome, then adding 1 to the element for each outcome as it is read in seems straightforward enough. (Would a FiniteMap be any better? I'm not sure how ...) Thanks for any help ... I'm starting to get the hang of Haskell, but some of these semantic subtleties are driving me nutty ... Luke Maurer jyrinx_list(a)mindspring.com

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Re: Interpret haskell within haskell.
by oleg＠pobox.com 24 Dec '02

24 Dec '02

Lauri Alanko wrote on Dec 22: > The magic is _here_: > (begin (define a 5) (eval '(set! a (+ a 1)) (interaction-environment)) a) > ==> 6 > Here (interaction-enviroment) is a run-time representation of the > compile-time environment. It makes possible two-way interaction between the > stages. Essentially, first-class environments are all the magic you need to > implement eval. I'm sorry to act as a Grinch, but there is no magic. First, according to R5RS, (interaction-environment) is not a first-class environment. R5RS says (Section 6.5): > [[optional procedure]] (interaction-environment) > This procedure returns a specifier for the environment that contains > implementation-defined bindings, typically a superset of those listed > in the report. The intent is that this procedure will return the > environment in which the implementation would evaluate expressions > dynamically typed by the user. There is nothing there about interaction between phases. And in fact, this interaction does not work. If you put the following code (module aaa) (begin (define a 5) (eval '(set! a (+ a 1)) (interaction-environment)) (display a) (newline)) into a file, _compile_ that file using Bigloo, and run the resulting a.out, you will see a message *** ERROR:bigloo:eval: Unbound variable -- a Nowhere the Scheme report talks about first-class environments. Please note that the result of (interaction-environment) is a _specifier_ for an environment. Some Scheme systems do provide first-class environments. For example, MIT Scheme has a form 'the-environment'. It comes at a price. In a post "Re: why isn't the environment explicit ?" on comp.lang.scheme on Mar 15, 2001, Joe Marshall wrote: > > Since you can (sort of) regard call/cc as making continuations > > explicit, why aren't environments explict ? > In MIT Scheme, they are. The special form (the-environment) returns > the current environment as a first-class object. > However. > The difficulty with using such a form is that it destroys any > possibility of compiler optimization. If you compile code with a > (the-environment) form in it (in MIT Scheme), the compiler essentially > throws up its hands and inserts code to call the interpreter. There is no Santa either.

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Re: Interpret haskell within haskell.
by Jonathan.C.Cast-1 23 Dec '02

23 Dec '02

----- Original Message ----- From: Matt Hellige <matt(a)immute.net> Date: Friday, December 20, 2002 0:20 am Subject: Re: Interpret haskell within haskell. > In any case, here are my guesses at why Haskell doesn't have "eval": > - It is difficult to make the kind of run-time safety guarantees > that we are used to with Haskell/ML if we introduce constructs of > this kind. > - It would seem to require considerable typing information to be > carried around at run-time, which in general Haskell compilers > seem to rely on being able to avoid. More generally, it may > render unsafe a wide range of optimizing program transformations. > - It would require the run-time for potentially every program to be > considerably larger, carrying around an entire interpreter. > - It has not, historically, been a priority for the community. The > FP community has tended to focus more on safety and static > analysis, as well as efficiency of compiled output. The general > answer has > been, "Well, if you want that, you should be using > lisp/scheme/etc." > Are those reasons basically right, basically wrong, somewhere in the > middle? If it's actually mostly the final reason, I wonder if the idea > deserves more consideration. Has the idea been revisited in light of > monadic control flow, type classes and other relatively recent > developments? Would it be possible, in this case, to have our cake > and eat it too? Are you trying to argue here that monads, type classes, etc. remove the difficulties you list? Otherwise, it seems to me that supporting eval would remove just about every advantage Haskell has over imperative languages. > Matt > > -- > Matt Hellige matt(a)immute.net > http://matt.immute.net Jon Cast

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Re: Interpret haskell within haskell.
by oleg＠pobox.com 21 Dec '02

21 Dec '02

David J. Sankel wrote: > I was referring to a haskell interpreter to be used > within haskell code. For instance: > main = do > user_configuration <- parseHaskell > title <- resolveFunction user_configuration "title" > :: String > putStr title This was exactly the gist of the message: http://www.haskell.org/pipermail/haskell-cafe/2002-September/003423.html The message gave the working example. The end of the message posed that GHCi is better for these kinds of things. Simon Marlow's message shows that this is indeed the case.

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