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RE: Newbern's Example 7
by Alson Kemp 13 Mar '05

13 Mar '05

Jeff, Perfect explanation. I got gummed up in the syntactic sugar and thought that >>= was sugar for the "do" notation, not vice versa. Thank you for the reminder and clarification. - Alson

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Solution to Thompson's Exercise 4.4
by Kaoru Hosokawa 13 Mar '05

13 Mar '05

I have been working through the exercises in Thompson's The Craft of Functional Programming 2nd Ed book. I am looking for a solution web site for Thompson's book. Or maybe the people here can help. In exercise 4.4, I am asked to define a function howManyOfFourEqual :: Int -> Int -> Int -> Int -> Int which returns the number of integers that are equal to each other. For example, howManyOfFourEqual 1 1 1 1 = 4 howManyOfFourEqual 1 2 3 1 = 2 howManyOfFourEqual 1 2 3 4 = 0 This is my solution. I give it here, since it's not an elegant solution. howManyOfFourEqual :: Int -> Int -> Int -> Int -> Int howManyOfFourEqual a b c d | a == b && howManyEqual b c d /= 0 = howManyEqual b c d + 1 | a == c && howManyEqual b c d /= 0 = howManyEqual b c d + 1 | a == d && howManyEqual b c d /= 0 = howManyEqual b c d + 1 | a == b && howManyEqual b c d == 0 = 2 | a == c && howManyEqual b c d == 0 = 2 | a == d && howManyEqual b c d == 0 = 2 | otherwise = howManyEqual b c d howManyEqual is a function from a previous exercise. howManyEqual :: Int -> Int -> Int -> Int howManyEqual a b c | a == b && b == c = 3 | a /= b && b /= c && a /= c = 0 | otherwise = 2 I hope to find a better solution. I googled but couldn't find the answer. Kaoru Hosokawa khosokawa(a)gmail.com

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RE: Newbern's Example 7
by Alson Kemp 13 Mar '05

13 Mar '05

Jeff, Thank you for your reply. >> After fn is lifted, where is "bind" used in allCombinations? >The bind happens inside the liftM2 function, which can be defined as >(taken from http://www.haskell.org/onlinereport/monad.html) >liftM2 f = \a b -> do { a' <- a; b' <- b; return (f a' b') } Using the above, I think that allCombinations can be rewritten: allCombinations fn (l:ls) = foldl (\a b -> do { a' <- a; b' <- b; return (fn a' b') }) l ls I'm not sure how "allCombinations (+) [[1,2][2,3]]" gets to the final result, since the above would seem to yield: allCombinations (+) [1,2]:[[2,3]] = foldl (\a b -> do { a' <- a; b' <- b; return (fn a' b') }) [1,2] [[2,3]] allCombinations (+) [1,2]:[[2,3]] = return ((+) [1,2] [2,3]) You mentioned on your site that the operation of the List monad is key to comprehending this function, but I still wasn't able to grok it. Any pointers would be appreciated. - Alson

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hFileSize vs length
by S. Alexander Jacobson 12 Mar '05

12 Mar '05

I am using GHC 6.2 on windows and am finding that when I open a file and use hFileSize I get a different number than I get from reading in the file and calculating the length. I assume this is not a bug, but I don't know why its happening. Also, why isn't there getFileSize function in System.Directory? -Alex- ______________________________________________________________ S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com

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fgl and Windows
by Tom Spencer 11 Mar '05

11 Mar '05

Does anyone have a build.bat that they have used to install fgl on windows that they could donate to me? Thanks Tom Spencer

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Newbern's Example 7
by Alson Kemp 11 Mar '05

11 Mar '05

All, I'm having substantial difficulty understanding Jeff Newbern's Example 7. Would love some help. My questions are: How does liftM2 know to lift fn into the _List_ monad? (Because it's the only possibility?) After fn is lifted, where is "bind" used in allCombinations? Basically, I'd some a detailed explanation of how allCombinations works, cause I'm not grokking it... - Alson --- import Monad -- allCombinations returns a list containing the result of -- folding the binary operator through all combinations -- of elements of the given lists -- For example, allCombinations (+) [[0,1],[1,2,3]] would be -- [0+1,0+2,0+3,1+1,1+2,1+3], or [1,2,3,2,3,4] -- and allCombinations (*) [[0,1],[1,2],[3,5]] would be -- [0*1*3,0*1*5,0*2*3,0*2*5,1*1*3,1*1*5,1*2*3,1*2*5], or [0,0,0,0,3,5,6,10] allCombinations :: (a -> a -> a) -> [[a]] -> [a] allCombinations fn [] = [] allCombinations fn (l:ls) = foldl (liftM2 fn) l ls -- print an example showExample :: (Show a) => String -> (a -> a -> a) -> [[a]] -> IO () showExample opName op ls = do putStrLn $ "opName over " ++ (show ls) ++ " = " putStrLn $ " " ++ (show (allCombinations op ls)) -- shows a few examples of using allCombinations main :: IO () main = do showExample "+" (+) [[0,1],[1,2,3]] showExample "*" (*) [[0,1],[1,2],[3,5]] showExample "/" div [[100, 45, 365], [3, 5], [2, 4], [2]]

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Joy Combinators (Occurs check: infinite type)
by Greg Buchholz 11 Mar '05

11 Mar '05

Recently, while investigating the (non-statically typed) stack-based functional language Joy (http://www.latrobe.edu.au/philosophy/phimvt/joy.html) I became interested in seeing if I could implement Joy's combinators in Haskell. I started with a stack based implementation using nested tuples as the stack. Things worked out well until I got to the recursion combinators. Even then, things didn't seem that bad, since Haskell was able to infer types for the the linrec and genrec combinators. Trouble popped up when I tried to apply the recursion operators to a function (in this case calculating the humble factorial). In the code below, fact3,4 & 5 all fail with an "Occurs check" that looks something like the following... joy3.hs:24: Occurs check: cannot construct the infinite type: t = (a, t) Expected type: ((a5 -> (a, (a, t)), a5) -> (a, t), ((a1, t3) -> (a1, (a1, t3)), ((a2, t2) -> (a2, t2), ((a3, t1) -> (Bool, t1),a4)))) -> c Inferred type: ((a5 -> (a, (a, t)), a5) -> (a, (a, t)), (a5 -> a5, (a5 -> (a, (a, t)), (a5 -> (Bool, b),a5)))) -> (a, (a, t)) ...Normally, I would have given up and declared that Joy wasn't an easily typed language, but I'm motivated to dig further because if you remove the type signature from "fact" below, it also fails with an "Occurs check". So, can anyone direct me towards more information on exactly what an "Occurs check" is, or does anyone see an easy fix to my problems? Thanks, Greg Buchholz P.S. The first thing you should note about the code below is the definition of the reverse composition operator (!), which I used to give the program a more Joy-like feel. (i.e. (!) f g = g.f) >--Joy combinators in Haskell > >main = do print $ ((lit 6) ! fact) bot -- factorial of 6 > print $ ((lit 2) ! square ! fact2) bot -- factorial of 4 > >bot = "EOS" -- end of stack >square = dup ! mult >cube = dup ! dup ! mult ! mult > >--In Joy: factorial == [0 =] [pop 1] [dup 1 - factorial *] ifte >fact :: (Integer, a) -> (Integer, a) >fact = (quote ((lit 0) ! eq)) ! > (quote (pop ! (lit 1))) ! > (quote (dup ! (lit 1) ! sub ! fact ! mult)) ! > ifte > >--In Joy: [1 1] dip [dup [*] dip succ] times pop >fact2 = (quote ((lit 1) ! (lit 1))) > ! dip ! (quote (dup ! (quote mult) ! dip ! suc)) > ! times ! pop > >{- fact3,4 & 5 don't type check, fails with... >-- Occurs check: cannot construct the infinite type: > >--In Joy: [null] [succ] [dup pred] [i *] genrec >fact3 :: (Integer, a) -> (Integer, a) >fact3 = (quote nul) ! (quote suc) ! (quote (dup ! pre)) ! (quote (i ! mult)) > ! genrec > >fact4 :: (Integer, a) -> (Integer, a) >fact4 = genrec.quote(mult.i).quote(pre.dup).quote(suc).quote(nul) > >--In Joy: [null] [succ] [dup pred] [*] linrec >fact5 :: (Integer, a) -> (Integer, a) >fact5 = quote(nul) ! quote(suc) ! quote(dup ! pre) ! quote(mult) ! linrec > >--} >nul = (lit 0) ! eq >suc = (lit 1) ! add >pre = (lit 1) ! sub > >linrec (rec2, (rec1, (t, (p, stack)))) > | fst (p stack) == True = (t stack) > | otherwise = rec2 (linrec (rec2, (rec1, (t, (p, (rec1 stack)))))) > >genrec (rec2, (rec1, (t, (b, stack)))) > | fst (b stack) == True = (t stack) > | otherwise = (rec2.quote(genrec.quote(rec2).quote(rec1). > quote(t).quote(b))) (rec1 stack) > >times (p, (n, stack)) | n>0 = times (p, (n-1, (p stack))) > | otherwise = stack > >(!) f g = g.f >i (a, b) = (a b) >add (a, (b, c)) = (b+a, c) >sub (a, (b, c)) = (b-a, c) >mult (a, (b, c)) = (b*a, c) >swap (a, (b, c)) = (b, (a, c)) >pop (a, b) = b >dup (a, b) = (a, (a, b)) >dip (a, (b, c)) = (b, (a c)) >eq (a, (b, c)) | a == b = (True, c) > | otherwise = (False,c) > >ifte (f, (t, (b, stack))) | fst (b stack) == True = (t stack) > | otherwise = (f stack) > >lit val stack = (val, stack) -- push literals on the stack >quote = lit >

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Card trick
by Mark Carroll 10 Mar '05

10 Mar '05

Having heard about an interesting card trick, I thought I'd try implementing it in Haskell. With luck, I didn't make any mistakes. I thought it was cool enough to be worth sharing with you guys. -- Mark

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Problem with Haddock
by Daniel Fischer 08 Mar '05

08 Mar '05

Hi, I have the problem that I can't haddock literate modules (latex-style), > haddock -h -o docs Test.lhs Test.lhs:1:1: Parse error the file begins with "\begin{code}". Does anybody know what's up and what to do about it? Furthermore, in the documentation of ghc-6.4.2005..., only four modules from base/GHC are included, in the documentation for ghc-6.2.2 there were 29. In particular, the absence of GHC.List, GHC.Show and GHC.Read is extremely annoying. Any suggestions how to remedy this? Thanks in advance, Daniel

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RE: [Haskell-cafe] Problem with Haddock
by Simon Marlow 08 Mar '05

08 Mar '05

On 08 March 2005 15:26, Daniel Fischer wrote: > I have the problem that I can't haddock literate modules > (latex-style), >> haddock -h -o docs Test.lhs > Test.lhs:1:1: Parse error > > the file begins with "\begin{code}". > > Does anybody know what's up and what to do about it? Haddock doesn't understand literate files on its own. The documentation describes how to get around this... er, well the documentation in CVS describes how to get around this, but the 0.6 docs on the web don't. Here's the relevant text: <section id="cpp"> <title>Using literate or pre-processed source</title> <para>Haddock only accepts plain, non-literate, Haskell source. This means that if you program in Literate Haskell, or you need to use the C pre-processor in your Haskell source, then you need to pre-process the files before feeding them to Haddock. This is easily accomplished using GHC; for example, suppose we have a Literate Haskell source file <filename>Foo.lhs</filename>, on which we also need to run the C pre-processor:</para> <screen> $ ghc -cpp -E -optP-P -D__HADDOCK__ Foo.lhs -o Foo.hs $ haddock -h Foo.hs ... </screen> <para>The <option>-E</option> option to GHC says "stop after pre-processing", the <option>-cpp</option> option turns on the C pre-processor, the <option>-optP-P</option> option tells the C pre-processor not to leave any extra dropping behind (see the description of the <option>-P</option> option in the gcc manual for details), and the <option>-D__HADDOCK__</option> option defines the symbol <literal>__HADDOCK__</literal> when pre-processing (this is sometimes handy if you need to any pre-processing conditionals in your source which depend on whether the source is going to be fed to Haddock).</para> </section> Cheers, Simon

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tuples and Show in GHC
by Stijn De Saeger 07 Mar '05

07 Mar '05

hi all, Is there a principled reason why in GHC tuples with up to five elements automatically derive from the Show class but from six elements and up they don't anymore? If this is not a bug I would be very curious to hear what the reasoning behind this is.... below is an example of said behaviour : > type MyInt1 = (Int,Int,Int,Int,Int) > myInt1 :: MyInt1 > myInt1 = (1,2,3,4,5) > type MyInt2 = (Int,Int,Int,Int,Int,Int) > myInt2 :: MyInt2 > myInt2 = (1,2,3,4,5,6) .... *HS> myInt1 Loading package haskell98 ... linking ... done. (1,2,3,4,5) *HS> myInt2 <interactive>:1: No instance for (Show MyInt2) arising from use of `print' at <interactive>:1 In a 'do' expression: print it *HS> stijn.

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RE: [Haskell-cafe] tuples and Show in GHC
by Simon Peyton-Jones 07 Mar '05

07 Mar '05

No principled reason. I wish there was no limit, which would involve generating code on the fly for instances that weren't pre-generated, but I never got around to implementing that. So you have to stop somewhere. Actually the Report says it should have Show instances up to 15 at least, but GHC doesn't. That isn't deliberate, and I guess we should fix it. Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Stijn | De Saeger | Sent: 06 March 2005 09:40 | To: haskell-cafe(a)haskell.org | Subject: [Haskell-cafe] tuples and Show in GHC | | hi all, | | Is there a principled reason why in GHC tuples with up to five | elements automatically derive from the Show class but from six | elements and up they don't anymore? If this is not a bug I would be | very curious to hear what the reasoning behind this is.... | below is an example of said behaviour : | | > type MyInt1 = (Int,Int,Int,Int,Int) | > myInt1 :: MyInt1 | > myInt1 = (1,2,3,4,5) | > type MyInt2 = (Int,Int,Int,Int,Int,Int) | > myInt2 :: MyInt2 | > myInt2 = (1,2,3,4,5,6) | | .... | | *HS> myInt1 | Loading package haskell98 ... linking ... done. | (1,2,3,4,5) | *HS> myInt2 | | <interactive>:1: | No instance for (Show MyInt2) | arising from use of `print' at <interactive>:1 | In a 'do' expression: print it | *HS> | | stijn. | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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ANN: HAppS -- Haskell Application Server v 0.3
by S. Alexander Jacobson 04 Mar '05

04 Mar '05

HAppS is a Haskell library for building Internet applications, featuring: * HAppS.ACID: Guarantee application integrity in the face of unplanned outages using this module's integrated write-ahead logging and checkpointing framework. * HAppS.DBMS: Do relational operations in Haskell (rather than SQL) on Haskell sets (outside the IO Monad). Define custom indices for your Haskell datatypes (e.g. geographic/geometric types). Use in combination with ACID for a robust relational DBMS customized for your application. * HAppS.Protocols: Expose your application using as an HTTP server and/or by recieving and sending SMTP. Changes since 0.2: HAppS Changes * Much cleaner interface for ACID transaction handlers * DBMS operators that look more like HaskellDB operators * SMTP send errors are exposed for use in future reliable deliver implementation * ACID callbacks disabled. Framework in place for total order on callbacks. * SideEffect queues are now algrebraic types rather than strings. * Parametrized HTTP using XML ToElement and HTTP ToMessage FromMessage and ToURI classes (you write transaction handlers in terms of Haskell types and the system takes care of exposing them for HTTP) * Parametrized HTTP response now has type (ToURL url,ToMessage msg)=>Response url message Framework takes care of automatically translating relative URLs to absolute URLs. Answers to questions a few people have asked: * Compare HAppS.DBMS with HaskellDB HaskellDB is really optimal if you want to talk to external databases. HAppS.DBMS is really good if you want to do relational operations on Haskell Sets without going to the IO monad. * Compare HAppS.ACID to STM Use STM if all your IO operations have a two-phase-commit interface. Use HAppS.ACID if your IO interfaces are not transactional. Instead HAppS.ACID gives you multi-queue, at-least-once execution of side-effects, and a total order on their callbacks. There is no rollback semantic because you can't unsend email. * Compare HAppS.Protocols.SMTP to Peter Simon's Postmaster Postmaster is trying to be a full fledged MTA. SMTP is trying simply to give you SMTP receive/send functionality in your applications. Neither yet has a queuehandler. We both hope to merge lessons learned soon. * Compare HAppS.Protocols.HTTP to Warrick Gray's Network.HTTP module Network.HTTP is more oriented towards building HTTP clients. HAppS..HTTP is more oriented towards building HTTP servers. Network.HTTP enforces a stronger separation between headers and body. HAppS..HTTP delivers a MIME message back to the app. With infrastructure to enable transparent conversion to and from application datatypes. -Alex- ______________________________________________________________ S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com

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State Monad
by Derek Elkins 04 Mar '05

04 Mar '05

> Thinking of stacks, I've often wondered if Haskell would be a good > language for implementing a PostScript interpreter. I actually have the beginning of a PostScript interpreter somwhere... And the core of a Joy interpreter is extremely small. It's pretty much 'foldl compose . map interpretWord'. Of course, I implemented a Joy interpreter in the slightly sugared lambda calculus that lambdabot's @eval module supports so... (Crap, I actually want to find it (the PostScript interpreter) now.)

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State Monad
by Sam G. 04 Mar '05

04 Mar '05

I need a Monad to represent an internal stack. I mean I've got a lot of functions which operates on lists and I would not like to pass the list as an argument everytime. Could you help me writing this monad? To start, I just need a + function which will return the sum of the 2 toppest elements of the stack. Thanks in advance, Sam.

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Corba and Haskell
by Maurício 04 Mar '05

04 Mar '05

Hi, Have you guys used any Corba library for Haskell? Which one would you recommend (if there are more than one)? Thanks, Maurício

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haskell DNS resolver w/o using FFI
by S. Alexander Jacobson 03 Mar '05

03 Mar '05

Peter Simons told me that someone has written a DNS resolver in Haskell that doesn't rely on an FFI call. I'd love to look at it if it is available... -Alex- ______________________________________________________________ S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com

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getting the name of a function for test diagnostic purposes
by Terrence Brannon 03 Mar '05

03 Mar '05

{- I have written a program (below) to run a test suite for a list of functions: [isTotalJunc, isPartialJunc] where each function receives a datum of type ApplyArg whose value slot is one element at a time of the list of types below: [JNone, JOne, JAny, JAll] I therefore must run 8 tests (the cross product of functions and types). Right now, here is the output: *Main> test_total_partial ["fun: (->)input: JNoneoutput: True","fun: (->)input: JOneoutput: False","fun: (->)input: JAnyoutput: False","fun: (->)input: JAlloutput: True","fun: (->)input: JNoneoutput: False","fun: (->)input: JOneoutput: True","fun: (->)input: JAnyoutput: True","fun: (->)input: JAlloutput: False"] -- One problem is that functions do not print as their name. The call to (show f) in showres simply yields (->) -- Another problem is that I cannot manage to separate each string with a carriage return. Any help with these problems is appreciated. -} test_total_partial = let funs = [isTotalJunc, isPartialJunc] types = [JNone, JOne, JAny, JAll] mkdat typ = ApplyArg "somename" (VJunc $ Junc typ emptySet emptySet) False showres f t r = "fun: " ++ (show f) ++ "input: " ++ (show t) ++ "output: " ++ (show r) runtest f t = do { retval <- f (mkdat t); return $ (showres f t retval) } in [ (showres f t (f (mkdat t))) | f <- funs, t <- types ] -- Carter's Compass: I know I'm on the right track when, by deleting something, I'm adding functionality.

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Re: [Haskell] specified or not
by Ross Paterson 03 Mar '05

03 Mar '05

[moving to the cafe] On Thu, Mar 03, 2005 at 09:39:17AM -0500, Scott Turner wrote: > Is the behavior of evaluating z unspecified? > z = f (0, z) > f x = case x of > (1,1) -> z > _ -> 0 > Hugs and GHC agree that z evaluates to 0. However, if the first line is > changed to > z = f (z,0) > then both implementations loop. In other words, the behavior depends on order > of evaluation, which AFAIK is not specified. The order of pattern matching is specified: pre-order, left to right. In detail: according to the rules of Section 3.17.3 of the Report, the definition of f is equivalent to f x = case x of (x1, x2) -> case x1 of 1 -> case x2 of 1 -> z _ -> 0 _ -> 0 _ -> 0 and hence f (0, anything) = 0 f (undefined, 0) = undefined

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Re: [Haskell-cafe] Re: Bound threads
by Wolfgang Thaller 03 Mar '05

03 Mar '05

Marcin Kowalczyk wrote: > Indeed, my brain is melting, but I did it :-) Congratulations. How about we found a "Bound-thread-induced brain melt victims' support group"? > [...] I have added some optimizations: I think we had thought of most of these optimizations, but things were already very complex, so I kept putting that off until we decided to do IO differently, at which point they were no longer necessary. Besides simplicity, one of the main reasons for moving our select() call from the run-time system to the libraries was to avoid the performance hit of having to call select() every time through the scheduler loop rather than only once per IO operation. Imagine having one or more (unbound) threads that spend most of their time waiting for IO, and a bunch of (also unbound) threads that do some computation. If select() is part of the scheduler loop, you will get a select() call whenever a thread-switch between the computation threads happens. If, on the other hand, the select() call happens in a separate OS thread, you will get extra inter-OS-thread messaging once the select wakes up, but that happens far less often as a thread-switch between the computation threads. > Bound threads introduced problems. They can partially be solved, > e.g. the worker pool, the wakeup pipe, epoll descriptor are correctly > recreated. But there is simply no way to return from callbacks because > the corresponding C contexts no longer exist. So I made them as > follows: > > All threads except the thread performing the fork become unbound. > [...] What happens when fork is called from an unbound thread? Does it become bound in the child process? When does the child process terminate? Does the thread that called fork gain a "main thread" status so that the process will exit as soon as the thread exits? For GHC, we side-stepped all those issues by only providing a simplified version of forkProcess: System.Posix.forkProcess :: IO () -> IO System.Posix.Types.ProcessID In the child process, only the IO action given as a parameter will run, and once it returns, the child process will terminate. This covers most use cases of fork and is, to the best of my knowledge, the most general version that can be implemented with the same semantics for both GHC's threaded RTS and GHC's non-threaded RTS. > I measured the speed of some syscalls on my system, to see what is > worth optimizing: > > - pthread_mutex_lock + unlock (NPTL) 0.1 us pthread_mutex_* is not necessarily a syscall. When there is no contention, the NPTL is able to do it entirely in user space without a context switch. The kernel only gets involved when a thread is actually suspended waiting for a lock. The situation is the same on Mac OS X, but Microsoft's multithreading API is rumored to be a lot slower (kernel calls for every lock/unlock - I haven't checked this, though). Cheers, Wolfgang

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RE: [Haskell-cafe] Re: Bound threads
by Simon Marlow 02 Mar '05

02 Mar '05

On 01 March 2005 11:21, Marcin 'Qrczak' Kowalczyk wrote: > Marcin 'Qrczak' Kowalczyk <qrczak(a)knm.org.pl> writes: > >> Why is the main thread bound? > > I can answer myself: if the main thread is unbound, the end of the > program can be reached in a different OS thread, which may be a > problem if we want to return cleanly to the calling code. > > I've now implemented a threaded runtime in my language Kogut, based > on the design of Haskell. The main thread is bound. The thread which > holds the capability performs I/O multiplexing itself, without a > separate service thread. We found that doing this was excessively complex (well, I thought so anyway). The problem is, when there is no other work to do but there are outstanding I/O requests to wait for, the thread holding the capability has to wait in select(). But then, another OS thread making an external call into Haskell has to somehow indicate that it needs the capability, and wake up the thread blocked in select(). It can do this with a pipe, but then you still have the problem that each call-in requires a context switch, which is sub-optimal if you're implementing a library in Haskell to be called from another language. > Producer/consumer ping-pong is 15 times slower between threads running > on different OS threads than on two unbound threads. You might also want to measure throughput of call-ins. Cheers, Simon

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how do I avoid excessive constructor application?
by S. Alexander Jacobson 02 Mar '05

02 Mar '05

For some reason, these two functions have different types. fun1 f (Left x)= Left (f x) fun1 _ r@(Right x) = Right x fun2 f (Left x) = Left (f x) fun2 _ r = r Is there a way to rewrite fun2 so that f has type (a->b)? In the general case, it seems wasteful to have to destruct and construct values just for type checking reasons, especially if your type has many more constructors than (Either a b). -Alex- ______________________________________________________________ S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com

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Numeric vs. relative precedences of infix operators
by Henning Thielemann 02 Mar '05

02 Mar '05

Is it widely accepted that the precedence of infix operators is defined by numbers? The numbers look arbitrary to me and it is not possible to introduce infix operators with interim precedences. What about defining relations such as "(*) has precedence over (+)"? The compiler could construct a topographically ordered graph from these relations. This way it would also be possible that from two infix operators none has precedence over the other (although the have not the _same_ precedence), thus the sub-expressions with these operators must be enclosed in parentheses.

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FFI and foreign function returning a structure
by Dimitry Golubovsky 02 Mar '05

02 Mar '05

Hi, If I have a C function returning a structure (not a pointer, but structure itself), can this be accomodated via FFI? I re-read the FFI Addendum, and my conclusion is most likely "No". I am asking just to make sure this is not only my finding. Dimitry Golubovsky Middletown, CT

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Browser-like UI
by Maurício 02 Mar '05

02 Mar '05

Hi, I've seen some options for GUI programming in Haskell libraries page, but what I really would like is to define my user interface using HTML (or, maybe, SVG). What are the options to do that in Haskell? I've read that Gtk2Hs has a mozilla rendering engine, but unfortunatly that won't build on my 64Mb computer. What else can I use? How does that work, can I handle HTML events with Haskell code? Thanks, Maurício

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RE: [Haskell-cafe] Re: Bound threads
by Simon Marlow 01 Mar '05

01 Mar '05

On 26 February 2005 12:14, Marcin 'Qrczak' Kowalczyk wrote: > Wolfgang Thaller <wolfgang.thaller(a)gmx.net> writes: > >>> Since the main thread is bound, and unbound threads are never >>> executed on an OS thread which has some Haskell thread bound, this >>> would imply that when the main thread spawns a Haskell thread and >>> they synchronize a lot with each other using MVars, the >>> synchronization needs OS-thread synchronization - the threads will >>> not execute on a the same OS thread. >> >> Correct. > > Is it important which thread executes Haskell code (I bet no) and > unsafe foreign calls (I don't know)? If not, couldn't the same OS > thread execute code of both threads until a safe foreign call is made? Actually in a bound thread, *all* foreign calls must be made using the correct OS thread, not just the safe ones. So your scheme would involve a context switch at every foreign call, which would end up being rather expensive. It's certainly a legitimate implementation, but I suspect not one that would lead to good performance. Cheers, Simon

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RE: [Haskell-cafe] how to avoid overlapping instance error?
by Simon Peyton-Jones 28 Feb '05

28 Feb '05

| Or I suppose, one could rephrase this question as why not | simplify instance declarations to be: | | instance ToSet where | toSet = Set.fromList | | And let the typechecker take care of figuring out what instance is | being specified here? That might be possible, but Haskell forces you to be a bit more explicit, that's all. Simon

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postgresql foreign function call segfaults
by Edwin Eyan Moragas 28 Feb '05

28 Feb '05

Hi Group, i'm trying to complete an haskell pgsql interface. all compiles well when using ghc's generated executable but it segfaults when i do a pqexec. -- PGresult *PQexec(PGconn *conn, const char *query); foreign import ccall "libpq-fe.h PQexec" pqexec::X_PGconn->CString->IO X_PGresult here's the offending code: create conn=do putStrLn "creating table products" putStrLn ("\t"++screate) q<- newCString screate -- segmentation fault here! rs<- pqexec conn q stat<- pqcmdstatus rs stats<-peekCString stat putStrLn ("\tstatus is: "++ show stats) ra<- pqcmdtuples rs ras<-peekCString ra putStrLn ("\trows affected is: "++ show ras) screate= "CREATE TABLE products (" ++ "product_no integer," ++ "name text," ++ "price numeric" ++")" i'm using pgsql 7.4, gcc 3.3.4, ghc 6.2.2 i tried compiling with -fvia-C and it doesn't finish compiling. i'm asking this question in the fear that i have done something wrong. i have posted my code here: http://www.eyan.org/haskell/Postgresql.hsc the makefile i'm using is here: http://www.eyan.org/haskell/Makefile the generated hc file is here: http://www.eyan.org/haskell/POstgresql.hc -- kind regards, eyan http://www.eyan.org

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RE: [Haskell-cafe] how to avoid overlapping instance error?
by Simon Peyton-Jones 28 Feb '05

28 Feb '05

Unfortunately, the context of an instance decl is not taken into account when matching instance decls. Yes, it would make sense to do so, but it'd make the system yet more complicated. So Show (table item) overlaps with Show ([] item) Overlap is checked lazily, so if you look for Show (MyTable t) you won't get overlap. Only if you ask for Show (table item), where the type checker still doesn't know what type 'table' is going to be, do you get a problem. Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of S. | Alexander Jacobson | Sent: 25 February 2005 17:09 | To: haskell-cafe(a)haskell.org | Subject: [Haskell-cafe] how to avoid overlapping instance error? | | | Currently, the HAppS.DBMS lib requires the user to provide a Show | instance for their table types. An example might be: | | instance (Show item) => Show (MyTable item) where | showsPrec d table = showsPrec d $ Set.toList $ myTableSet table | | But the Table class itself defines a toSet function so I think I | should be able to do this once for all Table instances: | | instance (Show item,Ord item, Table table item p) => Show (table item) where | showsPrec d table = showsPrec d $ Set.toList $ toSet table | | But I get an error telling me I am overlapping with Show [a]. | | Since [] is not an instance of Table, I don't see why there should be | an overlap. | | -Alex- | | ______________________________________________________________ | S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

2 1

Re: Bound threads
by Wolfgang Thaller 26 Feb '05

26 Feb '05

> I'm trying to understand the semantics and implementation of bound > threads > basing on the conc-ffi paper and others. > > Since the main thread is bound, and unbound threads are never executed > on an OS thread which has some Haskell thread bound, this would imply > that when the main thread spawns a Haskell thread and they synchronize > a lot with each other using MVars, the synchronization needs OS-thread > synchronization - the threads will not execute on a the same OS thread. Correct. > If I understand this correctly, doesn't it impose a significant > overhead compared to synchronizing two unbound threads? If not, > what am I missing? Yes, it does impose more overhead than synchronizing two unbound threads. Depending on how high your demands are and on how good your operating system is at OS-thread synchronization, it might or might not be significant. We need the main thread to be bound because the main thread is special for some libraries (Apple's Carbon and Cocoa libraries, and, to a lesser extent, Microsoft's Win32). Some calls to these libraries absolutely have to be executed in the main thread and not in any other thread. So treating the "call" to main the same way as other calls into Haskell code (using a bound Haskell thread) was the nicest solution to this; the performance disadvantage you pointed out can be easily worked around by just doing _all_ the work in unbound threads (and having the main thread just wait for the unbound threads to terminate). Hope that answers your question, Wolfgang

2 1

Compiling GHC from CVS
by Arjun Guha 26 Feb '05

26 Feb '05

I'm having trouble compiling GHC from CVS. Make seems to be looping infinitely while compiling the stage1 compiler. Make -d gives me: Pruning file 'main/Config.hs' On and on and on. The commands I used to make (from the fptools) directory were: autoreconf ./configure --prefix=/Users/arjun/local/fptools Make I'm using Mac OS X 10.3.8 with GHC 6.2.1. -Arjun

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Must be a FAQ - Can't make install Hugs98-Nov2003 on MacOSX 10.3.8
by Kaoru Hosokawa 25 Feb '05

25 Feb '05

I'm sure this must be a FAQ, but I can't make install hugs98-Nov2003 on MacOS 10.3.8. The error message I get is: ... gcc -c -g -O2 machine.c machine.c:1649: illegal expression, found `&&' machine.c:1649: illegal expression, found `&&' machine.c:1649: illegal expression, found `&&' machine.c:1649: illegal expression, found `&&' machine.c:1649: illegal expression, found `&&' machine.c:1649: illegal expression, found `&&' machine.c:1649: illegal expression, found `&&' machine.c:1649: illegal expression, found `&&' machine.c:1649: illegal expression, found `&&' machine.c:1649: illegal expression, found `&&' machine.c:1649: illegal expression, found `&&' machine.c:1649: illegal expression, found `&&' machine.c:1649: illegal expression, found `&&' machine.c:1649: illegal expression, found `&&' machine.c:1649: illegal expression, found `&&' machine.c:1649: illegal expression, found `&&' machine.c:1649: illegal expression, found `&&' machine.c:1649: illegal expression, found `&&' machine.c:1649: illegal expression, found `&&' machine.c:1649: illegal expression, found `&&' machine.c:1649: illegal expression, found `&&' machine.c:1649: illegal expression, found `&&' machine.c:1649: illegal expression, found `&&' machine.c:1673: illegal statement, missing `identifier' after `goto' machine.c:1673: syntax error, missing `;' after `*' machine.c:1677: illegal statement, missing `identifier' after `goto' machine.c:1677: syntax error, missing `;' after `*' machine.c:1681: illegal statement, missing `identifier' after `goto' machine.c:1681: syntax error, missing `;' after `*' machine.c:1685: illegal statement, missing `identifier' after `goto' machine.c:1685: syntax error, missing `;' after `*' machine.c:1689: illegal statement, missing `identifier' after `goto' machine.c:1689: syntax error, missing `;' after `*' machine.c:1694: illegal statement, missing `identifier' after `goto' machine.c:1694: syntax error, missing `;' after `*' machine.c:1698: illegal statement, missing `identifier' after `goto' machine.c:1698: syntax error, missing `;' after `*' machine.c:1707: illegal statement, missing `identifier' after `goto' machine.c:1707: syntax error, missing `;' after `*' machine.c:1715: illegal statement, missing `identifier' after `goto' machine.c:1715: syntax error, missing `;' after `*' machine.c:1729: illegal statement, missing `identifier' after `goto' machine.c:1729: syntax error, missing `;' after `*' machine.c:1738: illegal statement, missing `identifier' after `goto' machine.c:1738: syntax error, missing `;' after `*' machine.c:1742: illegal statement, missing `identifier' after `goto' machine.c:1742: syntax error, missing `;' after `*' machine.c:1748: illegal statement, missing `identifier' after `goto' machine.c:1748: syntax error, missing `;' after `*' machine.c:1756: illegal statement, missing `identifier' after `goto' machine.c:1756: syntax error, missing `;' after `*' machine.c:1759: illegal statement, missing `identifier' after `goto' machine.c:1759: syntax error, missing `;' after `*' machine.c:1763: illegal statement, missing `identifier' after `goto' machine.c:1763: syntax error, missing `;' after `*' machine.c:1771: illegal statement, missing `identifier' after `goto' machine.c:1771: syntax error, missing `;' after `*' machine.c:1791: illegal statement, missing `identifier' after `goto' machine.c:1791: syntax error, missing `;' after `*' machine.c:1814: illegal statement, missing `identifier' after `goto' machine.c:1814: syntax error, missing `;' after `*' cpp-precomp: warning: errors during smart preprocessing, retrying in basic mode ... What should I do? Thanks. Kaoru Hosokawa

4 3

Bound threads
by Marcin 'Qrczak' Kowalczyk 25 Feb '05

25 Feb '05

I'm trying to understand the semantics and implementation of bound threads basing on the conc-ffi paper and others. Since the main thread is bound, and unbound threads are never executed on an OS thread which has some Haskell thread bound, this would imply that when the main thread spawns a Haskell thread and they synchronize a lot with each other using MVars, the synchronization needs OS-thread synchronization - the threads will not execute on a the same OS thread. If I understand this correctly, doesn't it impose a significant overhead compared to synchronizing two unbound threads? If not, what am I missing? -- __("< Marcin Kowalczyk \__/ qrczak(a)knm.org.pl ^^ http://qrnik.knm.org.pl/~qrczak/

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how to avoid overlapping instance error?
by S. Alexander Jacobson 25 Feb '05

25 Feb '05

Currently, the HAppS.DBMS lib requires the user to provide a Show instance for their table types. An example might be: instance (Show item) => Show (MyTable item) where showsPrec d table = showsPrec d $ Set.toList $ myTableSet table But the Table class itself defines a toSet function so I think I should be able to do this once for all Table instances: instance (Show item,Ord item, Table table item p) => Show (table item) where showsPrec d table = showsPrec d $ Set.toList $ toSet table But I get an error telling me I am overlapping with Show [a]. Since [] is not an instance of Table, I don't see why there should be an overlap. -Alex- ______________________________________________________________ S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com

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RE: [Haskell-cafe] GHC compilation with postres libs
by Bayley, Alistair 25 Feb '05

25 Feb '05

> From: Edwin Eyan Moragas [mailto:haaktu@gmail.com] > > -compiled Main.hsc with: > $ hsc2hs -I/usr/local/pgsql/include Main.hsc > all went well with this > > - tried compiling this way: > $ ghc -o test -L/usr/local/pgsql/lib Main.hs Do you not also need the -l option? e.g. (wild guess, assuming library is libpg.so) > $ ghc -o test -L/usr/local/pgsql/lib -lpg Main.hs Alistair. ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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RE: [Haskell-cafe] GHC compilation with postres libs
by Bayley, Alistair 25 Feb '05

25 Feb '05

> From: Edwin Eyan Moragas [mailto:haaktu@gmail.com] > > eyan@tanis $ ghc -o test -L/usr/local/pgsql/lib/ -lpg Main.hs > compilation IS NOT required > /usr/lib/gcc-lib/i486-slackware-linux/3.3.4/../../../../i486-s > lackware-linux/bin/ld: > cannot find -lpg > collect2: ld returned 1 exit status > > still no go. :-) > thanks. Where is the library (the .so file) and what is it called? Can you show me the contents of /usr/local/pgsql/lib ? ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

1 0

YAWQ (Yet Another Wash Question)
by John Goerzen 24 Feb '05

24 Feb '05

Thanks for everyone that's helped me out with my Wash questions. I have one more. I have a textInputField that I only want to display on a form in certain cituations. I can't figure out how to make this work. For instance: ... do if cond then x <- textInputField ... else () ... submit ... Well, two problems there... first, the scope of the x doesn't reach to the submit. Second, there's a type issue. So I thought maybe I could figure out something like this: ... do x <- if cond then textInputField ... else () still stuck with the typing problem. So I thought maybe something like this... ... do let x = if cond then Just $ textInputField ... else Nothing Though I suspect this won't work either, since the textInputField won't actually be processed at the right place in the program. I then thought maybe I could kludge by this with a hidden field, but I can't find one of those, either. Ideas, anyone? Thanks, John

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new Haskell hacker seeking peer review
by Sean Perry 23 Feb '05

23 Feb '05

I am learning Haskell, so I decided to implement everyone's favorite, overused Unix command -- cat. Below is my simple implementation, comments about style, implementation, etc. are welcomed. In particular, is my untilEOF idiomatically ok? Is there a better way to accomplish this? Also, while talking about untilEOF, it is slightly annoying that hIsEOF returns IO Bool and that functions like 'not' only want Bool. Sure makes the logic tests feel like more work than they should be. cat.hs: module Main where import IO import System(getArgs) untilEOF :: Handle -> (Handle -> IO ()) -> IO () untilEOF hdl f = do eof <- hIsEOF hdl if eof then return () else do f hdl untilEOF hdl f cat :: Handle -> IO () cat hdl = do line <- hGetLine hdl putStrLn line catFile :: FilePath -> IO () catFile path = do hdl <- openFile path ReadMode untilEOF hdl cat main :: IO () main = do args <- getArgs if (length args) > 0 then mapM_ catFile args else untilEOF stdin cat

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module Crypt_Discordian - code critique requested
by Terrence Brannon 23 Feb '05

23 Feb '05

Hi, I am getting into Haskell so I decided to convert a Perl module of mine: http://cpan.uwinnipeg.ca/htdocs/Crypt-Discordian/Crypt/Discordian.html into Haskell. I was pleased at the cleanliness and conciseness of the Haskell code. However, I am sure that it can be improved on and am soliciting any feedback you may have about this module. Thanks, metaperl on #haskell module Crypt_Discordian where import List vowel_list = "aeiouAEIOU" is_vowel c = c `elem` vowel_list move_vowels lis = move_vowels' lis [] [] move_vowels' [] c v = v ++ c move_vowels' (x:xs) c v | is_vowel x = move_vowels' xs c (x:v) | otherwise = move_vowels' xs (x:c) v remove_spaces str = filter (\x -> x /= ' ') str encrypt str = List.sort $ move_vowels $ remove_spaces str {- The algorithm for Discordian text encryption is given at: http://www.principiadiscordia.com/book/78.php After implementing this, I realized that all the early steps are a farce. But anyway, here is the algorithm in case you don't enjoy tilting your head to read a page: Step 1. Write out message (HAIL ERIS) and put all vowels at the end (HLRSAIEI) Step 2. Reverse order (IEIASRLH) Step 3. Convert to numbers (9-5-9-1-19-18-12-8) Step 4. Put into numerical order (1-5-8-9-9-12-18-19) Step 5. Convert back to letter (AEHIILRS) This cryptographic cypher code is GUARANTEED TO BE 100% UNBREAKABLE .. so says the Principia Discordia. But I think we can generate and test to break it. Many thanks to kosmikus and Pseudonym for their help in developing this module -} -- Carter's Compass: I know I'm on the right track when, by deleting something, I'm adding functionality.

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GUI components for displaying Html pages
by David Owen 23 Feb '05

23 Feb '05

Hi all, I'm looking for a Haskell GUI library which supports the display of rendered Html pages in the same way that web browsers do. I've been getting to grips recently with wxHaskell as it was recommended on haskell-cafe. I was hoping that a special pane component (or something similar) existed in wxHaskell for the purpose. I've looked through the API but can't seem to see anything which looks suitable. Does anybody know if wxHaskell can support this? If not is anybody aware of a Haskell GUI library which caters for this purpose. Also, I am hoping to make use of HaXmL to deal with the Html parsing or failing that attempt a very simple home-made parser. Should anybody out there know of a better way of attacking this aspect then that would be much appreciated too! (Apologies, this email might have been better suited to the libraries mailing list!) Cheers all! David (Swansea, UK)

3 3

WASH defaults in inputFields
by John Goerzen 22 Feb '05

22 Feb '05

Hi, Hope this is not a stupid question, but I RTFM'd and couldn't find the answer. I am designing a form that will be used to edit some data that is in the database. I want users to pull up the form and have all the input fields pre-filled with the current state of the database (so they don't have to re-key all that), then the database gets updated with they hit submit. Simple to do in HTML, but I can't figure out how to do this with Wash. The inputFields don't seem to take a parameter giving a default, nor does there appear to be any way to set it later. Ideas? Thanks, John

4 5

ANN: HAppS -- Haskell Application Server v 0.2
by S. Alexander Jacobson 22 Feb '05

22 Feb '05

HAppS is a Haskell library for building Internet applications, featuring: * HAppS.ACID: Guarantee application integrity in the face of unplanned outages using this module's integrated write-ahead logging and checkpointing framework. * HAppS.DBMS: Do relational operations in Haskell (rather than SQL) on Haskell sets (outside the IO Monad). Define custom indices for your Haskell datatypes (e.g. geographic/geometric types). Use in combination with ACID for a robust relational DBMS customized for your application. * HAppS.Protocols: Expose your application using as an HTTP server and/or by recieving and sending SMTP. - License HAppS is released by HAppS.org under the terms of the GNU General Public License Version 2, a copy of which is enclosed with this package. - How do I get HAppS? More information is available at http://HAppS.org A tarball of HAppS is available at http://HAppS.org/HAppS/happs.tar.gz. HAppS is also available via darcs: darcs get http://happs.org/HAppS ______________________________________________________________ S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com

1 0

RE: [Haskell-cafe] Problems building GreenCard
by Simon Peyton-Jones 22 Feb '05

22 Feb '05

-fno-prune-tydecls is a dead flag. Just remove it. Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of | Dmitri Pissarenko | Sent: 15 February 2005 21:04 | To: haskell-cafe(a)haskell.org | Cc: sof(a)galois.com; alastair(a)reid-consulting-uk.ltd.uk | Subject: Re: [Haskell-cafe] Problems building GreenCard | | Hello! | | I could fix the problem by replacing the line | | GHC_OPTS = -cpp -fglasgow-exts -fno-prune-tydecls | | in file lib/Makefile by this one | | GHC_OPTS = -cpp -fglasgow-exts | | However, I don't know what this -fno-prune-tydecls does and whether it is | important (for using Green Card to access Java). | | Best regards | | Dmitri Pissarenko | -- | Dmitri Pissarenko | Software Engineer | http://dapissarenko.com | | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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Re: New to haskell: unresolved overloading question
by Christian Maeder 22 Feb '05

22 Feb '05

Blair Fraser wrote: > I'm new to haskell and I'm working through the class section of the > gentle tutorial. I decided to implement an ApproxEq class (just for > fun), but have run into problems. The code below works fine for units > and bools, but gives an unresolved overloading error for Floats and > Doubles. What's going on here? What does the error message mean and > what did I do wrong? (I'm in Hugs.) > > -- ApproxEq: My first class. > class (Eq a) => ApproxEq a where > (~=) :: a -> a -> Bool > x ~= y = x == y -- By default, check equality > > -- These two override default with same, just checking if it works > instance ApproxEq () where () ~= () = True > instance ApproxEq Bool where x ~= y = x == y > > -- These two override default with different > -- consider floating points equal if they differ by one or less > instance ApproxEq Float where x ~= y = (abs (x-y) <= 1) > instance ApproxEq Double where x ~= y = (abs (x-y) <= 1) More elegant seems to be: instance (Ord a, Num a) => ApproxEq a where x ~= y = (abs (x-y) < 1) However, this requires extensions to "Allow unsafe overlapping instances": hugs -98 +O ghci -fglasgow-exts -fallow-overlapping-instances -fallow-undecidable-instances -fallow-incoherent-instances > -- This one dosn't work either, but it just depends on the other two > instance ApproxEq a => ApproxEq [a] where > [] ~= [] = True > (x:xs) ~= (y:ys) = x ~= y && xs ~= ys > _ ~= _ = False > > Thanks, > Blair

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Re: New to haskell: unresolved overloading question
by Christian Maeder 22 Feb '05

22 Feb '05

Tomasz Zielonka wrote: > Why not forget about ApproxEq for () and Bool and simply define a > function? > > (~=) :: (Ord a, Num a) => a -> a -> Bool > x ~= y = abs (x-y) < 1 Indeed, this works best. The instance for lists is also not too important. Christian

1 0

Literate Haskell
by Dmitri Pissarenko 22 Feb '05

22 Feb '05

Hello! I'm curious what experienced Haskellers think about using literate Haskell in daily work. It seems to me like a good idea, since during coding it often helps to write down one's thoughts (often, I find a solution to a complicated problem in this way). What are your experiences with using literate Haskell? TIA Dmitri Pissarenko -- Dmitri Pissarenko Software Engineer http://dapissarenko.com

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Re: [Haskell-cafe] Problems with building GCJNI 1.2
by Antony Courtney 21 Feb '05

21 Feb '05

Hi, Sorry for not jumping in on this discussion sooner. I am the author of GCJNI. GCJNI was developed back when Hugs was the dominant Haskell implementation and GreenCard was the easiest/best way to do foreign language bindings for Haskell because the FFI proposal was still in the formative stages. These days, the Haskell FFI addendum is stable and widely implemented, ghc has become much more widely available and easy to install and use, and (without wishing to sound harsh), GreenCard's FFI code generator was broken and useless the last time I tried to use it (a few years ago). I actually ported GCJNI to the Haskell FFI a few years ago, getting rid of ALL use of GreenCard. I used ghc's 'hsc2hs' tool to help with the marshalling, and made no attempt to port to hugs. I called this newer system 'hsjni'. It worked very well for my needs (using the Java2D rendering engine as the back-end for a prototype Haskell GUI system). I never released hsjni simply due to lack of time while finishing my thesis. I invested a little effort in trying to test hsjni with the latest JDK and get a release together in December, but it got snagged on a silly little bug in hsc2hs's handling of paths which have embedded spaces. A minor bug, to be sure, but since the default installation location for the JDK under Windows is under "C:\Program Files", the bug meant hsjni couldn't work out-of-the-box for the most common installation of the prerequisites. Simon Marlow has since fixed this bug in CVS head; I'm just waiting for the next release of ghc with this fix available before I try again. If you think you'd be interested in having a shot at building 'hsjni' (or you are interested in maintaining it), let me know, and I'll make the sources available for you. Caveat is that I haven't tested it since JDK 1.3, because of this hsc2hs issue. My appologies for not mentioning any of the above on the GCJNI web page. Hopefully the whole GCJNI release and web pages can be deprecated soon, with hsjni offered in its place. Kind Regards, -Antony -- Antony Courtney email: antony(a)aya.yale.edu New York, NY WWW: http://www.apocalypse.org/~antony

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[darcs #222] performance problem with massive changes in a single file
by David Roundy via RT 21 Feb '05

21 Feb '05

I'm cc'ing haskell-cafe on this darcs bug, since I imagine there might be a student (or professor) hanging around there who might be interested in the implementation of an efficient LCS algorithm in haskell. [markjugg - Sun Feb 20 09:15:57 2005]: > Typically in software, 1000's of line aren't changed at the time, which > is why this hasn't been an big issue before. Indeed. The issue here is that darcs uses the exact Hunt-Szymanski LCS algorithm when it computes the diff, while GNU diff uses an approximate --but faster--algorithm, which works "almost" as well. The approximate algorithm is reasonably well-documented, but more complex to implement than the exact one, just because it's harder to understand. This would be a reasonable project for an interested haskell CS student. The LCS is implemented in pure haskell 98, and is of signature lcs :: Ord a => [a] -> [a] -> [a] It's an interesting problem that's easily testable, and has different scaling behavior in different limits (finite alphabet, infinite alphabet, permutations of unique objects). David (ever optimistic for new contributors to darcs!) http://darcs.net

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ANN: Haskell news/community site
by John Goerzen 21 Feb '05

21 Feb '05

Hi, I'm setting up a Haskell news & community site. Check it out at: http://sequence.complete.org/ It supports aggregation of Haskell blogs and discussion from various sources (including this list). Any registered user can submit content for the front page, and all registered users can vote to approve or reject such submissions. The first thing to do is pick a real name. I hope that eventually this can be a *.haskell.org site. -- John

1 0

Alternative lambda syntax (not entirely serious)
by Fritz Ruehr 21 Feb '05

21 Feb '05

I was introducing a fresh set of students to lambda abstraction in Haskell yesterday and had the following inspiration about a possible alternative syntax. I didn't end up showing the idea to the students -- too confusing -- but I thought I would float it here, as long as nobody takes it too seriously (this is the Cafe, after all :) ) . The idea arose from two things I'd done recently in class: introducing the underscore pattern as "an anonymous variable", and also commenting on the relationship between the final "->" in a type signature and the "=" of the corresponding function definition. So, since a lambda abstraction denotes an anonymous function, the underscore signifies anonymous things, and given that "->" of the current syntax connotes a type than a value, I thought "why not this syntax instead?": (_ x = x) -- the identity function (_ (x,y) = x) -- the fst function (_ f g x = f (g x)) -- composition In other words, at a lexical level we simply write "_" in place of "\" and "=" in place of "->". The use and placement of the "_" nicely indicates that what we are denoting is both anonymous and a function, and the use of the "=" makes it more parallel to the definition of a named function. This looks pretty nice in the right context, e.g., when talking about the kinds of code transformation one does when introducing lambda. For example: foo = map f xs where f x = blah VERSUS foo = map (_ x = blah) xs OK, so why not a serious proposal? Well first off, from a pedagogical perspective, we may not *want* to stress the similarity between the definition of a function and an expression which denotes one: this is a point of some confusion for newcomers, after all. Second, I imagine it could make parsing more difficult (although an *initial* underscore wouldn't otherwise come up in an expression context, right?). Third (and this is probably the death-knell), the use of "_" and "=" would be especially confusing in cases where an abstraction appears immediately on the RHS of a definition, or where one of the parameter patterns was itself an underscore. Or (and especially!) both; for example: const = (_ x _ = x) -- bleah! or worse yet: const x = (_ _ = x) -- yikes! Now, such definitions don't come up much in actual production code, I suppose, but they do tend to come up in classrooms, where the basic ideas are being introduced, and confusion would be especially unwelcome there. Finally, and not least, this proposal would further distance Haskell's syntax from the original lambda notation: this is not only a sad thing in general, but it would also make lambda-based T-shirt & merchandise logos even more obscure than they are now :) . -- Fritz

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