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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

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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

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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

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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. *****************************************************************

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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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