February 2005 - Haskell-Cafe

What is MonadPlus good for?
by Benjamin Pierce 16 Mar '05

16 Mar '05

I have seen lots of examples that show how it's useful to make some type constructor into an instance of Monad. Where can I find examples showing why it's good to take the trouble to show that something is also a MonadPlus? (I know there are many examples of things that *are* MonadPluses; what I want to know is why this is interesting. :-) Thanks, - Benjamin

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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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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

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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 25 Feb '05

25 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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