August 2001 - Haskell-Cafe

Pickling
by Mark Carroll 09 Sep '01

09 Sep '01

Am I right in thinking that there aren't currently any standard libraries for converting values to bitstreams and back, to be saved in files or squirted over network sockets or whatever to be resurrected later or elsewhere? I'd guess it would be hard to write, too - I've not noticed any introspection. How do people normally efficiently pass complex Haskell data structures to other co-operating Haskell processes? (I am thinking of something a bit like Java's serialisation.) -- Mark

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classes
by Cagdas Ozgenc 31 Aug '01

31 Aug '01

Hi, I am a newbie, so I might be making nonsense. In the following definiton: class Eq a where (==) :: a -> a -> Bool Why do we conlude that both of the "a" refer to the same type? Apparently using different types that both belonging to class Eq results in an error. For example 65 == 'a' It doesn't make much sense at first, however, how do you go by implementing multiple polymorphism? Secondly, why does Haskell distinguish between a Type and a Class? There seems to be no benefit of such approach. Thirdly, is it possible to define meta-classes in Haskell? Thanks

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Re: hiding import, tuple instances
by Ashley Yakeley 31 Aug '01

31 Aug '01

At 2001-08-31 09:27, Marcin 'Qrczak' Kowalczyk wrote: >> How do I define functions that work on all tuples? > >You can't, and this is unfortunate. You can make your own 'tuples' using cons = (,) and nil = (). Then you can use multi-parameter classes to manipulate them. -- Ashley Yakeley, Seattle WA

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Re: hiding import, tuple instances
by Marcin 'Qrczak' Kowalczyk 31 Aug '01

31 Aug '01

Fri, 31 Aug 2001 18:53:11 +0300, Cagdas Ozgenc <co19(a)cornell.edu> pisze: > 1) How can I hide an instance of a class imported from prelude? You can't. A given class/type pair must have at most one instance in the whole program. > How do I define functions that work on all tuples? You can't, and this is unfortunate. -- __("< Marcin Kowalczyk * qrczak(a)knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTÊPCZA QRCZAK

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hiding import, tuple instances
by Cagdas Ozgenc 31 Aug '01

31 Aug '01

Hi, 1) How can I hide an instance of a class imported from prelude? For example I would like to redefine instance (Ord a, Ord b) => Ord (a,b) where (<=) (x,y) (z,w) | x < z = True | x == z = if y <= w then True else False | otherwise = False for educational purposes. 2) I could not find an entry for this instance in Prelude instead I found instance Ord () where compare () () = EQ Is this suppose to mean "for all tuples"? How do I define functions that work on all tuples? Thanks for taking time

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Job advert: Compiler designer/architect
by Benedict R. Gaster 31 Aug '01

31 Aug '01

The following job advert has been sent to both the GCC and Pro64 mailing lists and I thought maybe of interest to people on this mailing list. SuperH are not looking for Haskell developers but rather are keen to recruit people with experience in language design and implementation. (The Pro64 is a highly optimizing compiler initially designed by SGI and targeted for the MIPS and IA64 architectures. Recently the Pro64 was made open source and SuperH is in the process of porting this technology to a selection of their current and future microprocessors.) Advert: SuperH, Inc is a new company formed by STMicroelectronics and Hitachi in order to market and develop the SuperH series of microprocessors as an independent company. The SuperH series will be offered as licensable cores for synthesis into embedded applications. See www.superh.com for more information. SuperH is headquartered in San Jose CA with R&D centers in Bristol UK and Tokyo Japan. SuperH will be developing software systems based on Open Source tecnology, including Gnu tools (i.e., gcc, gdb, etc.) and Pro64, and is looking for a range of engineers who have interests and/or experience in the following skill areas: o Code generation and optimization. Software pipelining, register allocation. Specific experience with gcc or Pro64 would be an advantage. o JIT compilation and dynamic optimization. o Linux development. o Architecture design. o Benchmarking. o Debugging technologies including gdb. SuperH will be developing products based on Gnu and Linux technogy as well as carrying out research into new innovative architectures and advanced compiler design, including the Pro64 and dynamic optimization. The main location for these posts is Bristol, UK. SuperH will be offering a comprehensive benefits package as well as an emloyee stock option scheme and bonus schemes. Job Title: Compiler designer/architect. Qualifications. Good knowledge of compiler optimizations, experience with Gnu or Pro64 would be useful. In depth knowledge of C & C++. Java an advantage. Experience in embedded system design. Please send CVs to: benedict.gaster(a)st.com Benedict R. Gaster SuperH, Inc.

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help me!!!!
by Garry Hadiwibowo 30 Aug '01

30 Aug '01

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RE: Numeric literals
by Simon Marlow 29 Aug '01

29 Aug '01

> On Tue, 28 Aug 2001, Simon Marlow wrote: > (snip) > > In GHC, you can do this: > > > > import Exception > > > > do result <- catch (evaluate (read "foo" :: Int)) > > (\error -> ... ) > (snip) > > I do like that. Is it likely to become standard someday, do you think? > Errors like this should certainly be catchable in such a way; > I was sad > when I was first looking into this and found that 'catch' seemed to be > inextricably linked to IO. Well, we'd like it to become standard :-) The exception stuff is really very easy to implement - you just use the standard stack-unwinding implementation of throw, and catch has to push a special stack frame. GHC's asynchronous exceptions are slightly harder, but I don't think it was more than a day or two's work. As a bonus you get to use a more efficient implementation of the IO monad, because it doesn't have to deal explicitly with exceptions. And then IO becomes a straightforward instance of ST. I guess I should also document the Exception extension in a self contained way, similar to the way the FFI extension is documented. Cheers, Simon

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RE: Numeric literals
by Simon Marlow 29 Aug '01

29 Aug '01

> I also haven't yet worked out how to tell if a string is "read"able or > not, yet - if (read "45")::Integer or whatever gives an error > (e.g. if I'd > put "af" instead of "45"), it seems to be pretty uncatchable, > which is a > pain. How do I tell if an instance of reading will work, or > catch that it > didn't? In GHC, you can do this: import Exception do result <- catch (evaluate (read "foo" :: Int)) (\error -> ... ) but unfortunately read doesn't raise a useful exception (just error "Prelude.read: no parse"), so you can't filter it very easily. However I don't think that read is likely to raise any other exceptions. Cheers, Simon

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RE: Numeric literals
by Simon Marlow 29 Aug '01

29 Aug '01

> Today's other question has to do with numeric literals. How do I write > literals for interesting IEEE values, e.g. > > Prelude> x > Infinity > Prelude> :type x > Double > Prelude> (read "Infinity") :: Double > *** Exception: Ratio.%: zero denominator > Prelude> This looks like a bug in GHC, which has now been fixed for version 5.02. Note that Haskell leaves the behaviour of these exceptional conditions undefined. Hugs for example will give a program error on divide by zero, and NHC has even more interesting behaviour: Prelude> 1/0 :: Float 34.02823669209384634633746074317682114560000000 Cheers, Simon

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