- Glasgow-haskell-users

mergesort. Reply
by Serge D. Mechveliani 21 Oct '02

21 Oct '02

Ketil Z. Malde <ketil(a)ii.uib.no> writes > I'll hereby argue for using a quicksort implementation akin to > >> sortBy' _ [] = [] >> sortBy' pc (x:xs) = let (l,e,g) = part3 (`pc` x) xs >> in sortBy' pc l ++ (x:e) ++ sortBy' pc g >> where >> part3 comp xs = p3 [] [] [] comp xs >> p3 ls es gs _ [] = (ls,es,gs) >> p3 ls es gs comp (x:xs) = case comp x of >> LT -> p3 (x:ls) es gs comp xs >> EQ -> p3 ls (x:es) gs comp xs >> GT -> p3 ls es (x:gs) comp xs > > (hopefully this is fairly bug-free) At least for my data (lots of > values, limited range), it appears to speed things up tremendously. I > haven't measured more general cases in any detail, though. And one > obvious drawback may be that it's not stable, which I think can be > alleviated by a few well placed 'reverse's. > > Comments welcome! But sortBy' (compare) [1 .. n] costs too much, even for n = 11000. It costs (on worst data) many times more than mergeSort. ----------------- Serge Mechveliani mechvel(a)botik.ru

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Sesso ma vero...
by Giada 19 Oct '02

19 Oct '02

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19 Oct '02

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more undecidable instances, but why
by Hal Daume III 18 Oct '02

18 Oct '02

I'm being hit by undecidable instances, but I can't see why. I read and reread the GHC docs on MPTCs, but to no avail. This is what I'm trying to do: newtype S a = S a instance MArray IOUArray a IO => MArray IOUArray (S a) IO where ... the instance declaration is basically just wrapping/unwrapping S and a few castIOArrays. However, GHC complains: /nfs/isd/hdaume/projects/MESumm/Linear2/Linear2.hs:76: Non-type variables in constraint: MArray IOUArray a IO (Use -fallow-undecidable-instances to permit this) In the context: (MArray IOUArray a IO) While checking the context of an instance declaration In the instance declaration for `MArray IOUArray (S a) IO' But I don't see why this is undecidable. As I understand it, undecidable instances basically arise when context reduction might not terminate. Here, it always will (AFAICS). Any thoughts? - Hal

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RE: A run-time problem?
by Simon Marlow 18 Oct '02

18 Oct '02

> I'm occasionally getting an error, when running a compiled program, I > get: > > | fatal error: GetMBlock: misaligned block 0x401fe000 returned when > | allocating 1 megablock(s) at 0xbff00000 > > apparently, this only happens on one computer (Red Hat 7.1), and not > others (Red Hat 7.2), which leads me to suspect it's a library issue. > > This is a known problem, yes? Is it possible to resolve this, apart > from upgrading the OS, or recompiling? It looks like your program is using a *lot* of memory, because the requested address is right near the top of the address space, and is probably running into the stack, hence the failure. Is that the case? I'm looking into fixing one problem that results in the getMBlock failure, but I don't think it'll help much in your case. Cheers, Simon

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A run-time problem?
by ketil＠ii.uib.no 18 Oct '02

18 Oct '02

Hi, I'm occasionally getting an error, when running a compiled program, I get: | fatal error: GetMBlock: misaligned block 0x401fe000 returned when | allocating 1 megablock(s) at 0xbff00000 apparently, this only happens on one computer (Red Hat 7.1), and not others (Red Hat 7.2), which leads me to suspect it's a library issue. This is a known problem, yes? Is it possible to resolve this, apart from upgrading the OS, or recompiling? -kzm -- If I haven't seen further, it is by standing in the footprints of giants

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RE: packages and profiling
by Simon Marlow 18 Oct '02

18 Oct '02

> I've been playing around with the package mechanism in ghc which > is great. > > However it took me quite a while to realise that when I compile > a module (say Foo.hs, for a package) with profiling, the .hi file > must be named Foo.p_hi, and not Foo.hi, when it appears in the > "import_dirs" directory. > > When the interface was called Foo.hi and I tried to use the package > the compiler complained: > > ghc-5.04: can't find module `Foo' (while processing "Main.hs") > > this is with ghc --make Thanks, I've added a note to the docs. Cheers, Simon

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packages and profiling
by Bernard James POPE 18 Oct '02

18 Oct '02

Hi all, I've been playing around with the package mechanism in ghc which is great. However it took me quite a while to realise that when I compile a module (say Foo.hs, for a package) with profiling, the .hi file must be named Foo.p_hi, and not Foo.hi, when it appears in the "import_dirs" directory. When the interface was called Foo.hi and I tried to use the package the compiler complained: ghc-5.04: can't find module `Foo' (while processing "Main.hs") this is with ghc --make I might have realised the problem earlier if the error message had said something about what file it was looking for, something like: "couldn't find Foo.p_hi in [wherever it was looking]" Although the documentation for packages mentions the "_p" suffix behaviour for library archives I don't think it mentions the suffix for interface files. Perhaps creating packages with profiling should have its own section in the documentation? Cheers, Bernie

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foralls in newtypes
by Hal Daume III 17 Oct '02

17 Oct '02

I was reading the HFL libs, namely Control.Monad.Logic, and there's a definition in there: newtype Logic a = Logic { mkLogic :: (forall b. (a -> b -> b) -> b -> b) } I'm curious why this is legal, but newtype Logic2 a = forall b . Logic2 ((a -> b -> b) -> b -> b) is not... - Hal -- Hal Daume III "Computer science is no more about computers | hdaume(a)isi.edu than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume

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Re: Database library?
by Ashley Yakeley 16 Oct '02

16 Oct '02

At 2002-10-16 09:49, Bryn Keller wrote: >Does anyone have a GHC-friendly library for accessing databases, >specifically Oracle and MSSQL? My HBase project includes HMySQL, for accessing MySQL databases. It's kind of a separate project from HBase (because it uses FFI), but it's in the same CVS repository. It's rather incomplete at the moment. <http://sourceforge.net/projects/hbase/> HBase proper includes an interesting monad for generating SQL "SELECT" statements. But HBase is it's own little world, replacing the Prelude... -- Ashley Yakeley, Seattle WA

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