February 2002 - Glasgow-haskell-users

Importing a reference to a non-function entity.
by George Russell 19 Feb '02

19 Feb '02

What is the politically correct way using the FFI and ghc5.03 (which I have just managed to compile from CVS) to import an externally defined object (not a function)? I have a source file containing the line foreign label "default_options" defaultOptions :: CString.CString for which ghc5.03 complains about "Warning: foreign declaration uses deprecated non-standard syntax", but which works anyway. However in my attempt to be a good little ghc user, I try instead foreign import "default_options" defaultOptions :: CString.CString (after deciphering with difficulty the Postscript version of the FFI document on this screen) but still get the deprecated warning. Then I try foreign import ccall "default_options" defaultOptions :: CString.CString and praise be, the deprecated message goes away. Only problem is, I now get a segmentation fault when I run the code . . .

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setFileTimes
by Markus Lauer 19 Feb '02

19 Feb '02

Hi, as the posix module isn't avalible under windows, is there any function to set the modification time of a file or directory under windows with ghc? Thanks, Markus Lauer

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RE: i368-pc-os2-emx port
by Russell O'Connor 19 Feb '02

19 Feb '02

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 One thing I could try would be to see if I can get a version of GHC to run under ODIN (ODIN is a Win32 API implementation under OS/2). Do you think this would be helpful? Or would it lead me down the path of making an OS/2 complier that produced Windows Executables (that would be bad). - -- Russell O'Connor roconnor(a)math.berkeley.edu <http://www.math.berkeley.edu/~roconnor/> ``This is not a time, as it is never a time, to seek vengeance, but a time to seek the courage to forgive'' -- George W. Bush -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.0.6 (SunOS) Comment: For info see http://www.gnupg.org iD8DBQE8cpifuZUa0PWVyWQRAu2RAKCXVoKOKLLX9Qd2cmSjWgMT/qyFxwCfSH2A dc5wBP4lUe3lTzmb/sknNW4= =63XF -----END PGP SIGNATURE-----

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RE: Strictness information?
by Simon Peyton-Jones 19 Feb '02

19 Feb '02

'V' means 'eValuates'. So your function is strict in both arguments. It's intended to be internal compiler stuff which is while it's not properly documented, I'm afraid. Simon | -----Original Message----- | From: Till Dörges [mailto:mai00dqr@informatik.uni-leipzig.de] | Sent: 17 February 2002 12:44 | To: Glasgow-haskell-users(a)haskell.org | Subject: Strictness information? | | | Hi folks, | | during the struggle for speed of my program, I've tried to | check the strictness of my functions, where most of the time | is consumed. Unluckily the ghc documentation doesn't mention | the strictness-types I found for the most part. :-( | | For the particular function in question it reads in the *.hi file: | | --- snip --- | isSubPosnOf :: Posn -> Posn -> PrelBase.Bool {-## __A 2 __C | __S VV __U (__inline_me (\ a2 :: Posn b :: Posn -> | PrelBase.zaza (zddmzsze1 a2 b) (isPrefixOf b a2))) ##-}; | --- snap --- | | If I understand it correctly I must look at '__S VV', but | what does V mean? (The doc only talks about L,S,E,P,U,A). | | Thanks -- Till | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users(a)haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-| haskell-users |

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fyi: haskell.org/ghc and Netscape win32 users
by Sigbjorn Finne 18 Feb '02

18 Feb '02

Hi, if you've run into problems installing haskell.org/ghc Windows Installers (.msi files) that you've downloaded with Netscape Navigator in the past couple of weeks, this is to let you know that the (new) web server setup has now been fixed, so downloading with Netscape should again work OK. Thanks to Christian Mann for pointing this out. --sigbjorn

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ghc-5.02.2 cannot parse string gaps
by Michael Marte 18 Feb '02

18 Feb '02

Hello *, Section 2.6. of the Haskell report says: A string may include a "gap"---two backslants enclosing white characters---which is ignored. This allows one to write long strings on more than one line by writing a backslant at the end of one line and at the start of the next. For example, "Here is a backslant \\ as well as \137, \ \a numeric escape character, and \^X, a control character." ghc-5.02.2 cannot parse string gaps. It accepts \ at the end of a line but not at the beginning of the following line. Michael

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RE: Minor suggestion
by Simon Marlow 18 Feb '02

18 Feb '02

> The need to recompile everything after upgrading was recently > mentioned, I think, on this list. After upgrading from .1 to .2 > (trying to get profiling on track), ghci gave me a slightly cryptic > message about an unknow symbol ("stg_gc_enter_1"). No big deal, and > after I removed the .o's, everything seems fine. > > But perhaps ghc could tag files with its version, so that ghci (and > possibly ghc when used for linking?) could be a bit more explicit? Yes, I'm planning to do this at some point. Cheers, Simon

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RE: The size of things
by Simon Marlow 18 Feb '02

18 Feb '02

> I'm building some stuff here that is basically a glorified and > specialized version of quicksort. It is very quick, and works like a > charm for my purposes, except that it consumes way too much space, > about 100 bytes times the size of the input list. Which means that, > for my data sets, the program exceeds available RAM and uses swap, > which in turn kills performance. > > Is there any guideline how much space the various constructs use? > E.g. how much for cons cells (or whatever that make up lists), how > much for a data type with only nullary data constructors -- should I > use Word8s instead? -- how much for tuples, and so on. I realize I > can (and I do somewhat) use profiling to determine heap use, but it'd > be nice to be able to do back-of-envelope calculations and have some > idea of what's the right approach. Ok, roughly speaking, in GHC every object requires one word plus one word for each field (a word is 4 bytes on a 32-bit machine). In other words, a cons cell will require 3 words. An Int requires two words. A Char usually requires no words (chars up to 256 are cached in the RTS). Nullary constructors require no dynamic storage. The smallest dynamic object is 2 words: ie. a Word8 requires 2 words, even though the useful data it contains is only a single byte. Of course, all of this just applies to fully-evaluated structures - any suspensions (thunks) will screw up the calculations, and certainly the amount of temporary allocation required to *generate* a particular structure is very rarely predictable. You can save storage by using strict constructor fields and using the -funbox-strict-fields flag. eg. data FloatList = FloatNil | FloatCons !Float FloatList requires 3 words for each float in the list, because its actual representation is data FloatList = FloatNil | FloatCons Float# FloatList (Float# is an unboxed float). We heartily recommend strict constructor fields: apart from saving space and time, they help avoid space leaks. Using -funbox-strict-fields is a good way to get the benefits of unboxed types without digging around in GHC's low-level primitive types & operations. Does that help? Cheers, Simon

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Re: Instances of IArray
by Albert Lai 18 Feb '02

18 Feb '02

I had looked into the source code of IArray a bit. Like you have found, (!) is not a class method. In fact, none of the claimed class methods in the doc is a class method, and no class method of IArray is documented. Instead, IArray class methods seem to pertain to implementation details (I mean, "unsafeAt"!), and user operations such as (!) are constrained polymorphic functions, e.g., (!) :: (IArray a e, Ix ix) => a ix e -> ix -> e >From the user's point of view, there is little difference between a class method and a class-constrained polymorphic function when the provided instances are abstract types. So the documentation is telling a white lie. Of course, you see the difference when you roll your own IArray instance. I think the white lie is a balance between exposition simplicity and implementation tuning.

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RE: The size of things
by Simon Marlow 18 Feb '02

18 Feb '02

> ketil(a)ii.uib.no (Ketil Z. Malde) writes: > > > Thanks for all your help, though! > > One interesting(?) observation, > > using a custom data type of > > data STuple = STuple !Int Foo > > is slightly less efficient than using a normal tuple > > (Int,Foo) Just checking... with -funbox-strict-fields, right? > Seems to go against theory, in which STuple should use three words > plus the Foo, while (,) should use three words plus the Foo, plus two > for the Int. Perhaps the Ints are constructed anyway, since they are > inserted from a function parameter? Sounds reasonable, I suppose; > memory use seemed to be the same (from "top"), and there was only a > speed difference. It's possible that the boxed Int is being reconstructed for some reason. You'll be able to see the difference more accurately using heap profiling. > And one more thing, would parametrised types make a difference? > E.g. my array is > > data MyArray label elts = MA label (Array Int elts) > > which means I have > > data Foo l a = Foo (MyArray l a) !Int > > and so on. As far as I can see, this shouldn't matter (a heap object > is a heap object), but am I seeing far enough, I wonder? A polymorphic strict field can't be unboxed - eg. data StrictList a = Scons !a | Snil will never result in the elements being unboxed, even in (StrictList Int). Apart from that, parameterised data types make no difference to the storage representation. Cheers, Simon

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