February 2006 - Haskell-Cafe

FFI question
by Frederico Franzosi 09 Feb '06

09 Feb '06

I'll try to make it short. I'm developping a package wich imports C functions. the fact is that when I try to compile if I call the compiler in the usual way, using -package and -llib it gives an undefined reference error... For example if I use: $ghc -package PKGname-PKGversion -fffi -o main Main.hs -llib it gives an error message (undefined references of objects from PKG to functions in lib) but if I use: $ghc -fffi -o main Main.hs -llib /path/to/PKGlib.a it compiles correctly. and also when I do: $ghc -package PKGname-PKGversion -fffi -o main Main.hs /path/to/lib.a it compiles correctly. I know that it is surely my mistake but I didn't find anything helpful in docummentation or in FFI list. I know that I should send this message to ffi mailing list but it is kind of stopped and I think I'll get my answers here... Thank you for the atention.

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Elementary HaXml question
by Koen.Roelandt＠mineco.fgov.be 09 Feb '06

09 Feb '06

Hi all, I'm new to Haskell and HaXml and I'm playing around with the latter to clean some (well-formed) 'legacy' html. This works fine except for the following cases. Some of the elements to be cleaned are: Hello World Hello World This should become: <h1 class="subtitle">Hello World</h1> I can build filters to find , or , but the combination does not seem to work. >From what I could gather from the documentation, it should be something like: foldXml (txt ?> keep :> (attrval("size",AttValue[Left "4"]) `o` tag "font") /> tag "i" ?> replaceTag "h1" :> children) This doesn't work. I am clearly missing something elementary here, so any hints are welcome. Cheers, K.

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Type inference
by Fred Hosch 09 Feb '06

09 Feb '06

Is type inferencing in Haskell essentially the same as in SML? Thanks. ---Fred Hosch

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Fast Mutable Variables for the IO and ST monads
by Bulat Ziganshin 08 Feb '06

08 Feb '06

Hello haskell-cafe, only for the souls interesting in writing efficient programs :) i included in this letter my own module used for fast variables - "DataVariables.hs". and "wc.hs" contains example of using these vars. DataVariables module contains many interesting beasts but for optimization purposes newVar/readVar/writeVar is especially interesting. these primitives mimics IORef interface, supports Int/Word/Bool variables, works in IO and ST monad and nevertheless they are compiled to the most-effective GHC primitives. moreover, they can be used outside GHC (of course, on other compilers just IORef/STRer are used to implement this) it was impossible to add Ptr's support with the same interface, so Ptrs supported by another functions - newPtr/readPtr/writePtr and don't work in ST monad. in other aspects fast Ptr variables are the same as fast general variables if you need to include these variables in data structures, use the types IntVar/WordVar/BoolVar/"MutPtr a" for the IO monad, and "MutInt s", "MutWord s", "MutBool s" for the ST monad another interesting beasts are unboxed Int and Bool values. although GHC can automatically unbox vars in some cases, using this module will guarantee unboxing. moreover, some operations such as bit shifts, are ineffective without explicit unboxing because their definitions don't use unboxed operations directly, but adds additional checks and conversions. using types FastInt and FastBool and associated operations guarantee unboxing on GHC and works on other compilers (where just boxed values are used to implement these types). one place from my lib where i used them to speed up program: putWord32be :: (Stream m h, Integral int, Bits int) => h -> int -> m () putWord32be h w = do let n = iUnbox (fromIntegral w) vPutByte h $! iBox ((n >># _ILIT 24) ) vPutByte h $! iBox ((n >># _ILIT 16) &# _ILIT 0xff) vPutByte h $! iBox ((n >># _ILIT 8) &# _ILIT 0xff) vPutByte h $! iBox ((n ) &# _ILIT 0xff) -- Best regards, Bulat mailto:bulatz@HotPOP.com

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Re: Re[2]: Tuple-like constructors
by Robert Dockins 08 Feb '06

08 Feb '06

[Moved to cafe; time to stop bothering the Haskell' committee...] On Feb 8, 2006, at 1:19 PM, Malcolm Wallace wrote: > Robert Dockins <robdockins(a)fastmail.fm> writes: > >> instance (Bin a,Bin b,Bin c,Bin d) => Bin (a,b,c,d) >> >> See the problem? Sooner or later (probably sooner) I'll get tired of >> typing. I have to write down an 'instance' declaration for each >> value of n. Clearly this can't generalize to all n. > > There has been a suggestion that the 'deriving' mechanism be de- > coupled > from the datatype declaration. Together with a generic default > definition, that means you could write something like > > deriving Bin for (,,,,,,,,,,,,,,,,,,,,) > > and hence not need to write the tedious instance header yourself, > since the compiler can easily infer it. Humm. That is nice, and it would help keep my fingers from cramping, but it doesn't solve the root objection. Consider machine-generated code using largish tuples. Should the generated code include the 'deriving' clause? If no, someone from outside has to supply it. If yes, I have to deal with overlapping instance from other generated code. If maybe, I have to know when to generate it and when not. Solvable? yes, but kind of ugly. Acceptable? probably. Now, (speculatively combining a whole host of language extensions) if I could write down: deriving Bin for (_::*) Which would mean "derive (as necessary) Bin for all types at kind * where the suitable preconditions can be found/derived" I'd be completely happy. Humph. That's more or less adding an axiom schema to the constraint solving system of the typehecker which could be realized at link-time via C++ style template thingies or at compile-time by whole-program analysis. I am simultaneously appalled by and attracted to this idea..... Rob Dockins Speak softly and drive a Sherman tank. Laugh hard; it's a long way to the bank. -- TMBG

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Spotted in Haskell Weekly News: February 06, 2006
by Graham Klyne 08 Feb '06

08 Feb '06

Donald Bruce Stewart wrote: > Haskell Weekly News: February 06, 2006 : > Oleg says, "The implementation of RSA on the type level is left > for future work". Methinks this gives a whole new meaning to "type security". :) #g -- Graham Klyne For email: http://www.ninebynine.org/#Contact

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Re: [Haskell-cafe] Why is $ right associative instead ofleftassociative?
by John Hughes 08 Feb '06

08 Feb '06

Lennart Augustsson wrote: > I now think :: for type signatures was a bad mistake. > I don't use lists very much. They are not the right data structure > for many things. So : is not as common as :: in my code. > I checked a small sample of code, about 20000 lines of Haskell. > It has about 1000 uses of ':' and 2000 of '::'. Just for interest, I analysed some of my code. Obviously my style is quite different to yours--my type specialiser of 3,500 lines has 240 conses, and only 22 occurrences of '::'. I seem to be using '::' a bit more lately, though, which I suspect is due to using classes much more. I also checked the Agda source code, about 14,000 lines, with about 500 occurrences of cons and 640 of '::'. I think the only conclusion one can draw is that style varies. > In my opinion all the special syntactic sugar for lists should go > away. I don't think lists are special enough to motivate it. What, no list comprehensions?? I'd disagree--sequencing is special, and lists represent it directly. Don't forget, also, that lists are also much more prevalent in beginners' code--and nice notation for beginners helps get people started on Haskell. > But this is not what Haskell' is about. It's supposed to be some > modest extensions to Haskell. Not designing a new perfect language. Right! John

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does haskell have plist's ?
by raptor 07 Feb '06

07 Feb '06

does Haskell have a property lists. Like Lisp ? any pointer to examples ? tia

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haskell reference card ?
by raptor 07 Feb '06

07 Feb '06

is there somewhre Haskell reference card ?

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EclipseFP (Haskell IDE) 0.9.1 released
by Thiago Arrais 07 Feb '06

07 Feb '06

EclipseFP 0.9.1 has been released since last Friday. It is an open-source development environment for Haskell code. EclipseFP integrates GHC with an Haskell-aware code editor and also supports quick file browsing through an outline view, automatic building/compiling and quick one-button code execution. Downloads and more information are available on the project home page http://eclipsefp.sourceforge.net/ We are open for comments and general discussion. Actually we would really appreciate comments from both newbie and veteran Haskell programmers. This is open-source development and everyone on the Haskell community is welcome to participate. Cheers, Thiago Arrais

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