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Basic Haskell Types Question
by Adam Wyner 16 Sep '05

16 Sep '05

Hi, I have a basic question about how output types are reported. I'm using Hugs. Suppose I have the following, where the type String is predefined in the Prelude as [Char]. type PropList = [String] atomicProps1 :: PropList atomicProps1 = ["prop1"] And I have a function on expressions of type PropList which takes every string from the input list and prefixes the string "neg-" to it. negationAtomicProps :: PropList -> PropList negationAtomicProps inPropsList = [ "neg-" ++ inString | inString <- inPropsList] So, with the following, I get the right result: > negationAtomicProps atomicProps1 ["neg-prop1"] The types for the parts are right: > :t negationAtomicProps negationAtomicProps :: PropList -> PropList > :t atomicProps1 atomicProps1 :: PropList But, what I don't understand or like is that the type of the whole is given as [[Char]], not what I thought it would be, namely PropList. Nor is it [String]. > :t negationAtomicProps atomicProps1 negationAtomicProps atomicProps1 :: [[Char]] I really want to get the following sort of report for the type: negationAtomicProps atomicProps1 :: PropList The reason I want this is that in more complex cases, it will be easier to see what is going wrong (as the functions are developed) if I can see the types I want. Am I supposed to be using newtype or data declarations here? How would I do this? I looked around for information in the usual sources, but haven't found an answer. Thanks, Adam Wyner

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Functional vs Imperative
by Dhaemon 15 Sep '05

15 Sep '05

Hello, I'm quite interested in haskell, but there is something I don't understand(intuitively). I've been crawling the web for an answer, but nothing talks to me... So I was hoping I could find some help here: "How is evaluating an expression different from performing action?" I'm puzzled... Doesn't it amount to the same thing? Maybe I have a wrong definition of "evaluating"(determine the value of an expression)? Examples would be appreciated. Also, just for kicks, may I had this: I read the code of some haskell-made programs and was astonished. Yes! It was clean and all, but there were "do"s everywhere... Why use a function language if you use it as an imperative one?(i.e. most of the apps in http://haskell.org/practice.html) Thanks in advance,

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Binary parser combinators and pretty printing
by Einar Karttunen 15 Sep '05

15 Sep '05

Hello I am trying to figure out the best interface to binary parser and pretty printing combinators for network protocols. I am trying to find the most natural syntax to express these parsers in Haskell and would like opinions and new ideas. As an example I will use a protocol with the following packet structure: 0 message-id 4 sender-id 8 receiver-id 12 number of parameters 16 parameters. Each parameter is prefixed by 32bit length followed by the data. We will use the following Haskell datatype: data Packet = Packet Word32 Word32 Word32 [FastString] 1) Simple monadic interface getPacket = do mid <- getWord32BE sid <- getWord32BE rid <- getWord32BE nmsg<- getWord32BE vars<- replicateM (fromIntegral nmsg) (getWord32BE >>= getBytes) return $ Packet mid sid rid nmsg vars putPacket (Packet mid sid rid vars) = do mapM_ putWord32BE [mid, sid, rid, length vars] mapM_ (\fs -> putWord32BE (length fs) >> putBytes fs) vars This works but writing the code gets tedious and dull. 2) Using better combinators packet = w32be <> w32be <> w32be <> lengthPrefixList w32be (lengthPrefixList w32be bytes) getPacket = let (mid,sid,rid,vars) = getter packet in Packet mid sid rid vars putPacket (Packet mid sid rid vars) = setter packet mid sid rid vars Maybe even the tuple could be eliminated by using a little of TH. Has anyone used combinators like this before and how did it work? 3) Using TH entirely $(getAndPut 'Packet "w32 w32 w32 lengthPrefixList (w32 bytes)") Is this better than the combinators in 2)? Also what sort of syntax would be best for expressing nontrivial dependencies - e.g. a checksum calculated from other fields. 4) Using a syntax extension Erlang does this with the bit syntax (http://erlang.se/doc/doc-5.4.8/doc/programming_examples/bit_syntax.html) and it is very nifty for some purposes. getPacket = do << mid:32, sid:32, rid:32, len:32 rest:len/binary >> ... The list of lists gets nontrivial here too... - Einar Karttunen

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RE: [Haskell] ANNOUNCE: ghc-src version 0.2.0
by Simon Marlow 15 Sep '05

15 Sep '05

On 30 August 2005 12:05, Arthur Baars wrote: > Daan is right, I wrote a parser for GHC using Doaitse Swierstra's > parsing combinator library > (http://www.cs.uu.nl/groups/ST/Software/UU_Parsing/index.html) > I needed a drop-in replacement for GHC's Happy parser, to make a > prototype for syntax macros. Syntax Macros allow a programmer to > extend a language with new syntax. Combinator based parsers parsers > can be dynamically extended, making them suitable for implementing > syntax macros. Unfortunately, I never had time to really finish the > syntax macro implementation. > > But I did finish the combinator based parser for GHC. I tested it by > having GHC( with combinator parser) compile itself and all the > libraries. This took about 10% longer than with the original GHC, so > in practice its speed is acceptable. With all due respect, a 10% increase in compile time isn't acceptable at all! And when you consider that parsing is less than 10% of compile time overall (probably much less), a 10% increase represents at least a factor of 2 in the parser. I'm not criticising the work at all - far from it, just the notion that we would consider adding 10% to GHC's compile times "acceptable". I've recently been struggling to shave a few percent off GHC's compile times, BTW :-) Cheers, Simon

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Doing Windows Programming
by Brian McQueen 15 Sep '05

15 Sep '05

How can I use Haskell to do general Windows programming, like you would be able to do if you were using one of those Windows IDEs: *moving data between windows apps *gaining access to windows registry *in general, access to the available Windows APIs I'm sure folks must be writing Windows apps in Haskell somewhere. How do I get started? Brian McQueen

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RE: Re[2]: [Haskell-cafe] Weak hashtable memoization code?
by Simon Marlow 15 Sep '05

15 Sep '05

On 15 September 2005 13:44, Bulat Ziganshin wrote: > Hello Simon, > > Tuesday, September 13, 2005, 7:42:52 PM, you wrote: > >> There's the memo table implementation in the util package: >> hslibs/util/Memo.lhs. Note that this is scheduled for demolition in >> GHC >> 6.6. > > why? It doesn't perform very well, there's a general lack of interest, and nobody interested in supporting it. The right thing to do is make it available as a Cabal package. Cheers, Simon

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Autrijus Tang interviewed by Perl.com
by John Goerzen 15 Sep '05

15 Sep '05

metaperl posted about this on the Haskell Sequence this morning and I thought all of you list readers might be interested as well. Autrijus Tang is well-known for developing the first working Perl 6 interpreter, Pugs. Pugs is written in Haskell. Perl.com has an interview with Autrijus, and page 2 of that interview gets particularly interesting. URL: http://www.perl.com/pub/a/2005/09/08/autrijus-tang.html?page=2 Favorite quote: "Haskell . . . is faster than C++, more concise than Perl, more regular than Python, more flexible than Ruby, more typeful than C#, more robust than Java, and has absolutely nothing in common with PHP." -- John

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RE: [Haskell-cafe] Weak hashtable memoization code?
by Simon Marlow 15 Sep '05

15 Sep '05

On 28 August 2005 16:39, Bulat Ziganshin wrote: > Hello David, > > Sunday, August 28, 2005, 4:19:07 PM, you wrote: > >> Hi all, > >> Does anyone have a nice bit of example code to implement memoization >> using weak pointers and hash tables? It would be nice to have a >> pre-packaged module that I could just use, which has already been >> tested. The contents of Data.WeakPtr seem a bit lower-level than >> I'd rather work with. > > http://www.haskell.org/~simonmar/papers/weak.pdf There's the memo table implementation in the util package: hslibs/util/Memo.lhs. Note that this is scheduled for demolition in GHC 6.6. Cheers, Simon

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How to call ICS from Haskell
by Huong Nguyen 15 Sep '05

15 Sep '05

Hello everybody, Do you know how to call ICS (Integrated Canonizer and Solver: www.icansolve.com <http://www.icansolve.com>) or PVS (Prototype Verification System) from Haskell ? Thank you in advance.

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Re: Functional vs Imperative
by Dhaemon 15 Sep '05

15 Sep '05

Hi again, Thanks to everyone for replying. I think I get it now; I was focusing on the wrong aspect of the question. I was totally overlooking referential transparency and the cascade of 'capabilities'(lazyness, etc) it creates. Thus evaluation is just getting values out of expressions, actions(which modify the state of the 'world') are never performed... Except when 'cheating' with monads. Hat-anim is really interesting, I'm playing with it at the moment... Thanks again, Best regards,

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