May 2007 - Haskell-Cafe

Re: [Haskell-cafe] The C Equiv of != in Haskell
by PR Stanley 29 May '07

29 May '07

At 03:02 29/05/2007, you wrote: >On 5/29/07, PR Stanley <prstanley(a)ntlworld.com> wrote: >>What is this, the Haskell Cafe Gang? > >Maybe. Moreover this list is not a prepaid service in which you can >unpolitely _demand_ answers. > >You got your question answered still complain ... I don't get it. Yes, the question has been answered but you still feel the need to keep the thread alive, hmm, you're right, it is very strange behaviour indeed.

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The C Equiv of != in Haskell
by PR Stanley 29 May '07

29 May '07

Hi What is the C equivalent of the inequality operator in Haskell? Thanks Paul

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Bug in GADT Implementation?
by Dominic Steinitz 28 May '07

28 May '07

I would expect foo r@(Range BITSTRING _ _) x = [] to give an error but it doesn't. Writing t = Range BITSTRING gives one so why not the pattern match? Dom. data BitString = BitString [Bool] deriving Show data ConstrainedType :: * -> * where INTEGER :: ConstrainedType Int BITSTRING :: ConstrainedType BitString Seq :: ConstrainedType a -> ConstrainedType b -> ConstrainedType (a,b) Range :: Ord a => ConstrainedType a -> a -> a -> ConstrainedType a foo :: ConstrainedType a -> a -> [Int] foo BITSTRING (BitString x) = map fromEnum x foo INTEGER x = [x] foo r@(Range INTEGER _ _) x = [upperInt r,x] foo r@(Range BITSTRING _ _) x = [] r1 = Range INTEGER 20 40 r2 = Range r1 25 45 upperInt :: ConstrainedType Int -> Int upperInt INTEGER = 256 upperInt (Range t l u) = min u (upperInt t)

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Curiose types
by Andrew Coppin 28 May '07

28 May '07

As I lay in bed last night, a curios fact occurred to me. (Yes, I don't get out very much...) Consider the map function: map :: (a -> b) -> [a] -> [b] There are two ways you can think about this function. First, you can see it as meaning map :: (a -> b) -> ([a] -> [b]) Which is beautifully symmetric. Alternatively, you can think about how you actually use it: map :: ((a -> b) -> [a]) -> [b] Now this got me thinking: is (->) associative? x_x Well now, let's consider a general 3-argument function: foo :: a -> b -> c -> d According to the rules, this is of course equivilent to foo :: a -> (b -> (c -> d)) If you understand what currying is and how it works, this makes perfect sense. Now let's bracket that the other way: foo :: ((a -> b) -> c) -> d Well, while that type *does* make sense, it's clearly a completely *different* type! So, clearly, (->) is not associative. Now I'm left wondering why you can bracket the type for map in two different ways, but not in general. Hmm...

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Re: [Haskell-cafe] Language extensions
by Andrew Coppin 28 May '07

28 May '07

Chaddaï Fouché wrote: > 2007/5/28, Andrew Coppin <andrewcoppin(a)btinternet.com>: >> Then again, later on in the very same book there's a chapter entitled >> "Fun with Phantom Types", which made precisely no sense at all... >> > > You seem to think you're extremely clever since if something doesn't > make sense to you it _obviously_ doesn't make sense at all... Well it > at least made sense for the authors of the article, and lots of person > who read it. Surely it makes sense to someone. (But then, there are people to whom number theory makes sense.) I didn't mean to imply that it is *impossible* to comprehend the article, only that it's very greatly beyond my capabilities. I like to think I'm an intelligent, but... maybe I'm just kidding myself...

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Random ramble
by Andrew Coppin 28 May '07

28 May '07

[You may want to just hit delete now? ;-) ] I don't know where everybody else is, but where I live, we have various science programs on TV from time to time. (Horizon being a fairly regular one.) They grab a bunch of eggheads, and try to get them to explain (in soundbites) some topic or other. Sometimes it's string theory, or quantum dynamics, or chaos theory, or something like that. (And sometimes it's economics - and then I change channel.) Usually the producers go to great lengths to make the subject seem "interesting" - that is, try to be as over-dramatic as possible. Often accompanied with copious computer graphics, which mainly just look pretty. Anyway, I was sitting here thinking to myself "hey, what if they got a bunch of people to talk about Haskell?" (That's *one* way to get publicity...!) I don't know how many of you are as neurotically familiar with The Matrix as I am, but... I found myself visualising that scene from about mid-way through the film. Morpheus: "This may feel... a little weird." [6 inch metal plug injected into the back of Neo's skull] Neo: "AAAAAAAAAAARRGH!!" [They turn on the computer.] [Neo finds himself floating in a sea of lambda functions, higher-kinded class instances and parametric polymorphism.] ...and then I find myself thinking of http://xkcd.com/c224.html Erm... OK, I'll go sit in the corner and be quiet now.

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GADT and typeclasses [was: Language extensions]
by oleg＠pobox.com 28 May '07

28 May '07

Philippa Cowderoy wrote: > For example, GADTs let you implement monads as interpreters by defining a > datatype representing the abstract syntax tree that describes a > computation - you can't get this to type without at a minimum existential > types and for many monad operations you need the full power of GADTs to > declare a corresponding constructor. I'm yet to see the example of that need. I have seen the examples that the need for GADT was _claimed_ -- but then it turns out the example is implementable without GADT after all. Here are a few such examples: implementing State monad in a free term algebra Initial (term) algebra for a state monad http://www.haskell.org/pipermail/haskell-cafe/2005-January/008241.html Implementing an interpreter in HOAS with fix Even higher-order abstract syntax: typeclasses vs GADT http://www.haskell.org/pipermail/haskell/2007-January/019012.html

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Parsec question
by Andrew Coppin 27 May '07

27 May '07

Greetings. I'd like to write a parser that takes some Haskell source code and seperates it into two piles - comments, and everything else. I have a parser that recognises single-line comments, and another that recognises multi-line comments. What I'd like to do is make a big parser that returns [Either String String], which all the comments in one side and all the rest in the other side. But I can't figure out how to construct such a parser... Any hints?

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Broadcast signals between threads
by Joachim Breitner 27 May '07

27 May '07

[Arg. Just lost the mail and have to re-type it. Sorry if I’m not as verbose as I should be]. Hi, I’m writing a TCP server app ATM. It has one thread per client. Some of the clients want to be notified if the internal state changes, while others are happily chatting with the server, possible modifying the internal state. What I need now is a way for the chatting thread to signal “anyone interested” that the state has changed. The semantics should be roughly these: * All listening threads run some wait-for-signal command that blocks until the signal is sent. * The state changing thread notifies the others by some signal-sending-command, which should not block. * It does not matter if no one is listening, or if threads miss signals because they are doing something else for the moment, or if several sent signals are handled only once. Here is my implementation so far: newtype MSignal a = MS (MVar a) newMSignal = MS `liftM` newEmptyMVar sendMSignal (MS mv) v = do forkIO $ takeMVar mv >> return () -- Cleanup afterwards putMVar mv v receiveMSignal (MS mv) = readMVar mv It builds on MVar’s promise that takeMVar and readMVar commands are handled FIFO, so by spawning a takeMVar thread, the sender ensures that exactly after all currently waiting threads are served the value is taken out of the MVar and everything is done. readMVar will both take the value and put it back, so that the next waiting thread (or the sending thread’s cleanup thread) can take the value. Do you think this is a sensible way to do it? Any pitfalls that I have overlooked? Might this be useful enough to be included in some mainstream library? Greetings, Joachim -- Joachim Breitner e-Mail: mail(a)joachim-breitner.de Homepage: http://www.joachim-breitner.de ICQ#: 74513189

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FFI and multi-threading
by Maurício 27 May '07

27 May '07

Do you guys know of a good example of Haskell calling functions to a thread-safe library (like an existing interface to a real library)? I have a function to which I need to alocate memory for each thread calling it, but I don't know how to do that. Thanks, Maurício

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