September 2007 - Haskell-Cafe

ANN: The Monad.Reader - Issue 8
by Wouter Swierstra 10 Sep '07

10 Sep '07

I am pleased to announce that the latest issue of The Monad.Reader is now available: http://www.haskell.org/haskellwiki/The_Monad.Reader Issue 8 consists of the following two articles: * Brent Yorgey Generating Multiset Partitions * Conrad Parker Type-Level Instant Insanity The Monad.Reader is a quarterly magazine about functional programming. It is less-formal than journal, but somehow more enduring than a wiki page or blog post. If you'd like to submit something to the next issue of The Monad.Reader, please get in touch. I haven't fixed the deadline for the next issue just yet; I hope to send out an official call for copy in the coming weeks. I'd also like to point out that you can now order TMR apparel from: http://www.cafepress.com/TheMonadReader. Enjoy! Wouter This message has been checked for viruses but the contents of an attachment may still contain software viruses, which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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Re: [Haskell-cafe] Tiny documentation request
by manu 10 Sep '07

10 Sep '07

Neil Mitchell wrotes : > Replicating actual tables with CSS is a nightmare - you shouldn't use > table's for lots of things, I agree > but there are sometimes when it really is > the best option. Which isn't the case here ! Nested lists would easily do the trick... > Fixing up the CSS and still keeping tables is a > perfectly valid option. yes it's true... E.D

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Re: [Haskell-cafe] Speed of character reading in Haskell
by Miguel Mitrofanov 10 Sep '07

10 Sep '07

> I started with the obvious > main = getContents >>= print . tokenise > where tokenise maps its way down a list of characters. This is very > simple, very pleasant, and worked like a charm. > However, the language has an INCLUDE directive, so I'm going to have > to call readFile or something in the middle of tokenising, so the > main tokeniser loop can't be a pure String -> [Token] function any > more. What about > tokenise :: [String] -> ([Token],[FilePath]) > main = print . fst =<< mfix process where > process (tokens,paths) = do > mainContents <- getContents > includes <- mapM readFile paths > return $ tokenise $ mainContents : includes I guess, it would be useful to replace ([Token],[FilePath]) with Writer [FilePath] [Token] > Method 1A (pure list processing) > main = getContents >>= print . doit 0 > doit n ('\n':cs) = doit (n+1) cs > doit n ( _ :cs) = doit n cs > doit n [] = n I think, you should use something like (doit $! n+1) cs here. > In *retrospect*, it is really obvious why this was > necessary, but I must say that in *prospect* I wasn't expecting it. In fact, I was expecting this to be an issue even for 1A. I suppose, GHC is smart enough to suppress lazyness in the first method.

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Elevator pitch for Haskell.
by Paul Johnson 10 Sep '07

10 Sep '07

This page (http://www.npdbd.umn.edu/deliver/elevator.html) has a template for an "elevator pitch". This is what you say to someone when you have 30 seconds to explain your big idea, for instance if you find yourself in an elevator with them. I thought I'd try instantiating it for Haskell. For software developers who need to produce highly reliable software at minimum cost, Haskell is a pure functional programming language that reduces line count by 75% through reusable higher order functions and detects latent defects with its powerful static type system. Unlike Ada and Java, Haskell allows reusable functions to be combined without the overhead of class definitions and inheritance, and its type system prevents the hidden side effects that cause many bugs in programs written in conventional languages. Comments? Paul.

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Re: [Haskell-cafe] Where would I find fromInt?
by Tom Harper 09 Sep '07

09 Sep '07

Hi Paul -- The function you want is called fromIntegral, and works for all Integral types. Using it, you can add a type signature to specify what you want to change the number to (Float, Double, other Integral type, etc. Example: > fromIntegral (4 :: Int) :: Double 4.0 Hope this helps! -- Tom Harper rtharper(a)aftereternity.co.uk +1 949 235 0185 Public Key: http://aftereternity.co.uk/rth.asc On 9/9/07, PR Stanley <prstanley(a)ntlworld.com> wrote: > Hi > Where would I find the fromInt function, please? > Better still, would anyone know how to write a func for converting > int to float and vice versa? > Thanks, Paul > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- Tom Harper rtharper(a)aftereternity.co.uk +1 949 235 0185 Public Key: http://aftereternity.co.uk/rth.asc

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Help understanding type error
by Levi Stephen 09 Sep '07

09 Sep '07

Hi, I'm after some help understanding either what I'm doing wrong, or why this error occurs. I have a data type: > data T a = forall b. (Show b) => T b a and I want to use/extract 'b' from this. > extShow (T b _) = b This gives the following compilation error: extest.hs:5:0: Inferred type is less polymorphic than expected Quantified type variable `b' escapes When checking an existential match that binds $dShow :: {Show b} b :: b The pattern(s) have type(s): T t The body has type: b In the definition of `extShow': extShow (T b _) = b I tried adding a type signature: > extShow' :: (Show b) => T a -> b > extShow' (T b _) = b Now I get a different error: extest.hs:8:19: Couldn't match expected type `b' (a rigid variable) against inferred type `b1' (a rigid variable) `b' is bound by the type signature for `extShow'' at extest.hs:7:18 `b1' is bound by the pattern for `T' at extest.hs:8:10-14 In the expression: b In the definition of `extShow'': extShow' (T b _) = b It seems (to newbie me ;) ) like it should be possible to extract the first part of T, and deduce that it is an instance of the class 'Show'. The following seems ok: > doShow (T b _) = putStrLn (show b) Thanks, Levi

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Hackage and GHC 6.8
by Neil Davies 09 Sep '07

09 Sep '07

Given that GHC 6.8 is just around the corner and, given how it has re-organised the libraries so that the dependencies in many (most/all) the packages in the hackage DB are now not correct. Is there a plan of how to get hackage DB up to speed with GHC 6.8 ? Cheers Neil

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((a -> b) -> c) -> (a -> m b) -> m c
by Stuart Cook 09 Sep '07

09 Sep '07

(Inspired by this[1] reddit thread.) When combining monadic and non-monadic code, I've often wished for a magical combinator of type (Monad m) => ((a -> b) -> c) -> (a -> m b) -> m c which would let me inject a monadic function into a pure one, then wrap the ultimate result to ensure that no nastiness escapes. Now, I'm pretty sure that writing such a combinator as-is is impossible. My question is, what restrictions/changes would be necessary for a similar combinator to be definable and well-behaved? (Obviously it would be impossible to ensure that the (m b)s get chained in any particular order, for order-sensitive monads, but it would be the caller's responsibility to ensure order-irrelevance in this case.) As a thought experiment, I imagined an implementation for IO using unsafePerformIO. This makes it easy to give the injected function the right type, and it's possible to use return to give the result the correct type too. Unfortunately, the semantics fail: if the injected function is used lazily, there's no way to ensure that the unsafe IO actions actually take place before the rest of the IO-based program, and it's possible for them to show up later than expected. I suspect that even in a strict language, lambda-abstractions would cause similar trouble. [1] http://programming.reddit.com/info/2n6eh/comments Stuart Cook

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ICMP in Haskell (was Elevator pitch for Haskell)
by Dominic Steinitz 09 Sep '07

09 Sep '07

> > Does it enable you to, say, send raw ICMP packets? > > AFAIK, Haskell supports TCP, and nothing else. (A while back I wanted to > write an automated pinging program. But the only way I could figure out > how to do it is to call the OS "ping" utility and attempt to parse what > it writes to stdout. Oh, did I mention that's different on WinNT, WinXP > and UNIX?) > Andrew, I thought this was relatively straightforward. Look in http://www.haskell.org/networktools/src for multi-threaded versions. Beware I wrote ping and traceroute about 5 years ago. They're pretty much straight from Stevens' book. On the other hand, I quite liked this: sequenceWhile_ :: Monad m => (a -> Bool) -> [m a] -> m () sequenceWhile_ p [] = return () sequenceWhile_ p (x:xs) = x >>= \c -> if (p c) then sequenceWhile_ p xs else return () I'd forgotten about them and I'm not particularly proud of them. I guess I could put them in hackage so that they are at least a starting point for someone. Dominic.

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"Cofunctor" as a name
by Conal Elliott 09 Sep '07

09 Sep '07

I've heard contravariant functors referred to as "cofunctors", and I've also heard objections to that term. As I'm preparing to release some libraries using contravariant functors, I'd appreciate hearing preferences and/or reasons. For instance, another name would be "Contrafunctor". -- | Contravariant functors. often useful for /acceptors/ (consumers, -- sinks) of values. class Cofunctor acc where cofmap :: (a -> b) -> (acc b -> acc a) Thanks, - Conal

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