- Haskell-Cafe - Haskell.org

((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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Serial Communications in Haskell
by Ronald Guida 08 Sep '07

08 Sep '07

I'm on a Windows box and I'm looking for a way to talk to a serial port (for example, RS-232) from Haskell. I couldn't find a library to do this, so I am wondering how to create one. I have a fairly thorough understanding of how to open and use a serial port with the Windows API. In particular, to open a serial port, I have to use CreateFile, which is the same API call that opens files. In fact, if I call openFile from GHC, and pass "COM1:" as the filename, then I can get a writable serial port. > module Serial > where > import System.IO > > main = do > h <- openFile "COM1:" ReadWriteMode > hPutStrLn h "Hello World" I can't read from the port (I always get an immediate EOF), and I have no way of configuring things like the baud rate or the parity settings. Nevertheless, this demonstrates that openFile can at least open the serial port. What I would like to do is create some functions that would allow me to open and configure a serial port, and get a Handle back so that I can use the existing IO functions like hGetChar and hPutChar. I am assuming that hGetChar eventually calls win32::ReadFile and hPutChar eventually calls win32::WriteFile. These same two API calls would work for serial ports. In Windows, there are 23 API functions that apply specifically to serial ports. Out of these 23 functions, only a few of them are actually necessary if I just want to send and receive data. Of course, I don't know how to call Windows API functions from Haskell, and I have no idea how to hook things to the IO library so that I can use a Handle for a serial port. I'm looking for some advice on how to proceed. -- Ron

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generate a news summary page from web services
by brad clawsie 08 Sep '07

08 Sep '07

i recently worked up a little example program i use to generate a general news summary page using yahoo web services if you are interested in an example of XHT and Text.XHtml.Strict in action, you can see it here http://www.b7j0c.org/content/haskell-newspage.html

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Block-wise lazy sequences in Haskell
by Henning Thielemann 08 Sep '07

08 Sep '07

I want to have a data structure like Data.ByteString.Lazy, that is block-wise lazy, but polymorphic. I could use a lazy list of unboxed arrays (UArray) but the documentation says, that the element types are restricted. But I will need (strict) pairs of Double and the like as elements. It seems that I need generalized instances in order to use these arrays. I thought it must be possible to define an unboxed array type with Storable elements. I cannot find such a type in the standard libraries. Further on, I wonder why pairs are not instances of Storable.

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Extending the idea of a general Num to other types?
by Peter Verswyvelen 08 Sep '07

08 Sep '07

I like the fact that Haskel treats numbers in a generic way, so you can lift them to any other datatype by instantiating Num. Can the same be done on other builtin constructs? For example, if I have [a], can this list be lifted to other types? I guess not, because no type class exists for the list type? Does this make sense? I'm still searching for correct terminology... Thanks, Peter

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Learning advice
by Chris Saunders 08 Sep '07

08 Sep '07

I wish to learn Haskell. I purchased a book - An Introduction to Functional Programming Systems Using Haskell but I think this might be too outdated. I'm using GHC and it seems that some of the functions used in the book are no longer a part of Haskell. I'm looking for advice on books or tutorials that might be more up to date. So far I'm finding Haskell difficult - I may be too thick. Regards Chris Saunders

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Fwd: [Haskell-cafe] turning an imperative loop to Haskell
by Stephen Dolan 07 Sep '07

07 Sep '07

Sorry, forgot to cc list > I think this is called taking a good thing too far, but cool too: > > > f1 u = u + 1 > f2 u v = u + v > f3 u v w = u + v + w > > -- functions renamed for consistency) > zipWith1 = map > zipWith2 = zipWith > > -- and hey presto! > us1 = 3 : zipWith1 f1 us1 > us2 = 2 : 3 : zipWith2 f2 (drop 1 us2) us2 > us3 = 2 : 3 : 4 : zipWith3 f3 (drop 2 us3) (drop 1 us3) us3 How about this: import Data.List -- for transpose f = sum zipWithN fn = map fn . transpose us k = s where s = [2..k+1] ++ (zipWithN f $ x $ map (flip drop s) [0..k-1]) take 10 (us 1) [2,2,2,2,2,2,2,2,2,2] take 10 (us 2) [2,3,5,8,13,21,34,55,89,144] take 10 (us 3) [2,3,4,9,16,29,54,99,182,335] take 10 (us 4) [2,3,4,5,14,26,49,94,183,352] We can always take it further :D Stephen

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Typeclasses and implicit parameters
by ajb＠spamcop.net 07 Sep '07

07 Sep '07

{-# OPTIONS -fglasgow-exts #-} -- G'day everyone. -- This is okay. f1 :: (?foo :: String) => String f1 = ?foo -- So is this. f2 :: (Show a, ?foo :: a) => a -> String f2 _ = show ?foo -- Hugs allows this. GHC rejects it on the grounds that "a" is unused -- on the right-hand side of the (=>). I think this is arguably a bug -- in GHC. f3 :: (Show a, ?foo :: a) => String f3 = show ?foo -- GHC rejects this. Hugs compiles it, but I can't call it as -- let ?foo = "Hello" in show Foo -- -- Is there a good reason to disallow this? data Foo = Foo instance (?foo :: String) => Show Foo where showsPrec _ Foo = showString ?foo . showString "Foo" -- Cheers, -- Andrew Bromage

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