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Query
by vishy anand 10 Feb '07

10 Feb '07

hi i am going through yaht tutorial and exercise 4.6 and 4.7..i understood 4.6,but not 4.7 in which fromTuple (One a ) = Left (Left a ) and fromTuple (Two a b ) = Left (Right (a,b) ) function r written..why use Either type..cant i just say fromTuple (Two a b )=(a,b)

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What's wrong with "cgi-undecidable"?
by keepbal 10 Feb '07

10 Feb '07

http://darcs.haskell.org/packages/cgi-undecidable/ There only 3 or 4 lines of source code, but it can't be installed. The ghc version is 6.6 . Undecidable.hs:23:0: Duplicate instance declarations: instance [overlap ok] (MonadTrans t, MonadCGI m, Monad (t m)) => MonadCGI (t m) -- Defined at Undecidable.hs:23:0 instance [overlap ok] (MonadTrans t, MonadCGI m, Monad (t m)) => MonadCGI (t m) -- Defined in Network.CGI.Monad

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Random numbers without IO
by Dougal Stanton 10 Feb '07

10 Feb '07

Well, nobody likes tainting their beautiful pure code with IO, so I rewrote the Random module to take advantage of the latest research [1]: > module Random where > > getRandom = 4 Cheers, D. [1] <http://xkcd.com/c221.html> -- Dougal Stanton

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learning Haskell
by Bulat Ziganshin 09 Feb '07

09 Feb '07

Hello, i'd a discussion in russian programmer's forum (rsdn.ru) about haskell and learning it. one interesting point was that imperative programmers imagine that multi-threading programming should require special knowledges. they was really amazed when i showed the following program that creates thread and interacts with it - without locks, semaphores, synchronized access and all other old stuff -- Thread, that cyclically read commands from channel, and executes them graphThread c = do cmd <- getChan c cmd graphThread c -- Creates graphThread and sends a few commands to it for execution main = do c <- newChan forkOS $ graphThread c putChan c (print "Hi") tid <- forkOS $ putChan c (print "Hi from other thread") putChan c (print "Bye") waitThread tid i think that some example like it would be appropriate in haskell book for *novices* - just because it's easy to do in Haskell. so, i vote for adding Multi-threading chapter to such books (and it may be especially important nowadays when everyone are crazy about multi-cores :) -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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weak references & caches
by Thomas Conway 09 Feb '07

09 Feb '07

Hi All, I'm hacking some (external) B-Tree code, and amongst the numerous interesting problems I've come up against[*], is to do with managing which pages/nodes are in memory and which are not. We use TVars to point from one (in-memory version of a) page to another, so we have a type like the following: data Node = Leaf [(Key,Value)] | Node NodePtr [(Key,NodePtr)] type NodePtr = TVar (Maybe Address, Maybe Node) type Address = Integer Addresses are just page numbers or file offsets, or similar. NodePtr is the interesting bit - the following table summarizes what the combination of values mean: explain (Just addr, Nothing) = "An external node we have not read in yet" explain (Nothing, Just node) = "A new node for which we have not yet allocated a page" explain (Just addr, Just node) = "A node we've read in" explain (Nothing, Nothing) = "Emit nasal daemons" Now, we want a cache to hang on to nodes that we've read in to hold them in memory, to save repeatedly reading them off disk unnecessarily. But more than that, we want to make sure that when we create a NodePtr pointing to a particular Address, that all references to this particular address go through the same NodePtr, otherwise in a concurrent situation we could well end up with two copies of the page in memory, both recieving updates leading to havoc. At the same time, we'd like to be able to evict pages according to some policy (the Oracle hot-list cold-list cache is attractive, if implemented carefully). So one scheme that comes to mind is to keep a table mapping addresses to (Weak NodePtr). When we ask for a node, first we consult the cache (if we have one) and get the NodePtr right back. If the addressed node is not in the cache then we check the table to see if there is/was a known NodePtr for that address. If there is none then we can allocate a new one and stick it in the table, and if it is there then we return it. This is also nice because we can attach finalizers to the (Weak NodePtr)s to write them out if dirty. So the question is, is there a simpler scheme (though this one isn't tooooo bad), and more importantly, is this likely to be horribly slow? How efficient is dereferencing a weak reference, and what is the likely impact on performance of having 10^3-10^5 weak references around? cheers, Tom [*]Film at 11. I'm planning to write it all up, maybe for a conference, maybe just for the Monad Reader, or something.

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Portable implementation of unsafeCoerce
by Alfonso Acosta 08 Feb '07

08 Feb '07

Hi all, Reading the sources of Lava (a Haskell Hardware Description DSL) I run into this definition ... unsafeCoerce :: a -> b unsafeCoerce a = unsafePerformIO $ do writeIORef ref a readIORef ref where ref = unsafePerformIO $ do newIORef undefined Is this actually equivalent to the infamous unsafeCoerce# ? Thanks in advance, Alfonso Acosta

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Monolithic module tool
by Chad Scherrer 08 Feb '07

08 Feb '07

Maybe a year or so ago, I came across a tool for Haskell that takes a collection of modules, does some name-mangling, and gives a single module that only needs to export main. There were wonderful reductions in the size of the resulting executable, and potentially more optimizations available to GHC. Anyway, I can't find the link now, and I'm wondering if anyone knows of it. Thanks! Chad

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still wrestling with io
by Martin DeMello 08 Feb '07

08 Feb '07

Code here: http://zem.novylen.net/anagrid.html I've got an instance of IO appearing unexpectedly and I can't figure out where from. It throws up the following error: $ ghc --make test.hs Chasing modules from: test.hs Compiling Main ( test.hs, test.o ) test.hs:38:15: Couldn't match `StaticText ()' against `IO (StaticText ())' Expected type: StaticText () Inferred type: IO (StaticText ()) In the application `staticText p [text := (labelText a b)]' In the definition of `textOf': textOf p a b = staticText p [text := (labelText a b)] martin

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How did you stumble on Haskell?
by Alexy Khrabrov 08 Feb '07

08 Feb '07

How do people stumble on Haskell? I've taught ML at UPenn, and many of my colleagues in Amazon are in SeaFunc -- switching from C funk to func funk. I've got Hudak's book a while ago, but didn't have time/excuse to delve into it until recently. Then the most fantastic chain if events triggered it: -- finally switched to Intel Mac -- got Parallels -- got a recent Linux, openSUSE 10.2, to stick into Parallels -- decided finally to try Gentoo -- found equery slow, came across Adelie/FQuery as "fast equery" -- emerge adelie To my surprise, saw it emerge GHC! Realized Adelie is a Haskell way to hack Portage. Dug all things Haskell! I also used Darcs for a long time. It probably takes several pro et contra factors to push away from boring things and to be inducted into interesting FP things. Again, UPenn focus on ML was invaluable, and Ullman's book on ML superb -- and thin! What's folks most interesting ways to arrive at FP? Cheers, Alexy

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currying vs partial application
by Steve Downey 07 Feb '07

07 Feb '07

I think I finally have it. Partial application is taking a function of N parameters, binding a value to one of them, and turning it into a function of N-1 parameters. Currying is where ask is that a function that takes two ints and returns an int, or is that a function that takes one int and returns a function that takes an int and returns an int, and the answer is yes. Currying implies partial application, but not all partial application is currying.

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