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Bug in "Haskell for C programmers" tutorial?
by Erik de Castro Lopo 02 Jan '06

02 Jan '06

Hi all, I'm working through the "Haskell for C programmers" tutorial and in this section": http://www.haskell.org/~pairwise/intro/section2.html#part3 it has: fib :: Num a, Num b => a -> b fib n = fibGen 0 1 n fibGen :: Num a, Num b => b -> b -> a -> b fibGen a b n = case n of 0 -> a n -> fibGen b (a + b) (n - 1) which doesn't work with GHC, but it does if I add parentheses around both instances of "Num a, Num b". Is this a bug in the tutorial or is there another explanation? Cheers, Erik -- +-----------------------------------------------------------+ Erik de Castro Lopo +-----------------------------------------------------------+ "Anyone who considers arithmetical methods of producing random digits is, of course, in a state of sin." - John Von Neumann (1951)

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Project postmortem II /Haskell vs. Erlang/
by Dominic Steinitz 02 Jan '06

02 Jan '06

I'd like to second Joel's comments on records very strongly. In an application where you need lots of big records (such as supporting X.509), you end up with very unnatural names for components. It works but it's not pretty. Dominic.

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(no subject)
by Henk-Jan van Tuyl 01 Jan '06

01 Jan '06

L.S., I used the link checker of the World Wide Web Consortium to check the links on haskell.org; there are several links broken. As the checks take a long time, I have put the results on my website: http://members.chello.nl/hjgtuyl/computing/Haskell/checklinks/checklink.htm The check is a recursive check with depth 3, starting from the home page; this will probably not cover all of the website. To repeat this check, go to: http://validator.w3.org/checklink -- Best wishes for the new year, Henk-Jan van Tuyl -- http://members.chello.nl/hjgtuyl -- Using Opera's revolutionary e-mail client: https://secure.bmtmicro.com/opera/buy-opera.html?AID=789433

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expanding do notation / StateMonad example from tutorial all about monads
by Marc Weber 31 Dec '05

31 Dec '05

Hi.. I'm still struggling with monads ;-) In the tutorial "All about monads" there is the function getOne used: getOne:: (Random a) => (a,a) -> State StdGen a getOne bound = do g <-get (x,g')= return $ randomR bounds g put g' return x In the haskell "Yet another Haskell tutorial" by Hal Daume it is said that do {fp <- e; es} is translated to: let ok p = do {es} ; ok _ = fail "..." in e >>= ok Using this rule I was able to expand the first "<-" of getOne: getOne2 :: (Random a) => (a,a) -> State StdGen a getOne2 bounds = let ok g = do (x,g') <- return $ randomR bounds g put g' return x ok _ = fail "asd" in get >>= ok but I couldn't manage to also expand the second. My attempt is: getOne3 :: (Random a) => (a,a) -> State StdGen a getOne3 bounds = let ok g = do let ok2 (x,g') = do { put g'; return x } -- ok2 _ = fail "2" in (return $ randomR bounds g >>= ok2 ok _ = fail "1" in get >>= ok But I get this compile error in at the line marked by a trailing --: "The last statement in a 'do' construct must be an expression" Can you help me? Sincerly Marc

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OpenAL bindings / compiling ghc 6.5
by Michael Benfield 31 Dec '05

31 Dec '05

I see here: http://www.haskell.org/HOpenGL/newAPI/ OpenAL bindings listed as part of the Hierachical Libraries. And when I download the source to a development snapshot of GHC, there they are. Is there a way to install this on GHC 6.4? Alternatively... I can't get GHC 6.5 to compile. I do ./configure && make and it gets to this step: ------------------------------------------------------------------------ ==fptools== make all -r; in /Users/mike/source/ghc-6.5.20051221/ghc/compiler ------------------------------------------------------------------------ and sits forever (well, I only let it sit a day and a half, actually...). And the only relevant process that seems to be running is gmake (no ghc or gcc or anything). I've tried a few different versions of 6.5, including HEAD and STABLE branch. I'm on Mac OS 10.3.9. Thanks. Mike Benfield

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Testing serialization/deserialization.
by Pupeno 31 Dec '05

31 Dec '05

Hello, I am working with NewBinary, I have made a lot of datatypes which are Binary instances. Now I want to write some testing for the serialization/de-serialization. I've started with QuickCheck but it seems I can't do IO inside a property of QuickCheck, am I right ? If not, how is it done ? Not being able to work with QuickCheck, I tried HUnit only to find another problem (which I would have found with QuickCheck anyway). How do I generate a BinHandle where I can write and read what I written ? Tring to do that I wrote a test for Int: intSerialization = TestCase (do bh <- openBinMem 1024 [] let i :: Int i = 3 put_ bh i i2 <- (get bh) :: IO Int assertEqual "i == i2" i i2) it didn't work: *Network.Server.DNS> runTestTT tests ### Error in: 1 user error (HUnit:i == i2 expected: 3 but got: 209944584) In general, what method/way do you recommend to test serialization/de-serialization (note: I'd like to use QuickCheck if possible)? Thank you. -- Pupeno <pupeno(a)pupeno.com> (http://pupeno.com)

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Re: Haskell Speed
by Isaac Gouy 31 Dec '05

31 Dec '05

--- Isaac Gouy <igouy(a)yahoo.com> wrote: > We'll be happy to also show a Haskell program that > uses Data.HashTable - first, someone needs to > contribute that program. Someone did: k-nucleotide Haskell GHC #2 http://shootout.alioth.debian.org/gp4/benchmark.php?test=knucleotide&lang=a… __________________________________ Yahoo! for Good - Make a difference this year. http://brand.yahoo.com/cybergivingweek2005/

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Can this be improved?
by Chris Kuklewicz 30 Dec '05

30 Dec '05

Happy Holidays, I was wondering if it this small snippet of code could be altered to require fewer OPTIONS -fallow-... switches. It creates a show-what-i-mean function called "swim" that takes a variable number of arguments, and treats strings as-is, calling show on the other arguments. > {-# OPTIONS -fglasgow-exts #-} > {-# OPTIONS -fallow-undecidable-instances #-} > {-# OPTIONS -fallow-incoherent-instances #-} > {-# OPTIONS -fallow-overlapping-instances #-} > > -- Want to show strings "as-is" > > class (Show a) => MyShow a where > myShow :: a -> String > myShow = show > > instance MyShow String where > myShow = id > > instance (Show b) => MyShow b > > -- Want to take variable number of arguments > > class PVShow a r where > pvShow :: String -> a -> r > > instance (MyShow b) => PVShow b String where > pvShow oldString b = oldString ++ (myShow b) > > instance (MyShow b,PVShow a r) => PVShow b (a->r) where > pvShow oldString b = pvShow (oldString ++ (myShow b)) > > -- swim is short for Show What I Mean > > swim :: (PVShow a r) => a -> r > swim = pvShow "" > > -- Demonstates of polymorphic and first class nature of swim > > test foo = foo " and " 7 " are " > > -- Tests > > main = do putStrLn $ swim "Hello" " " "World #" [17,18,19] "!" > putStrLn $ test (swim "I also think " [4,5,6]) "cool" "." GHCI session: Main> main Hello World #[17,18,19]! I also think [4,5,6] and 7 are cool. *Main> Could this be improved by TypeEq machinery? Is there a Haskell 98 way to make myShow work this way? -- Chris Kuklewicz

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binary IO
by Pupeno 30 Dec '05

30 Dec '05

It seems I have found a hole in Haskell... :( Before I start to develop a library with functions such us those on http://haskell.org/hawiki/BinaryIo (hGetWord8, hGetWord16le, hGetWord16be, etc), is there some reliable library that can help me ? I basically need a set of functions to read binary data out of a Handle (a higher lever of hGetBuf and hPutBuf). What I am doing is implementing a DNS server, so, my typical need is: First two bytes are an integer. The next bit is a boolean. So are the following three bits. Then I have 4 bits which is also an integer. Etc. Any help is greatly appreciated. Thank you. -- Pupeno <pupeno(a)pupeno.com> (http://pupeno.com)

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module names
by Scherrer, Chad 30 Dec '05

30 Dec '05

Something about the way module names are specified in a file seems really strange to me. When I first started learning Haskell (I had used OCaml previously), I tried things like module Main where import A import B main = A.f >> B.f module A where f = ... module B where f = ... in a single file. This example is straight from chapter 5 of the Report, and no mention is made (that I could find) about modules needing to be in separate files. But this won't load in ghci! (Even if ... is changed to putStr "hi"). Eventually I figured out that it works fine if it's split over three separate files. So here's what I'm trying to figure out: If every file corresponds to exactly one module (is that true?), then why must the module name be given again in the text of the file? When I'm using ghci, I have lots of modules that I sometimes want to load "as Main", and sometimes I only want them loaded as a dependency from another module. Currently, I have to go into each file to change the "module Foo where" line to do this. Chad Scherrer

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