July 2006 - Haskell-Cafe

ghc hackathon start time?
by Greg Fitzgerald 26 Jul '06

26 Jul '06

What time on September 14th will the GHC Hackathon begin? I don't need a precise answer, just a quick one. Plus or minus an hour will do. Thanks, Greg

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Re: [Haskell-cafe] Newbie Q: Deriving MyOrd from Eq problem
by voigt.16734551＠bloglines.com 26 Jul '06

26 Jul '06

Hi Dmitri, I don't have answers for all your questions. But some, at least. --- Dmitri O.Kondratiev" <dokondr(a)gmail.com wrote: > -- Ok, then I can derive MyOrd class directly from Ord: > > class Ord a => MyOrd a where > (%<), (%<=), (%>), (%>=) :: a -> a -> Bool > x %< y = x < y > x %<= y = (x < y || x == y) > x %> y = y < x > x %>= y = (y < x || x == y) > > instance (MyOrd a, MyOrd b) => MyOrd (a,b) where > (x1, y1) %< (x2, y2) = (x1 %< x2) && (y1 %< y2) > (x1, y1) %> (x2, y2) = (x1 %> x2) && (y1 %> y2) > (x1, y1) %<= (x2, y2) = (x1 %<= x2) && (y1 %<= y2) > (x1, y1) %>= (x2, y2) = (x1 %>= x2) && (y1 %>= y2) > > greaterMyOrd :: (MyOrd (a, b)) => (a, b) -> (a, b) -> Bool > greaterMyOrd (x,y) (z,t) = (x,y) %> (z,t) > > -- This should work, right? Yet I get this error message: Only if you tell the compiler to allow constraints of the form (C t), where C is a class and t is not a variable. That's exactly what GHC tells you to try, using -fglasgow-exts. > -- Notwithstanding :) when i comment out declaration: > > -- greaterMyOrd :: (MyOrd (a, b)) => (a, b) -> (a, b) -> Bool > > -- program gets compiled and checking type of 'greaterMyOrd' gives: > > *ClassTest> :t greaterMyOrd > greaterMyOrd :: (MyOrd (a, b)) => (a, b) -> (a, b) -> Bool > > -- which is the same as I tried to declare in the program source. > What is hapenning here? That is indeed a bit funny. Using your instance declaration, GHC could have reduced the constraint "MyOrd (a,b)" to "MyOrd a, MyOrd b". I think there was a discussion on why it does not do so very recently. Maybe you can find it in the archive. > -- Now, when trying to use function 'greaterMyOrd' I get: > > *ClassTest> greaterMyOrd (2, 3) (1, 2) > > <interactive>:1:0: > Ambiguous type variable `a' in the constraints: > `MyOrd a' arising from use of `greaterMyOrd' at <interactive>:1:0-11 > `Num a' arising from the literal `2' at <interactive>:1:14 > Probable fix: add a type signature that fixes these type variable(s) That is because GHC doesn't know which type your literal 2 comes from. So it doesn't know whether there is a MyOrd instance for it, or which to choose. The Num comment comes from GHC only knowing that literal 2 will be of some type in the Num class. But that doesn't tell it anything about possible MyOrd instances. > -- Then I try to declare argument types explicitely and get this: > > *ClassTest> greaterMyOrd (2::MyOrd, 3::MyOrd) (1::MyOrd, 2::MyOrd) > > <interactive>:1:17: > Class `MyOrd' used as a type > In an expression type signature: MyOrd > In the expression: 2 :: MyOrd > In the first argument of `greaterMyOrd', namely `(2 :: MyOrd, 3 :: MyOrd)' > *ClassTest> Well, just as GHC tells you, you should use a type for the signature, not a typeclass. Try something like greaterMyOrd (2::Int, 3::Int) (1::Int,2::Int). Maybe the annotations on the first tuple suffice. > -- I am lost. Please enlight me, what am I doing wrong trying to > create my own class, its instance and then using it. Hope that helped. BTW, you might get problems with overlapping instances. Because with your instance declaration there are two possible ways, for example, to get a MyOrd instance for (Int,Int). The first is via: Ord Int => MyOrd Int => MyOrd (Int,Int) The second is via: Ord Int => Ord (Int,Int) => MyOrd (Int,Int) Ciao, Janis.

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Frantk
by Alvaro Galan 26 Jul '06

26 Jul '06

Hi, Im trying to use Frantk from winhughs but i dont know how to import the file, where to put the sources to work, how to import other libaries, etc. If i wanto to import Frantk.lh where i have to place (somo folder, inside the winhugs tree, where?) it or import it? Thanks in advance. Alvaro. -- It's only Rock'n'Roll (..but I like it)

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scripting in haskell
by mvanier 26 Jul '06

26 Jul '06

Hi, I was playing around with runhaskell (runghc to be precise), and I discovered the limitation wherein you have to use the file suffix ".hs". Don't get me wrong, runhaskell is great, but if you didn't have that restriction it would make haskell much more attractive to many programmers who write standalone scripts. Of course it's always possible to do e.g. (file: hello) #! /bin/sh runhaskell Hello.hs (file: Hello.hs) module Main where main :: IO () main = putStrLn "hello, world!" but that's pretty inconvenient. What I'd like is: (file: hello) #! /usr/bin/env runhaskell module Main where main :: IO () main = putStrLn "hello, world!" which is _almost_ doable; you have to name the file "hello.hs". What would really be optimal is something like (file: hello) #! /usr/bin/env runhaskell -fglasgow-exts -- extra arguments to runhaskell !# module Main where main :: IO () main = putStrLn "hello, world!" Looking through the mailing list I see something from May 2005 on this topic about adding the equivalent of gcc's "-x" option to ghc. What's the status of that? Cheers, Mike

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Universal machines in Haskell
by dons＠cse.unsw.edu.au 26 Jul '06

26 Jul '06

Now the ICFP contest is now over, several of the entrants have been interested in seeing if we can't write a decent universal machine (the 'hardware' of this year's contest), in Haskell. This is an interesting problem for Haskell, since the spec encourages the use of mutable variables and mutable arrays, as well as a bit of IO. Getting C-like performance from Haskell, for this problem, is a bit of a challenge, and highlights some corners in our libraries. So, we're collecting any UMs in Haskell, with the goal to breed some good solutions, and learn a bit about writing low level machines in Haskell. The website is here, http://www.cse.unsw.edu.au/~dons/um.html If you wrote a Haskell UM, or a faster C UM, free to submit it! Just mail me a darcs repo url, a source module url, or the source itself. -- Don

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Why Haskell?
by Matthew Bromberg 25 Jul '06

25 Jul '06

I am currently in the middle of a reasonably large software simulation of a wireless network that I'm programming in Haskell and in C. Progress has been slower than anticipated and I occasionally (probably because of looming deadlines) ask myself did I take the right approach? Here are the reasons why I chose Haskell initially 1) It's mathematical elegance, I thought would lend itself to the complex mathematical expressions I am implementing. 2) It rates reasonably high in performance comparisons with other languages http://shootout.alioth.debian.org/gp4sandbox/benchmark.php?test=all&lang=all 3) It has a very active community who are courteous and quite helpful 4) It has an interactive shell. 5) It integrates easily with C. The idea would be to implement hooks to some fast matrix libraries and some plotting routines in C and then develop the higher level logic in Haskell. Since marshalling arrays and such, is a bit of a pain I structured my C matrix library in the form of stack based machine, using reference counting to handle memory management and to minimize the amount of actual data I need to pass between C and Haskell. This seems to work fairly well. However I have been somewhat disabused of my original thought about how development would proceed. I had hoped that I could get within a factor of two or so of the fast prototyping, development times of a good scripting language such as Matlab, Python or Lua, but I don't think I'm achieving this unfortunately. Here are some reasons why 1) Lack of debugging support. Yes there are print statements and trace, but I would like to set a breakpoint. It would be nice to do so and launch the GHCi interpreter with all the variable context supported. A google search revealed that there is current work on this, but unfortunately the package is in cabal, which has spotty support in windows it seems. 2) Recompiling binaries (necessary in order to link in foreign object code into GHCi) is slow using GHC. Moreover I have to restart GHCi if I want to reload a changed DLL (unless there is a way to unload a DLL in GHCi). It also requires jumping around between several console windows to get the job done. (I'm not using an IDE does one exist?) 3) Lack of automatic type coercion for closely related types. In particular I have to use CInt and CDouble to go into and out of C. Unfortunately only a small subset of library functions will actually support CInt so I have to explicitly coerce this values. I usually have a host of compile errors initially because of this silly issue and it makes the code ugly. What would help would be an automatic type conversion from CInt to Int and CDouble to Double. Perhaps some warning level could flag these if this were 'too dangerous'. 4) GHCi is really not as useful as I'd hoped. You can not just cut and paste Haskell code from a text file and run it in the interpreter. There is also this context issue concerning what modules are actually in scope. So although in Haskell once I import a module, all of its functions are in scope, in GHCi, if I load this module only the exported functions from that module are in scope. The result, again, is that I can not get an apples to apples idea of what is happening in my code. Thus I have been using GHCi primarily as a syntax checker for Haskell constructs. 5) Learning new programming patterns may have a role to play, but sometimes things like iteration is cleaner than recursion. I worry about performance issues however. I had to create a 50 Mbyte matrix and I used what I thought, initially was an elegant contruction technique in Haskell. Something like this do ... sequence $ [ reffill b s | s <- [0..(fi temits)-1], b <- [0..(fi nc)-1]] ...(push list on to matrix stack) The refill function generates a matrix based on basestation index b and SU index s and pushes it on the Matrix stack in C, inside an IO monad. So it returns IO(). The list comprehension ends up doing a cartesian product of all (b,s) pairs which in this case is a large number. fi is my shorthand for doing an annoying CInt to Int conversion. The problem with this code is that it takes about 500 Mbytes in memory instead of 50 Mb. Apparently it loads up all the potential IO actions into the huge list before executing it, taking the factor of 10 or so hit. There is probably a nice space saving way of doing this, that executes each IO action before generating the next one, but I was in too much of a hurry to figure it out. This also makes me wonder about the efficiency of the C interface. Each IO action calls a C function (using the unsafe specifier in the import declaration). I hope there isn't too much overhead with this. In my case I do almost no marshalling. Most calls have no arguments and no returns, or at most require a handful of CInts or CDoubles. My application has 60-70% of it's statements in an IO monad calling lots of C code. I begin to wonder if all those statements shouldn't in fact just be in C to begin with. Thus I begin to wonder why I'm using Haskell. Maybe I just need some encouragement that my choice will pay off in the end.

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Internships on GHC and Haskell at MSR Cambridge
by Simon Peyton-Jones 25 Jul '06

25 Jul '06

Gentle Haskellers Would you be interested in working for three months at Microsoft Research, Cambridge, on a project related to GHC or Haskell? MSR Cambridge now takes interns *year-round*, not just in the summer months. Simon Marlow and I are keen to attract motivated and well-qualified folk to work with us on our research, and on improving or developing GHC. If you or one of your students is interested, you can find more details here http://hackage.haskell.org/trac/ghc/wiki/Internships Our next empty slot is the Oct-Dec period, but you are welcome to apply for some later period. Simon PJ PS: There are lots of other programming-language folk at MSR (F#, security, probabilistic languages...), and stuff beyond that (machine learning, systems and networks...); see http://research.microsoft.com/aboutmsr/labs/cambridge/

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Re: Re[4]: [Haskell-cafe] REALLY simple STRef examples
by Chad Scherrer 22 Jul '06

22 Jul '06

The IO monad hasn't given me too much trouble, but I want to be sure to structure things the way "they should be". If I get everything running using IO first and then have type-checking problems with ST, it will be tempting to just slap on an unsafePerformIO and call it good. Sure, it's really doing the same thing anyway, but it just comes out looking like a hack. On 7/20/06, Bulat Ziganshin <bulat.ziganshin(a)gmail.com> wrote: > Hello Chad, > > Friday, July 21, 2006, 12:26:58 AM, you wrote: > > > Ok, I see now why the return is necessary. > > btw, it may be helpful to read "IO inside" material. ST monad is not > very different from IO monad - it only limited to operations on STRef > and STArray, so that it can't have side-effects visible outside of > runST statement used to run ST computation >

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incoherent instance selection when it should be still coherent
by Ralf Lammel 21 Jul '06

21 Jul '06

The following pain is sort of ghc (6.4.2) specific. (The same behavior is achievable with hugs -98 +O which is Ok in so far that +O (as opposed to +o) is not strongly supposed to be coherent.) Note the following GHC options preceding the code. They do not contain -fallow-incoherent-instances. {-# OPTIONS -fglasgow-exts #-} {-# OPTIONS -fallow-undecidable-instances #-} {-# OPTIONS -fallow-overlapping-instances #-} -- A single parameter class with two silly instances class Foo x where foo :: x -> (); foo = const () instance Foo () instance Foo Bool -- A two-parameter class with a generic and a non-generic instance class (Foo x, Foo y) => Bar x y where bar :: x -> y -> String instance (Foo x, Foo y) => Bar x y where bar _ _ = "generic instance" instance Foo z => Bar () z where bar _ _ = "non-generic instance" -- An existential wrapper around foos data Wrap = forall x. Foo x => Wrap x -- A wrapper-based variation on the type-class member bar uuh :: Wrap -> Wrap -> String uuh (Wrap x) (Wrap y) = bar x y -- Let's try all unwrapped and wrapped combinations of bar and uuh t1 = () t2 = True w1 = Wrap t1 w2 = Wrap t2 main = do print $ bar t1 t1 print $ uuh w1 w1 -- uuh! print $ bar t1 t2 print $ uuh w1 w2 -- uuh! print $ bar t2 t1 print $ uuh w2 w1 print $ bar t2 t2 print $ uuh w2 w2 We get: {- "non-generic instance" "generic instance" "non-generic instance" "generic instance" "generic instance" "generic instance" "generic instance" "generic instance" -} This means that the generic instance is consistently chosen by uuh. This is clearly incoherent. I would also complain that uuh type-checks in the first place. Opinions? Regards, Ralf

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HXT - problems with runIOSLA or xread
by Niklas 21 Jul '06

21 Jul '06

Hi everybody, and especially Uwe Schmidt et al. I ran into some problems trying to process some xml from a string source, illustrated in the code below. From file, using readDocument, everything works great though. I've probably missed a finer detail or two in the arrow handling, or it could be a bug in HXT (if I run the code in ghci, it will crash hard). Output: -->main.exe NTree (XTag (QN {namePrefix = "", localPart = "small", namespaceUri = ""}) []) [ NTree (XTag (QN {namePrefix = "", localPart = "xml", namespaceUri = ""}) []) [], NTree (XTag (QN {namePrefix = "", localPart = "example", namespaceUri = ""}) []) []] --> So it seems that the string gets read, since it shows up as a result from procString, but I get no output from writeDocument in writeStrDoc. Could anybody enlighten me? I'm using GHC 6.5 and HXT 6.0 on Windows XP. Regards, /Niklas ---8<--- module Main where import Text.XML.HXT.Arrow wa = [(a_indent, "1"), (a_remove_whitespace, "1")] main = procString tststr >>= mapM_ print . snd procString = runIOSLA writeStrDoc (initialState ()) writeStrDoc = xread >>> writeDocument wa "-" tststr = "<small><xml/><example/></small>" --->8---

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