unsafePerformIO to give warnings
Hi all If I want to give warnings when doing something and I don't care too much about the order they appear in, can I use this? foo x = if success x then Just x else warn "Working out x went wrong" Nothing warn :: String -> a -> a warn s x = unsafePerformIO (hPutStrLn stderr s) `seq` x The hslibs docs say If the I/O computation wrapped in unsafePerformIO performs side effects, then the relative order in which those side effects take place (relative to the main I/O trunk, or other calls to unsafePerformIO) is indeterminate. but it's not entirely clear on whether or not I could end up with 2 warnings interspersed? And is it guaranteed that the warnings will be printed at some point? hugs and ghci only seem to print the first warning, but ISTR similar problems happen with threads and got the impression that they were in the wrong. Thanks Ian ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
installation problems with graphics-2.0.4
The graphics lib of the new December release does not compile with the shipped makefile. The makefile does not do anything at all??? Maybe there is a problem with wildcard extensions? To be honest, I have no clue. I am using a Sparc5/Solaris machine. Any help appreciated. Thanks in advance, Bernhard ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Working Conference on Generic Programming, 2nd call for papers
WCGP '02 IFIP WG2.1 Working Conference on GENERIC PROGRAMMING http://www.generic-programming.nl/wcgp/cfp.html Organised in conjunction with MPC'02 July 8 - July 13, 2002 Dagstuhl, Germany CALL FOR PAPERS Generic programming is about making programs more adaptable by making them more general. Generic programs often embody non-traditional kinds of polymorphism; ordinary programs are obtained from them by suitably instantiating their parameters. In contrast with normal programs, the parameters of a generic programs are often quite rich in structure. For example they may be other programs, types or type constructors, class hierarchies, or even programming paradigms. Generic programming techniques have always been of interest, both to practitioners and to theoreticians, but only recently have generic programming techniques become a specific focus of research in the functional and object-oriented programming language communities. This working conference will bring together leading researchers in generic programming from around the world, and feature papers capturing the state of the art in this important emerging area. We welcome contributions on all aspects, theoretical as well as practical, of generic programming, aspect-oriented programming, polytypic programming, adaptive object-oriented programming, generic components, and so on. SUBMISSION Full papers should be submitted in Postscript or pdf format by e-mail to reach [EMAIL PROTECTED] by February 16, 2002. The details of the submission procedure can be found at http://www.generic-programming.nl/wcgp/submit.html Although there is no page limit, submissions should strive for brevity and clarity. IMPORTANT DATES Submission February 16, 2002 Notification April 12,2002 Final version dueMay 24, 2002 PROGRAMME COMMITTEE Matt Austern Eerke Boiten Ulrich Eisenecker Jeremy Gibbons (co-chair) Ralf Hinze Johan Jeuring (co-chair) Gary Leavens Karl Lieberherr Lambert Meertens Eugenio Moggi Bernhard Moeller Oege de Moor David Musser Martin Odersky Ross Paterson Simon Peyton Jones Colin Runciman Doaitse Swierstra Stephanie Weirich LOCAL ORGANISATION Jeremy Gibbons Johan Jeuring Bernhard Moeller CORRESPONDENCE Jeremy Gibbons ([EMAIL PROTECTED]) Johan Jeuring ([EMAIL PROTECTED]) ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: global counters
> basically, i want a function getVar :: () -> String which returns a new
> string every time. i tried this:
>
> > curVarId = newSTRef (0 :: Integer)
> >
> > {-# NO-INLINE newVar -}
> >
> > newVar = \_ -> ('\0' : show x)
> > where y = unsafePerformIO (stToIO curVarId)
> > x = unsafePerformIO $ stToIO $
> > do x <- readSTRef y
> > writeSTRef y (x + 1)
> > return x
>
> and in ghci it works wonderfully, but when i actually compile, all i get
> is "\NUL0".
>
> what's the proper way to write this?
I think (haven't tested this one) you should write (modulo layout):
| curVarIdRef :: IORef Integer
| curVarIdRef = unsafePerformIO $ newIORef (0 :: Integer)
|
| {-# NO-INLINE newVar -}
|
| newVar :: () -> String
| newVar = \_ -> ('\0' : show x) where
| x = unsafePerformIO $
| do x <- readIORef curVarIdRef
| writeIORef curVarIdRef (x + 1)
| return x
Because in your original version, curVarId allocates a new reference to 0
every time newVar is called.
Hope this works, Jan de Wit
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x^y. Reply
Toralf Wittner <[EMAIL PROTECTED]> writes > [..] > data PowerNum = INT Integer | DBL Double deriving (Eq, Show, Read) > [..] > Now it basically works. However wouldn't it have been easier to write > something like this: > > powPos :: Integer -> Integer -> Integer > [..] > powNeg :: Integer -> Integer -> Double > [..] > | y < 0= 1 / fromInteger x * powNeg x (y + 1) > > Initially I wanted something as terse as the Python version, now I either > have to write two functions or I need to define a type. Is there not an > easier way to come as close as possible to the Python version? > > Thanks anyway - learnt a lot! For this particular task, the most natural solution is, probably, pow :: Fractional a => a -> Integer -> a pow a n = if n > 0 then a^n else (1/a)^(- n) (^) is of standard, only pow adds the facility of negative n. Then, you need each time to convert the argument to appropriate type of Fractional: pow (fromInteger 2 :: Ratio.Rational) 2 --> 4 % 1 pow (fromInteger 2 :: Ratio.Rational) (-2) --> 1 % 4 pow (2 :: Double) (-2) --> 0.25 pow (2 :: Integer) (-2) --> ... No instance for (Fractional Integer) If you replace standard (and not lucky) Fractional with some class Foo (with multiplication mul and division div), make Integer an instance of Foo (where div may fail for some data), and program pow :: Foo a => a -> Integer -> a via mul, div, then it would work like this: pow (2 :: Integer) 2--> 4 pow (2 :: Integer) (-2) --> "Error: cannot invert 2 :: Integer" pow (2 :: Rational) (-2) --> 1%4 Another way is to try to straggle with overlapping instances by defining something like this: class Pow a b where pow :: a -> Integer -> b instance Num a => Pow a a where pow = (^) Fractional ? instance Num a => Pow Integer a where pow = ? If this succeeds, there will be also no need in new type constructors. - Serge Mechveliani [EMAIL PROTECTED] ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
