November 2001 - Haskell

IO library report
by Ian Lynagh 28 Nov '01

28 Nov '01

The report has the following example: import IO main = do hSetBuffering stdout NoBuffering putStr "Enter an integer: " x1 <- readNum putStr "Enter another integer: " x2 <- readNum putStr ("Their sum is " ++ show (x1+x2) ++ "\n") where readNum :: IO Integer -- Need a type signature for -- readLn to avoid ambiguity readNum = readLn However, I believe that due to the monomorphism restriction and type defaulting the type signature isn't actually required. The next example: import IO import System main = do [f1,f2] <- getArgs h1 <- openFile f1 ReadMode h2 <- openFile f2 WriteMode copyFile h1 h2 hClose h1 hClose h2 copyFile h1 h2 = do eof <- hIsEOF h1 if eof then return () else do c <- hGetChar h1 hPutChar h2 (toUpper c) copyFile h1 h2 also requires Char to be imported for toUpper, as does the final example import System main = do [f1,f2] <- getArgs s <- readFile f1 writeFile f2 (map toUpper s) The section also doesn't follow the convention of the portable part of the module definition being given at the end of the section. Thanks Ian

1 0

Possible bug/omission in Numeric library?
by Malcolm Wallace 28 Nov '01

28 Nov '01

There is something strange about the Haskell'98 Numeric library, which I think could be considered a bug of sorts. There are functions readDec, readOct, readHex :: (Integral a) => ReadS a which read an integer from a string in base 10, 8, or 16, but there are no corresponding show functions to convert an integer to a string using base 8 or 16. The sole function given is showInt :: (Integral a) => a -> ShowS which shows a number in base 10 only. I think you'll agree that it is odd that you can read a certain number format but cannot show it? There are several obvious generalisations of showInt. One might be: showInt :: (Integral a) => a -> a -> ShowS showInt base n r | n < 0 = error "Numeric.showInt: can't show negative numbers" | otherwise = let (n',d) = quotRem n base r' = Char.intToDigit (fromIntegral d) : r in if n' == 0 then r' else showInt base n' r' which shows a number in any base up to 16. Another might be: showInt :: (Integral a) => a -> (Int->Char) -> a -> ShowS showInt base intToDig n r | n < 0 = error "Numeric.showInt: can't show negative numbers" | otherwise = let (n',d) = quotRem n base r' = intToDig (fromIntegral d) : r in if n' == 0 then r' else showInt base intToDig n' r' which more closely resembles the dual function readInt and could be likewise accompanied by specific instances for the three most common bases: showDec, showOct, showHex :: (Integral a) => a -> ShowS showDec = showInt 10 Char.intToDigit showOct = showInt 8 Char.intToDigit showHex = showInt 16 Char.intToDigit However, I would guess that changing the type signature of the current showInt function is unacceptable for Haskell'98. Maybe we should consider adding the more general version under a new name like showIntBase, together with show{Dec,Oct,Hex}? This would break no existing code, and clear up the slight anomaly in the current capability of the Haskell'98 library. Regards, Malcolm

2 1

RE: Possible bug/omission in Numeric library?
by Simon Marlow 28 Nov '01

28 Nov '01

> There is something strange about the Haskell'98 Numeric library, > which I think could be considered a bug of sorts. There are functions > > readDec, readOct, readHex :: (Integral a) => ReadS a > > which read an integer from a string in base 10, 8, or 16, but there > are no corresponding show functions to convert an integer to a string > using base 8 or 16. The sole function given is > > showInt :: (Integral a) => a -> ShowS > > which shows a number in base 10 only. I think you'll agree that it > is odd that you can read a certain number format but cannot show it? Yes indeed. GHC and Hugs have showHex, showOct, showBin and showIntAtBase in NumExts: http://www.haskell.org/ghc/docs/latest/set/sec-numexts.html Cheers, Simon

1 0

Where to ask questions regarding categories and datatypes
by Peter Douglass 28 Nov '01

28 Nov '01

Hi, I have a number of questions regarding categories and datatypes. I know that many of the folk in this mailing list could answer these question, but I wonder if there is a more appropriate forum. (i.e. the question are not Haskell specific). Thanks, --Peter Douglass

2 1

runtime effect of polymorphic Array.!
by Johannes Waldmann 28 Nov '01

28 Nov '01

Of course (!) is polymorphic (!) :: Ix a => Array a b -> a -> b but on the other hand the GHC manual says Overloaded functions are not your friend: Haskell's overloading (using type classes) is elegant, neat, etc., etc., but it is death to performance if left to linger in an inner loop. I assume (!) is inlined and specialized by GHC, but what does hugs do? Does it build and pass around dictionaries all the time I index an array? (In my case, I am forced to use (run)hugs.) Just curious, -- -- Johannes Waldmann ---- http://www.informatik.uni-leipzig.de/~joe/ -- -- joe(a)informatik.uni-leipzig.de -- phone/fax (+49) 341 9732 204/252 --

1 0

Wanted: Hash Tables
by Johannes Waldmann 28 Nov '01

28 Nov '01

Dear all, I would like to use hash tables in my Haskell programs (especially in places where I use `FiniteMap String a' now). I remember there was a `class Hash' in hbc (?) and I imagine this should be combined with STArray or the like. Is there already some re-usable code for doing this? Thanks - -- -- Johannes Waldmann ---- http://www.informatik.uni-leipzig.de/~joe/ -- -- joe(a)informatik.uni-leipzig.de -- phone/fax (+49) 341 9732 204/252 --

1 0

RE: Library report examples
by Simon Peyton-Jones 27 Nov '01

27 Nov '01

| The library report defines | | -- Diagonal of a square matrix | diag :: (Ix a) => Array (a,a) b -> Array a b | diag x = ixmap (l,u) (\i->(i,i)) x | where ((l,l'),(u,u')) | l == l' && u == u' = bounds x And that is indeed a stupid definition. It's like saying x = if x>2 then 1 else 10 (the guard is really just an 'if'.) So of course l, u etc are all bottom. The guard needs to be on diag itself diag x | l==l' && u==u' = ixmap ...as before where ((l,l'),(u,u')) = bounds x thanks for pointing this out | I am also curious why, for example, | | row :: (Ix a, Ix b) => a -> Array (a,b) c -> Array b c | row i x = ixmap (l',u') (\j->(i,j)) x where ((l,l'),(u,u')) | = bounds x | | isn't written as | | row :: (Ix a, Ix b) => a -> Array (a,b) c -> Array b c | row i x = ixmap (l,u) (\j->(i,j)) x where ((_,l),(_,u)) = bounds x | ~~~ ~~~ ~~~ you can write it either way S

2 1

Postdoctoral Research Associate, "Refactoring Functional Programs"
by C.Reinke 27 Nov '01

27 Nov '01

Postdoctoral Research Associate in Refactoring Functional Programs Applications are invited for a three year Postdoctoral Research Associate position at the University of Kent at Canterbury, to work under the direction of Professor Simon Thompson and Dr Claus Reinke on an EPSRC funded project entitled "Refactoring Functional Programs". Refactoring is the process of redesigning existing code without changing its functionality. In our project we will explore refactoring for functional programs and in particular we will construct a catalogue of functional refactorings, and build a prototype tool in Haskell to support refactoring of Haskell programs. Applicants should be able to demonstrate extensive experience of programming in a functional programming language, preferably Haskell, and have good communication skills. The appointment will be made within the salary ranges of £17,278-£26,229 p.a. on the Research IA scale. Please telephone the Personnel Office for further particulars on 01227 827837 (24 hours) or email: Personnel(a)ukc.ac.uk , quoting the reference number R02/15. Text phone users please telephone 01227 823674. The closing date is Friday 11 January 2002. Informal enquires can be directed to either Simon or Claus: {S.J.Thompson,C.Reinke}@ukc.ac.uk. More details of the project and further particulars for the position can be found at: http://www.cs.ukc.ac.uk/people/staff/sjt/Refactor/ Simon and Claus

1 0

Re: Asynchronous Exceptions
by Steinitz, Dominic J 27 Nov '01

27 Nov '01

---------------------- Forwarded by Dominic Steinitz/HEATHROW/BRITISH AIRWAYS/GB on 27/11/2001 09:59 --------------------------- From: Dominic Steinitz on 27/11/2001 08:16 To: uk1o cc: haskell bcc: Subject: Re: Asynchronous Exceptions Hannah, I was thinking of where you really want to write something to a transaction log so that you know an action really did complete. Is there a way of doing this? Dominic. uk1o(a)rz.uni-karlsruhe.de 26/11/2001 23:53 To: haskell@haskell.org@INTERNET cc: bcc: Dominic Steinitz/HEATHROW/BRITISH AIRWAYS/GB Subject: Re: Asynchronous Exceptions Hello! On Fri, Nov 23, 2001 at 09:38:35AM -0000, Simon Marlow wrote: > [...] > However, I agree that sometimes you really want to be able to do this, > so perhaps we need another form of 'block' which doesn't allow *any* > exceptions to be delivered, for those times when you know that the time > spent waiting in an interruptible operation is going to be bounded. But even things like putStr can block for an unbounded amount of time. E.g. stdout is a pipe and the reader just doesn't read it for n hours (n unknown), etc. Kind regards, Hannah. _______________________________________________ Haskell mailing list Haskell(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell ------------------------------------------------------------------------------------------------- To receive our special fares directly by email, register at http://www.britishairways.com/registration

1 0

RE: Asynchronous Exceptions
by Simon Marlow 27 Nov '01

27 Nov '01

> Can someone help me understand how this works? I've been > reading the paper "Asynchronous Exceptions in Haskell". This > gives a combinator > > finally :: IO a -> IO b -> IO s > > finally a b = > block (do { > r <- catch (unblock a) (\e -> do { b; throw e }); > b; > return r; }) > > Now suppose we have > > finally (putStrLn "Test Started") (putStrLn "Test Terminated") > > then looking at the semantics, putStrLn can become stuck and > therefore can be interrupted. So the interrupt could occur > whilst "Test Terminated" is being output and we could end up with > > Test Started > Test Term > > Is this what could happen? If so, is there a way of making > sure that "Test Terminated" is output? Yes, that's correct. putStrLn is interruptible, so you can't force it to run to completion - to do so would introduce a possible deadlock. However, I agree that sometimes you really want to be able to do this, so perhaps we need another form of 'block' which doesn't allow *any* exceptions to be delivered, for those times when you know that the time spent waiting in an interruptible operation is going to be bounded. Cheers, Simon

2 1