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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

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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 --

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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 --

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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

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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

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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

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RE: Asynchronous Exceptions
by Simon Marlow 26 Nov '01

26 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

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List module in library report exports []((:), [])
by Ian Lynagh 26 Nov '01

26 Nov '01

Hiyas A few more things I'm afraid :-( The List module in the library report exports []((:), []) which, as I mentioned in a previous thread WRT the GHC prelude, the report doesn't allow in the exports list. The block at the top listing the exports and types has been split into 2 at an apparently arbitrary point for no obvious reason? deleteFirstsBy has no corresponding deleteFirsts and is not mentioned in the text. Actually, having looked at the definitions it seems to be what we would like to call \\By, so this should be noted in the "By" section. insert and the zip/unzip stuff is also not explained in the text - don't know if this is deliberate or not (insert is also mentioned with sort). sortBy is mentioned before the "By" section unlike the other By variants for no obvious reason. "exmaple" genericIndex seems to be a generic !!, but The prefix "generic" indicates an overloaded function that is a generalised version of a Prelude function. and index is something completely different and defined in Ix. Clearly the function can't be called generic!!, so I suggest the non-uniformity be noted. I am also curious as to why other functions don't have generic equivalents, e.g. elemIndex, findIndices - is it because they were also not considered important enough (as per the By functions)? The indentation of unfoldr, maximumBy and minimumBy seems to have gone wrong with some things not being indented at all (only looked at the HTML version). I think unionBy eq xs ys = xs ++ foldl (flip (deleteBy eq)) (nubBy eq ys) xs may be clearer written as unionBy eq xs ys = xs ++ deleteFirstsBy eq (nubBy eq ys) xs Thanks Ian

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Library report examples
by Ian Lynagh 26 Nov '01

26 Nov '01

Hi guys, 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 but ghc, hugs and nhc98 all loop (trying to get and test the value of l I believe). 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 ~~~ ~~~ ~~~ Thanks Ian

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RE: heap and walking through a very long list
by Simon Peyton-Jones 26 Nov '01

26 Nov '01

You can do it specifically for GHC, not not in standard Haskell (except grossly inefficiently). To see how to do it in GHC, look at the implementation in the source code (ghc/lib/std), which is online (check the "CVS repository" link on GHC's home page). Meanwhile you could use the IArray stuff. Simon | -----Original Message----- | From: Joe English [mailto:jenglish@flightlab.com] | Sent: 25 November 2001 00:12 | To: haskell(a)haskell.org | Subject: Re: heap and walking through a very long list | | | | Simon Peyton-Jones wrote: | | > There should really be a strict accumArray, just as there | should be a | > strict foldl. | | Yes, please! | | Is there a way to write a strict version of accumArray in | Haskell 98, or does this need to be done by the implementation? | | | --Joe English | | jenglish(a)flightlab.com | | _______________________________________________ | Haskell mailing list | Haskell(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell |

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