- Haskell-Cafe - Haskell.org

RE: Treating POSIX signals as exceptions?
by Simon Marlow 11 Nov '03

11 Nov '03

> I was wondering why System.Posix.Signals is as it is, and > whether it could > be rewritten to use exceptions, which seems like the obvious > way to handle > signals. Of course, that requires asynchronous exceptions, but that > doesn't seem like a severe problem. > > What I currently do is to use the following function > > withSig :: Signal -> IO a -> IO a -> IO a > withSig s handler job = do > id <- myThreadId > installHandler s (Catch $ throwDynTo id s) Nothing > job `catchDyn` catcher > where catcher s' | s' == s = handler > catcher s' = throwDyn s' Hmm, there is clearly a choice between having a signal spawn a new thread or raise an asynchronous exception, and it's not at all clear which is best. But, since we can implement either using the other one, it doesn't really matter too much which one is primitive. I've attached a more robust implementation of your withSig. This one: - doesn't suffer from race conditions around the point at which the signal handler is installed - it uses a datatype for the signal exception (Typeable can be derived these days) - it allows several threads to request notification of the same signal - it doesn't support nesting use of withSig with the same signal. (that could be added, but I'm not sure it's worth it). > Is there any reason all of System.Posix.Signals couldn't be > done in this > manner? Have the RTS always add a handler for each signal, which just > throws an exception. Off the top of my head, all we'd need > is one function > > acceptSignals :: IO () -- tells RTS to send signal exceptions to this > -- thread. You can implement this too using the module I attached. I guess we should provide this in the POSIX library. Would you like to play around with the API and submit it for inclusion? Cheers, Simon

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Value space transformations
by Graham Klyne 11 Nov '03

11 Nov '03

I just found myself writing some code like this: processFoo processBar = toFoo . processBar . fromFoo where fromFoo foo = ... toFoo bar = ... and noticed some similarity in style to certain kinds of transformation used in linear algebra, such as for diagonalization: D = T M inv(T) This got me to wondering if this is a common pattern in functional programming. There seems to be some similarity, for example, with the way that Monads are sometimes used, where operations are "lifted" into (?) a Monad. So my questions are: (a) is there a common functional programming pattern that corresponds to vector space transformations so that a function defined over one space can be used in another, and (b) if so, are there any not-too-heavy papers or articles discussing this pattern? #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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Is there an easier way to use guards in an expression...
by Graham Klyne 11 Nov '03

11 Nov '03

<aside> This is almost a reprise of a question that came up back in July [1]. At the time, Mark Jones posted a reference [2], but I'm not seeing how it is relevant to this issue of case expressions. [1] http://haskell.org/pipermail/haskell-cafe/2003-July/004708.html [2] ftp://ftp.research.bell-labs.com/dist/smlnj/papers/92-tr-aitken.ps </aside> The following code works, but it seems a bit clumsy: [[ mapXsdBoolean = DatatypeMap { -- mapL2V :: String -> Maybe Bool mapL2V = \s -> case s of s | matchT s -> Just True | matchF s -> Just False | otherwise -> Nothing -- mapV2L :: Bool -> Maybe String , mapV2L = Just . (\b -> if b then "true" else "false") } ]] The case expression seems rather an unwieldy way of getting guards into an expression. If this was an ordinary variable declaration rather than a field definition, I'd use a function definition with guards, but I can't see how to make that work with a lambda expression. Is there a neater way? And a supplementary question... do the prelude or standard libraries define any function like this: cond :: Bool -> a -> a -> a cond True a _ = a cond False _ b = b #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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determining if a int is a power
by ddekker6＠chello.nl 10 Nov '03

10 Nov '03

hello all, im new to haskell and have to do some excersises. Well i ran into this problem. I have to determin if a list of int's contains a int that is a power. So i have been trying using "any" with a helper function which return a Bool if the int is a power and applying that to the list. I think that this aproach should work. Anyway the problem im having is in the helper function. I have been trying this: isPower n = let powerF = floor(sqrt(n)) in if (powerF*PowerF == n) then True else False but this doesnt work. Prelude says that the definition of isPower needs a fractional int (if i remember correctly) anyway it looks like something is wrong with my type's? Could you guys help me out? Is there a simple standard function i overlooked. Or what should i change? Thanks in advance guys/gals

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Converting IO Int --> Int
by Tim Stitt 10 Nov '03

10 Nov '03

Can anyone tell me how to convert a value of type IO Int to a value of type Int. I wrote a simple script to read an integer from the user using IO and then the value is returned from the IO function using return value with a return type of IO Int. In the calling function I wish to do a comparison with the user value and a hard-coded integer. Unfortunately I keep getting type errors between the Int and the IO Int. I would be grateful for any assistance in this. Thanks in advance, Tim Stitt

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Beta Reduction, undefined vs Type Classes
by Jared Warren 10 Nov '03

10 Nov '03

Consider: > class Thing t where > thing :: t > > instance Thing Int where > thing = 0 > > instance Thing Char where > thing = 'a' Can someone please explain why > fst (1,thing) ...gets an 'ambiguous type variable' error, but > fst (1,undefined) ...doesn't? And is there anything I can change to make the former work as well? Even modifying fst doesn't work: > fst' :: Thing b => (a,b) -> a > fst' (~a,~b) = a -- ~ Jared Warren <warren(a)cs.queensu.ca> Computing Science, Queen's University

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RE: automaticly create the ana, cata, hylomorphisms
by Marta Isabel Oliveira 08 Nov '03

08 Nov '03

Ok, i read the page but i'm still stuck. I'm reading some papers about cata, ana and hylomorphisms but i need to know where to start putting it to code. So, 1. in order to have a pre-processor, i need to have a module with ALL data types? 2. from this module, how could i create the cata, ana,... i mean, a)how can i represent the result in haskell b) how can i create the cata(whatever) without knowing all of the possible functions that may be inserted in order to it's cata be created? thansk a lot, lesina.

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RE: automaticly create the ana, cata, hylomorphisms
by Marta Isabel Oliveira 07 Nov '03

07 Nov '03

Thanks and sorry. Beside new in Haskell i'm also new in this archive lists. So, i was not aware i had sent with sucess the mail to the first list. lesina > -----Original Message----- > From: Keith Wansbrough [SMTP:Keith.Wansbrough@cl.cam.ac.uk] > Sent: Friday, November 07, 2003 3:00 PM > To: Marta Isabel Oliveira > Cc: haskell(a)haskell.org; haskell-cafe(a)haskell.org > Subject: Re: automaticly create the ana, cata, hylomorphisms > > > Hello programmers, > > > > I have a problem to solve and don't have a clue where to start. > > This is a homework question. Please read > > http://www.haskell.org/hawiki/HomeworkHelp > > for some helpful hints. > > By the way, you shouldn't post the same message to multiple lists - you should choose the right one (haskell-cafe in this case), or if you must, cross-post one message to both. This means people who subscribe to both won't see your message twice. > > --KW 8-) > -- > Keith Wansbrough <kw217(a)cl.cam.ac.uk> > http://www.cl.cam.ac.uk/users/kw217/ > University of Cambridge Computer Laboratory. >

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Re: automaticly create the ana, cata, hylomorphisms
by Keith Wansbrough 07 Nov '03

07 Nov '03

> Hello programmers, > > I have a problem to solve and don't have a clue where to start. This is a homework question. Please read http://www.haskell.org/hawiki/HomeworkHelp for some helpful hints. By the way, you shouldn't post the same message to multiple lists - you should choose the right one (haskell-cafe in this case), or if you must, cross-post one message to both. This means people who subscribe to both won't see your message twice. --KW 8-) -- Keith Wansbrough <kw217(a)cl.cam.ac.uk> http://www.cl.cam.ac.uk/users/kw217/ University of Cambridge Computer Laboratory.

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Haskell pre-processor
by Marta Isabel Oliveira 07 Nov '03

07 Nov '03

Hello programmers, I have a problem to solve and don't have a clue where to start. I need to do a program in haskell that creates automaticly the ana, cata and hylomorphisms to arbitraty types. It's sort of a pre-processer. How can i start? I have the HaPar.hs that has uses Haskell parser to extract the type declarations from a file containing a Haskell module. I also have the TDec1.hs that has some dataTypes defined(TDec1, CDec1, Type) and some Show instances. Can anyone help me? I'm totally new to Haskell :(

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