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Programming style (or: too clever by half?)
by Graham Klyne 12 Nov '03

12 Nov '03

I just spotted a possible idiom for something that seems to pop up from time to time: foldr const But I can't help feeling that this code is perversely obscure. Does it have any advantages over the more obvious form suggested below? [[ headOrSomething = foldr const headOrSomething1 _ (x:_) = x headOrSomething1 x [] = x h = headOrSomething -- h = headOrSomething1 test1 = h 0 [1,2,3] == 1 test2 = h 0 [] == 0 test3 = h 0 (repeat 42) == 42 ]] This suggests, e.g., an alternative coding of Maybe.listToMaybe: [[ altListToMaybe = foldr (const . Just) Nothing ]] Opinions? #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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Why doesn't GHC do the following optimisation?
by andrew cooke 12 Nov '03

12 Nov '03

Hi, This question is purely out of curiousity - I'm no expert on this. I wrote the following code: sum1 :: [Int] -> Int sum1 = foldl (+) 0 foldl' :: (a -> b -> a) -> a -> [b] -> a foldl' f v l = foldr (\x -> \fld -> \v' -> fld $ f v' x) id l v sum2 :: [Int] -> Int sum2 = foldl' (+) 0 main :: IO () main = do n <- readLn; print $ sum1 [1..n]; print $ sum2 [1..n] and ran it through ghc with: ghc Folds.hs -ddump-simpl -O I was expecting the two calls to be optimized to something equivalent, or even for the result of a single calculation to be printed twice. However, peering at the Core output, it seems that the two sum functions exist separately and have different structure (sum1 appears to have a simple recursive definition, while sum2 involves two lamba expressions). I did all this because I read a paper introducing folds that showed how you can express foldl in terms of foldr and giving the derivation. Unfortunately I don't have the paper/derivation, but I believe my foldl' is correct (that may be the problem, of course - that sum1 and sum2 are not equivalent either because of a coding error, or because of some subtlety in the types, although I carefully gave them explicit types to try and avoid that). The derivation wasn't that complicated (althugh I find it simpler to simply write the fold down). If it's not an error on my part, why doesn't ghc do the same kind of transformation itself? Or am mistaken in thinking that if derivation is easy then the revrese transformation as an optimisation should also be easy? Thanks, Andrew -- http://www.acooke.org/andrew

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Why no findM ? simple Cat revisited
by Ahn Ki-yung 12 Nov '03

12 Nov '03

Simple Cat (revisitied) \begin{code} import IO findM f [] = return Nothing findM f (x:xs) = do { v <- x; if f v then return (Just v) else findM f xs } isLeft (Left _) = True isLeft _ = False main = findM (isLeft) (hCat stdin) where hCat h = try (hGetLine h) : hCat h \end{code} This is my answer for the question of my own, which is posted a couple There are mapM, filterM in the Haskell 98 Standard Library. But why no findM there ? As you can see from simple cat, it seems quite useful. I think fildM should be added to the module Monad. -- Ahn Ki-yung

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Treating POSIX signals as exceptions?
by David Roundy 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' which just catches a given signal and throws an exception, and then catches that exception with a given default handler. This has the nice effect that my 'job' can't be rudely interrupted by the user hitting ^C at just the wrong time (since I use this to catch sigINT). Thus, this makes bracket work in the presence of signals, allows me to use block to avoid being interrupted by a ^C when this could cause data corruption, etc. (Yes, I know that the above isn't very clean, since any other process that uses {catch,throw}Dyn on a CInt will mess things up, but creating a new Typeable instance seemed like too much trouble.) 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. We'd also need some sort of SignalException type (and of course, to define all the signals), and the rest would be done using the existing Control.Exception code. And, of course, default signal handlers would need to be converted to default exception handlers. Is there a problem with this idea? -- David Roundy http://www.abridgegame.org/darcs

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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 10 Nov '03

10 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 10 Nov '03

10 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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