August 2005 - Haskell-Cafe

Weak hashtable memoization code?
by David Roundy 28 Aug '05

28 Aug '05

Hi all, Does anyone have a nice bit of example code to implement memoization using weak pointers and hash tables? It would be nice to have a pre-packaged module that I could just use, which has already been tested. The contents of Data.WeakPtr seem a bit lower-level than I'd rather work with. -- David Roundy http://www.darcs.net

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Comtrol.Monad.when
by Juan Carlos Arevalo Baeza 27 Aug '05

27 Aug '05

Any reason why "Control.Monad.when :: Monad <Control.Monad.html#t%3AMonad> m => Bool <Data.Bool.html#t%3ABool> -> m () -> m ()" doesn't have type "Monad <Control.Monad.html#t%3AMonad> m => Bool <Data.Bool.html#t%3ABool> -> m a -> m ()" instead? Just seems a bit of a waste to have the restriction without need. I find myself needing to add the extra spurious "return ()" at the end more often than not. I made an alternate "when_" for my own use, but... JCAB

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RE: [Haskell-cafe] How to use STArray?
by Bayley, Alistair 26 Aug '05

26 Aug '05

> From: Udo Stenzel [mailto:u.stenzel@web.de] > > You messed the argument order to readArray up. Yes, that won't help :-) > Even with that repaired, > you'll need a type signature somewhere to help ghc resolve the > overloading of newArray and readArray, which is surprisingly > tricky due > to the "s" that must not escape. This works: > > compute :: Int -> Int > compute n = runST ( do > arr <- newArray (-1, 1) n :: ST s (STArray s Int Int) > readArray arr 1 > ) Thanks. This is what I needed. The thing that worked best for me was to wrap newArray (and newArrayList) with functions that I could give signatures to. I was hoping to keep the program as signature-free as possible, to avoid committing heavily to particular implementations. e.g. makeArray :: Int -> Int -> a -> ST s (STArray s Int a) makeArray lower upper val = newArray (-lower, upper) val makeListArray :: Int -> Int -> [a] -> ST s (STArray s Int a) makeListArray len lst = newListArray (1, len) lst Alistair. ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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RE: [Haskell-cafe] How to use STArray?
by Bayley, Alistair 26 Aug '05

26 Aug '05

> From: Ketil Malde [mailto:ketil+haskell@ii.uib.no] > > Greg Buchholz <haskell(a)sleepingsquirrel.org> writes: > > > Diff arrays have an immutable interface, but rely on > internal updates in > > place to provide fast functional update operator //. > > While a cool concept, ISTR that somebody benchmarked these some time > ago, and found the performance to be fairly poor in practice. Has > that changed lately, or am I just misremembering? DiffArray certainly improved the execution time of my program (the execution time was dominated by array updates, so no surprise there). I should have said that I'm also concerned with space usage; I'd like to allocate the arrays at the beginning and expect that to be all of the memory required, so that's another reason to be interested in mutable arrays. Alistair. ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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RE: [Haskell-cafe] How lazy can "peek" be?
by Bayley, Alistair 25 Aug '05

25 Aug '05

> From: Juan Carlos Arevalo Baeza > > If I don't want to pay for that cost, then I have > to convert the code as in: > > func p = > b <- peekByteOff p 4 > case b of > B1 -> doSomething > B2 -> do { a <- peekByteOff p 0; doSomethingElse a } > B3 -> do { a <- peekByteOff p 0; doAnotherThing a } > > Repetition, repetition, repetition. This does get tedious. Would this work? > func p = do > let get_a = peekByteOff p 0 > b <- peekByteOff p 4 > case b of > B1 -> doSomething > B2 -> do a <- get_a; doSomethingElse a > B3 -> do a <- get_a; doAnotherThing a Or, with marginally less duplication: > B2 -> get_a >>= doSomethingElse > B3 -> get_a >>= doAnotherThing Alistair. ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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How lazy can "peek" be?
by Juan Carlos Arevalo Baeza 25 Aug '05

25 Aug '05

Hi! One of the nice things of laziness is that it allows us to express things concisely. For instance: func a b = let ta = transmogrify a in case b of B1 -> doSomething B2 -> doSomethingElse ta B3 -> doAnotherThing ta It is clear why having this let-expression is a good thing. We can "transmogrify a" only once, and then use the "transmogrified a" only when (if) needed. In this case, one branch of the case expression doesn't need it, in which case we've effectively paid nothing for it, no matter how complex and time/resource-consuming "transmogrify" is. So far, so good. Now, I need to interoperate with external code using FFI. I have a function that gets called from outside, and gets its parameters as a pointer to a structure, so: func p = do a <- peekByteOff p 0 b <- peekByteOff p 4 case b of B1 -> doSomething B2 -> doSomethingElse a B3 -> doAnotherThing a Now I see myself with a little dilemma. This code looks a lot like the first version, where instead of "transmogrifying", we just read the value from the structure through the use of the pointer. Reading the value from memory can't be very time consuming, but it has to cost something. If I don't want to pay for that cost, then I have to convert the code as in: func p = b <- peekByteOff p 4 case b of B1 -> doSomething B2 -> do { a <- peekByteOff p 0; doSomethingElse a } B3 -> do { a <- peekByteOff p 0; doAnotherThing a } Repetition, repetition, repetition. This does get tedious. Especially when the case expression has many branches, and there are many variables in the structure that might or might not be needed. So you see my dilemma. It's _very_ tempting to do something like: func p = do let a = unsafePerformIO $ peekByteOff p 0 b <- peekByteOff p 4 case b of B1 -> doSomething B2 -> doSomethingElse a B3 -> doAnotherThing a I mean... the contents of "p" are never going to be modified, so this "feels" right on some level. But we're effectively breaking the pureness of the language. I mean... this function could be called twice with the same _pointer_ but different data stores in the structure, which can (and maybe will) cause problems depending on how evaluation proceeds. Is there a way to handle this nicely? I recently read about the Clean language. It seems like it allows (and relies) in an extension to the type system, that allows the program to specify uniqueness of values, so that two pointers values might be represented using the same bits, but they'd still be considered distinct values. But no such thing in Haskell. Maybe a good thing... trying to understand the whole explanation of uniqueness in Clean made my head spin. But still... Is it known what GHC, for instance, will do to this code when optimizing? Might it just do the right thing? Probably the C optimizer would take care of it. It feels to me that's my only hope. But it'd be great if the "haskell" part of the compiler could take care of it natively. This is something that I've been thinking about. I mean... the IO monad does seem to impose too much sometimes. Any IO action always is assumed to modify the "external world" (affect the execution of the actions that come after it), whether it does or not: do peekByteOff p 0 -- totally useless, but it needs to be performed anyway before "returning" return () do a <- peekByteOff p 0 b <- peekByteOff p 4 -- could be done before "a" Somehow, it feels like it'd be a good thing to be able to limit the scope of IO actions. Going too far would complicate it enormously: do a <- readFile "file" b <- peekByteOff p 4 -- could be done in any order, really, but how would you express it... maybe "domains of influence", like "filesystem" and "memory"... not very clear. do a <- peekByteOff p1 0 pokeByteOff p2 0 1234 -- could be done in any order, as long as p1 and p2 don't alias each other (yuck!) So I wouldn't want to go that far... yet :). A global thing might be an improvement. I'm aware it wouldn't be a monad anymore, but could it be something else? I hope this is all pointless :-). JCAB

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Pattern match failure, then inconsistent function
by Adam Wyner 24 Aug '05

24 Aug '05

Hi, I am getting a "pattern match failure", and then subsequent functions which worked right work wrong. I am using Hugs and Trex. While most of the other functions work fine, the function "findCoordinatesUtil" generates an error, then misbehaves; it takes a record and searches a list of records to find other records in a specified relationship with the input function. In the first example, it works fine: LexicalSemantics03> findCoordinatesUtil (giveActionFromLabel "Action6" testLexActions03) testLexActions03 [(label="Action7", xcond=["prop1","prop7","prop5"], ycond=["prop3","pr op4","neg-prop6"]),(label="Action08", xcond=["prop1","prop7","prop5"], ycond=["prop3","neg-prop6","prop5"]),(label="Action09", xcond=["prop1 ","prop7","prop5"], ycond=["prop3","neg-prop6","neg-prop1"])] However, when I run the function on a larger list of records (here testList01), it generates an error message. LexicalSemantics03> findCoordinatesUtil (giveActionFromLabel "Action33 " testList01) testList01 Program error: pattern match failure: map_v780 (instEq_v20 instEq_v11 Eq_== "neg-prop2") map_v780 INTERNAL ERROR: Error in graph Moreover, when I subsequently run the function, I get a different (and wrong) result -- label-"Action08" is missing. LexicalSemantics03> findCoordinatesUtil (giveActionFromLabel "Action6" testLexActions03) testLexActions03 [(label="Action7", xcond=["prop1","prop7","prop5"], ycond=["prop3","pr op4","neg-prop6"]),(label="Action09", xcond=["prop1","prop7","prop5"], ycond=["prop3","neg-prop6","neg-prop1"])] When I reload the module, it correctly works again. LexicalSemantics03> :r LexicalSemantics03> findCoordinatesUtil (giveActionFromLabel "Action6" testLexActions03) testLexActions03 [(label="Action7", xcond=["prop1","prop7","prop5"], ycond=["prop3","pr op4","neg-prop6"]),(label="Action08", xcond=["prop1","prop7","prop5"], ycond=["prop3","neg-prop6","prop5"]),(label="Action09", xcond=["prop1 ","prop7","prop5"], ycond=["prop3","neg-prop6","neg-prop1"])] I don't understand the pattern match failure or how to correct it. What is this error message telling me? Thanks, Adam Wyner

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Overlapping instances
by Frank 24 Aug '05

24 Aug '05

I would like to state that a class Sup is exhaustively broken down in two subclasses Sub1 and Sub2 (meaning, for every instance of Sub1 and every instance of Sub2, the methods in Sup apply). I try to code this as: instance Sub1 x => Sup x instance Sub2 x => Sup x And get the (expected) error message ExhaustiveClass.hs:22:0: Duplicate instance declarations: ExhaustiveClass.hs:22:0: instance (Sub1 x) => Sup x ExhaustiveClass.hs:25:0: instance (Sub2 x) => Sup x (I have allowed overlapping instances, undecidable instances) Is there another way of achieving this? Any help appreciated! Andrew ----------------- complete code I tried with: class Sub1 a where op1 :: a -> a class Sub2 a where op2 :: a -> a -> a class Sup a where op3 :: a -> Bool instance Sub1 Int where op1 = id instance Sub2 Float where op2 x y = y instance Sub1 x => Sup x where op3 _ = True instance Sub2 x => Sup x where op3 _ = False But get Andrew U. Frank Professor, Head of Department Geoinformation and Cartography E127 phone: +43 1 588 01 12710 TU Vienna secr. +43 1 588 01 12700 Gusshausstrasse 27-29 fax +43 1 588 01 12799 A-1040 Vienna Austria cellular phone +43 676 41925 72 http://www.geoinfo.tuwien.ac.at/persons/frank/frank.html skype:AndrewUFrank

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RE: [Haskell-cafe] Re: Oracle + Haskell advice?
by Bayley, Alistair 23 Aug '05

23 Aug '05

> From: Brian Strand [mailto:bstrand@switchmanagement.com] > > > > > Reviewing at the bind variable code has highlighted a > glaring design flaw: > > you can only pass homogenous lists of bind values! > > I was wondering about that; I thought maybe you just passed > everything to OCI > as String and let Oracle sort it out, but that doesn't seem > very Haskell-ish. I've fixed this now, so it should be quite a bit more usable. The code's in the experimental branch, but it's all quite safe, except for the MS Sql Server stuff, which is work-in-progress. I've updated the Haddock docs, which should explain how to use everything. Please tell me of anything that's unclear, or omitted. Again, I'm happy to help with any problems or questions. Alistair. ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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Category theory monad <----> Haskell monad
by Carl Marks 23 Aug '05

23 Aug '05

Is there any text/article which makes precise/rigorous/explicit the connection between the category theoretic definition of monad with the haskell implementation?

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