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Re: Haskell-Cafe Digest, Vol 33, Issue 9
by Alberto G. Corona 12 May '06

12 May '06

Hi, I´m trying to make a searchable transactional cache using STM. The whole idea is to use indexed TVar variables using a FiniteMap. another TVar holds the finitemap . This last TVar has to be a global variable. I found that when handled as global, a TVar does not keep the state. For example: stmcache= newTVar 0 main= do v<- val v<- val print v val = atomically $! do cache <- stmcache x <- readTVar cache writeTVar cache (x+1) return x this code print 0 instead of 1 as the result of incrementing cache and retrieving it the second time The same happens with: iocache= atomically $ newTVar 0 `debug` "newTVar" main= do v<- val v<- val print v getChar val = do cache<- iocache atomically $! do x <- readTVar cache writeTVar cache (x+1) return x here i used a IO TVar global instead of a STM TVar This other version , that passes TVar as parameter, does work: iocache= atomically $ newTVar 0 `debug` "newTVar" main= do cache<-iocache v<- val cache v<- val cache print v val cache= atomically $! do x <- readTVar cache writeTVar cache (x+1) I need the global version since the cache acces may be in any location on the program.... Anyone know a solution using STM? The MVar version works OK, but blocking the entire index is not the best solution....

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RE: Template Haskell Person/Paper entry
by Simon Peyton-Jones 11 May '06

11 May '06

Can anyone help Christoph with uploading a Postscript file to a haskellwiki page? Thanks | -----Original Message----- | From: Ch. A. Herrmann [mailto:herrmann@uni-passau.de] | Sent: 11 May 2006 14:29 | To: Ian Lynagh | Cc: Simon Peyton-Jones | Subject: Re: Template Haskell Person/Paper entry | | Hi, | | I was able to edit | | http://haskell.org/haskellwiki/Template_Haskell | | and add a paragraph to it, but there are still two problems. | | (1) When I try to upload a Postscript file (597453 bytes) I get | the error message "file contains no data" |

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Porting GHC to OSX86?
by Deling Ren 10 May '06

10 May '06

Hi there, Has anyone made any attempt to port GHC to Mac OS X on x86? Wolfgang Thaller’s binary package runs over Rosetta but slow (not surprising). It can not be used to compile a native version either (I got some errors related to machine registers). I tried to do a bootstrap but can't find the ".HC" files mentioned in the manual. They don't seem to be on the download page of GHC. Any ideas? Thanks. Deling

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Re: [Haskell] FP-style vs. OO-style AST structure
by Robert Dockins 10 May '06

10 May '06

[moved to haskell-cafe] On Wednesday 10 May 2006 11:09 am, Doug Kirk wrote: > Hi, > > I'm a Haskell newbie, but not new to programming, and I have a > question regarding style (I think). > > I'm writing a parser for OMG's OCL, and have two ways of defining the > AST model of a constraint. Each constraint in OCL has the following 4 > characteristics: > > 1. name :: Maybe String > 2. context :: UmlElement > 3. expr :: OclExpression > 4. type :: OclConstraintType > > Now, having come from an O-O background, this looks right; however, in > an FPL, it may not be. OclConstraintType is essentially an enumeration > of the values: > > Invariant > > | Precondition > | Postcondition > | InitialValue > | Derivation > | Body > > The question is this: is it better to create a single type as above > with a 'type' attribute, or would it be better to use the types as > separate constructors of a Constraint, each constructor taking the > same attributes? Opinions may differ on this, bit I'm going to go out on a limb and say that using separate constructors with the same field names is the more idiomatic approach in Haskell. Having a product type or a big record with an enumerated tag field is usually just a way to simulate sum types. Why do that if the language supports them directly? > I'm looking to avoid any pitfalls that could occur with either > decision, and at this point I don't know the benefits of doing it one > way vs. the other. So long as every constraint has exactly the same fields, there isn't much practical difference. If later you discover that different kinds of constraints need different fields, you'll be better off with the separate constructors. > BTW, I am using UUST Parser Combinator AG for the parser definition, > in case that makes a difference. > > Thanks! > --doug

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GHC 6.4.1 x86-64 does not compile
by Dusan Kolar 09 May '06

09 May '06

Hello all, I've install "universal binary" for x86_64 of GHC 6.4.1. The installation was done on AMD dual core machine. Uname for the machine gives: Linux <machine name> 2.6.16.5 #1 SMP Thu Apr 13 09:08:22 CEST 2006 x86_64 x86_64 x86_64 GNU/Linux While ghci was running some tests well, the ghc ended compilation with a long list of errors starting like this: Chasing modules from: queen-main Compiling QueensDK ( ./QueensDK.hs, ./QueensDK.o ) /tmp/ghc27286.s: Assembler messages: /tmp/ghc27286.s:26: Error: bad register name `%r12' /tmp/ghc27286.s:27: Error: bad register name `%r15' /tmp/ghc27286.s:29: Error: bad register name `%r12)' /tmp/ghc27286.s:30: Error: bad register name `%r12)' /tmp/ghc27286.s:31: Error: bad register name `%r12)' /tmp/ghc27286.s:32: Error: bad register name `%rax' /tmp/ghc27286.s:33: Error: bad register name `%r13)' /tmp/ghc27286.s:34: Error: bad register name `%rbp' /tmp/ghc27286.s:37: Error: bad register name `%rbx)' /tmp/ghc27286.s:38: Error: bad register name `%rbx)' /tmp/ghc27286.s:45: Error: bad register name `%rbp)' /tmp/ghc27286.s:46: Error: bad register name `%r14' /tmp/ghc27286.s:48: Error: bad register name `%rbp)' /tmp/ghc27286.s:49: Error: bad register name `%r13' ... The list is quite long. I've truncated it. Well, OK, the x386 version is running, but it won't compile GHC-6.4.2 and, moreover, it's not optimized for the HW. ;-) Is there any way out, or what may I be doing wrong way? Regards, Dusan

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Predicate Library?
by Neil Mitchell 08 May '06

08 May '06

Hi, I'm trying to manipulate predicates in Haskell, and was wondering if there was a predicate library available that I could use? My predicates consist of `and` and `or` over some constraints. I have various rules which can be used to collapse certain constraints. Currently I have written my own predicate code, but its not great, and the simplification is overly hacky. The main facility I want out of the predicate library is some simplification. Thanks Neil

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Partial Derivatives
by Gerhard Navratil 08 May '06

08 May '06

Hi, I need a library that provides partial derivatives for functions. The solution I came up with is based on a datatype using reversed polish notation to store the function: Type VarName = String data Fkt a = Val a | Var VarName | Add (FktElem a) (FktElem a) | Sub (FktElem a) (FktElem a) | Mul (FktElem a) (FktElem a) | ... I can then use pattern matching to compute the derivative: derivative :: VarName -> Fkt b -> Fkt b derivative name (Var n) = if n == name then Val 1.0 else Val 0.0 derivative name (Add a b) | (containsVar name a) && (containsVar name b) = Add (derivative name a) (derivative name b) | (containsVar name a) && (not(containsVar name b)) = (derivative name a) | (not(containsVar name a)) && (containsVar name b) = (derivative name b) | otherwise = Val 0.0 derivative name (Sub a b) | (containsVar name a) && (containsVar name b) = Sub (derivative name a) (derivative name b) | (containsVar name a) && (not(containsVar name b)) = (derivative name a) | (not(containsVar name a)) && (containsVar name b) = (derivative name b) | otherwise = Val 0.0 derivative name (Mul a b) | (containsVar name a) && (containsVar name b) = Add (Mul (derivative name a) b) (Mul a (derivative name b)) | (containsVar name a) && (not(containsVar name b)) = Mul (derivative name a) b | (not(containsVar name a)) && (containsVar name b) = Mul a (derivative name b) | otherwise = Val 0.0 ... Where the function containsVar :: VarName -> Fkt a -> Bool tests if a function contains the variable. The solution works but is not very elegant. The complete module is appended to the mail. Does anyone have a more elegant solution or is there a package that provides derivatives in a similar way? Thank you, Gerhard ======================================== Gerhard Navratil Teaching- And Research-Assistant Technical University Vienna, Austria Institute of Geoinformation and Cartography Gusshausstr. 27-29, 1040 Vienna Tel.: ++43 (0) 1 / 58 801 - 12712 Fax.: ++43 (0) 1 / 58 801 - 12799 Cel.: ++43 (0) 699 / 197 44 761 http://www.geoinfo.tuwien.ac.at

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Re: [Haskell] how to 'accumulate' values efficiently (time and space) ?
by dons＠cse.unsw.edu.au 08 May '06

08 May '06

noteed: > Hi all, > > the problem is simple but i can't do it : > > I want to generate some values and 'accumulate' them. The meaning of > 'accumulating' is not so important. The only point is that to > 'accumulate' the values, I only have to know one value and the > accumulator (and not all the values). > > The most simple example is that of adding a list of numbers. > > I thought that I could use a haskell list to represent the values > so I could use foldr or mapM to do the accumulation (and not an > imperative-style loop). > > Below are four attempts to solve the problem. It's not solved : in > ghci, I have '*** Exception: stack overflow' with 'test1 5000000'. > > 5000000 is not so much for my application. > > How can I achieve what I want (and efficiently)? > ---------------------------------------------------------------------} > > import Control.Monad.State > import Control.Monad.ST > import Data.STRef > > acc1, acc2, acc3 :: [Int] -> Int > > ---------------------------------------------------------------------- > > acc1 ints = foldr (+) 0 ints Strictify the accumulator? Prelude List Data.List> foldl1 (+) (replicate 5000000 1) *** Exception: stack overflow versus Prelude List Data.List> foldl1' (+) (replicate 5000000 1) 5000000 (Redirected to haskell-cafe@) -- Don

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throwing sugar into the void.
by Marc A. Ziegert 07 May '06

07 May '06

Hi@all! I'd like to hear some comments about the following ideas; maybe they are someway obsolete or even useless nonsense. A few days ago, I thought about abstracting the instance of an object away, like used in Foreign.Storable.sizeOf::(Storable a)=>a->Int, where only the type of an object is used. The problem with the function sizeOf is, that the result should be constant per definition(*), but how can we make the compiler know this? (*) <http://www.haskell.org/ghc/docs/latest/html/libraries/base/Foreign-Storable…> > sizeOf::a->Int > "Computes the storage requirements (in bytes) of the argument. The value of the argument is not used." So I thought we may need a new (abstract) datatype to wrap the type of an object, just to contain no dynamic data: data Type a typeOf :: a -> Type a typeOf = undefined #define TYPE(a) (undefined::Type (a)) ... sizeOf :: (Storable a) => Type a -> Int Maybe this is a little bit too ugly and cumbersomely, just to make sizeOf constant. This version of sizeOf is not really comfortable anymore; but it isn't comfortable to write "(undefined::a)", too. To be able to write "(::a)" instead, would be really nice, IMHO; but how about "(_::a)"? ---------- <trying to be equivocating> undefined is another way to implement ...nothing. </trying to be equivocating> More comfortable would it be to write a single symbol instead of the long word "undefined": _ :: a _ = undefined Using "_" as a function should not be such a problem: it cannot be mixed with the joker "_" as parameter. To define this function may be a problem: It needs a compiler-patch to allow this, ATM. ---------- By the way, this symbol is not used at type-level... How about praefix/postfix operators instead of only infix? -- praefix newtype BigLambda a = BigLambda a (/\) :: _ -> a -> BigLambda a /\ a = BigLambda a -- postfix newtype Lifted a = Lifted a (!^) :: a -> _ -> Lifted a a !^ = Lifted a May this be confusing? Or even impossible? Am I insane? Well, I don't know how hard it would be to implement those language features. I would not like to make anyone implement all this, if it is only a niceToHave-butIDontUseIt, like the implicit parameters feature. - marc

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Haddock seems to generate wrong types in newtype deriving
by Brian Hulley 06 May '06

06 May '06

Hi - I have the following code: data MState = MState -- details omitted type MonadStateMState = MonadState MState -- necessary for Haddock newtype ManagerM a = ManagerM (StateT MState IO a) deriving (Monad, MonadIO, MonadStateMState) which means that ManagerM is an instance of Monad, MonadIO, and MonadState MState. However, the Haddock docs look like: data ManagerM a Instances ??? a => Monad (ManagerM a) ??? a => MonadIO (ManagerM a) ??? a => MonadStateMState (ManagerM a) which doesn't seem at all right to me. I'd have thought it should say: data ManagerM a Instances Monad ManagerM MonadIO ManagerM MonadStateMState ManagerM Is this just a bug in Haddock or am I misunderstanding something about Haskell? Thanks, Brian.

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