- Haskell-Cafe - Haskell.org

Functional Dependencies Help
by John Creighton 01 May '10

01 May '10

I've been trying to apply some stuff I learned about functional dependencies, but I run into one of two problems. I either end up with inconsistent dependencies (OverlappingInstances doesn't seem to apply) or I end up with infinite recursion. I want to be able to do simple things like if a is a subset of b and b is a subset of c then a is a subset of c. If a is a is a subset of b and b is a c then a is a c. Before I added the equality functions I had infinite recursion. Once I put them them in then I have trouble with overlapping isntances. {-# LANGUAGE TypeOperators #-} {-# LANGUAGE OverlappingInstances #-} {-# LANGUAGE UndecidableInstances #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE FunctionalDependencies #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE FlexibleContexts #-} data Noun = Noun deriving (Show) --10 data Verb = Verb deriving (Show) -- data Adjactive = Adjactive deriving (Show) data Animal=Animal deriving (Show) data Feline=Feline deriving (Show) data Cat = Cat deriving (Show) data Taby_Cat=Taby_Cat deriving (Show) data T=T deriving (Show) data F=F deriving (Show) --20 class Isa a b c | a b->c where isa::a->b->c instance Isa Animal Noun T where isa a b = T --25 class IsSuperSet a b c | a b->c where isSuperSet :: a->b->c instance IsSuperSet Feline Cat T where --30 isSuperSet a b=T instance IsSuperSet Animal Feline T where isSuperSet a b=T instance IsSuperSet a Animal F where isSuperSet a b=F --35 class TypeNotEq d b c | d b->c where typeNotEq :: a->b->c instance (IsSuperSet d b c, --40 IsSuperSet a d c, TypeNotEq a d T, TypeNotEq b d T, TypeEq c T T )=> IsSuperSet a b c where isSuperSet a b=undefined::c instance TypeNotEq a a c where typeNotEq a b = undefined::c --50 instance TypeNotEq a b c where typeNotEq a b = undefined::c class TypeEq a b c | a b->c where typeEq :: a->b->c instance TypeEq a a c where typeEq a b = undefined::c instance TypeEq a b c where typeEq a b = undefined::c class ToBool a where toBool :: a->Bool instance ToBool T where toBool a = True instance ToBool F where toBool a = False myCat=Cat bla=isSuperSet Animal Cat

1 3

Asynchronous exception wormholes kill modularity
by Bas van Dijk 01 May '10

01 May '10

Dear all, (sorry for this long mail) When programming in the IO monad you have to be careful about asynchronous exceptions. These nasty little worms can be thrown to you at any point in your IO computation. You have to be extra careful when doing, what must be, an atomic transaction like: do old <- takeMVar m new <- f old `onException` putMVar m old putMVar m new If an asynchronous exception is thrown to you right after you have taken your MVar the putMVar will not be executed anymore and will leave your MVar in the empty state. This can possibly lead to dead-lock. The standard solution for this is to use a function like modifyMVar_: modifyMVar_ :: MVar a -> (a -> IO a) -> IO () modifyMVar_ m io = block $ do a <- takeMVar m a' <- unblock (io a) `onException` putMVar m a putMVar m a' As you can see this will first block asynchronous exceptions before taking the MVar. It is usually better to be in the blocked state as short as possible to ensure that asynchronous exceptions can be handled as soon as possible. This is why modifyMVar_ unblocks the the inner (io a). However now comes the problem I would like to talk about. What if I want to use modifyMVar_ as part of a bigger atomic transaction. As in: block $ do ... modifyMVar_ m f ... >From a quick glanse at this code it looks like asynchronous exceptions can't be thrown to this transaction because we block them. However the unblock in modifyMVar_ opens an asynchronous exception "wormhole" right into our blocked computation. This destroys modularity. Besides modifyMVar_ the following functions suffer the same problem: * Control.Exception.finally/bracket/bracketOnError * Control.Concurrent.MVar.withMVar/modifyMVar_/modifyMVar * Foreign.Marshal.Pool.withPool We can solve it by introducing two handy functions 'blockedApply' and 'blockedApply2' and wrapping each of the operations in them: > import Control.Exception > import Control.Concurrent.MVar > import Foreign.Marshal.Pool > import GHC.IO ( catchAny ) > blockedApply :: IO a -> (IO a -> IO b) -> IO b > blockedApply a f = do > b <- blocked > if b > then f a > else block $ f $ unblock a > blockedApply2 :: (c -> IO a) -> ((c -> IO a) -> IO b) -> IO b > blockedApply2 g f = do > b <- blocked > if b > then f g > else block $ f $ unblock . g Control.Exception: > finally :: IO a -> IO b -> IO a > a `finally` sequel = blockedApply a $ \a' -> do > r <- a' `onException` sequel > _ <- sequel > return r > bracket :: IO a-> (a -> IO b) -> (a -> IO c) -> IO c > bracket before after thing = blockedApply2 thing $ \thing' -> do > a <- before > r <- thing' a `onException` after a > _ <- after a > return r > bracketOnError :: IO a -> (a -> IO b) -> (a -> IO c) -> IO c > bracketOnError before after thing = blockedApply2 thing $ \thing' -> do > a <- before > thing' a `onException` after a Control.Concurrent.MVar: > withMVar :: MVar a -> (a -> IO b) -> IO b > withMVar m io = blockedApply2 io $ \io' -> do > a <- takeMVar m > b <- io' a `onException` putMVar m a > putMVar m a > return b > modifyMVar_ :: MVar a -> (a -> IO a) -> IO () > modifyMVar_ m io = blockedApply2 io $ \io' -> do > a <- takeMVar m > a' <- io' a `onException` putMVar m a > putMVar m a' > modifyMVar :: MVar a -> (a -> IO (a,b)) -> IO b > modifyMVar m io = blockedApply2 io $ \io' -> do > a <- takeMVar m > (a',b) <- io' a `onException` putMVar m a > putMVar m a' > return b Foreign.Marshal.Pool: > withPool :: (Pool -> IO b) -> IO b > withPool act = blockedApply2 act $ \act' -> do > pool <- newPool > val <- catchAny > (act' pool) > (\e -> do freePool pool; throw e) > freePool pool > return val I'm not proposing to make this change (yet) because I first would like to have some discussion on this. Thanks for reading this rather long mail, Bas

13 46

Location of library documentation with 6.12.1?
by Roly Perera 01 May '10

01 May '10

Hi, I've just upgraded from 6.10.1 to 6.12.1 and can't seem to find all the library documentation. This page is apparently the documentation root: http://www.haskell.org/ghc/docs/6.12.1/html/ but if I click through to the Haskell Hierarchical Libraries page http://www.haskell.org/ghc/docs/6.12.1/html/libraries/index.html I can't for example find Control.Monad.State. I guess I'm missing something obvious about how things are organised? thanks in advance, Roly

4 6

Typing Haskell in Haskell
by Aaron Gray 01 May '10

01 May '10

Hi, I am relatively new to Haskell. I am attempting to get "Typing Haskell in Haskell" to work on HUGS or GHC. http://web.cecs.pdx.edu/~mpj/thih/ I am getting an error on loading SourcePrelude :- Hugs> :l SourcePrelude ERROR ".\PPrint.hs" - Can't find imported module "Pretty" And I cannot find a Pretty module, the module and "Language.Haskell.Pretty" does not seem to be what is required. Any help welcome, Aaron

3 2

profiling uniplate
by Ozgur Akgun 01 May '10

01 May '10

Hi all, I have a code which makes use of the uniplate package. When I try to compile with profiling enabled, I get the following error: $$ ghc --make -O2 -prof -auto-all test.hs test.hs:10:7: Could not find module `Data.Generics.Uniplate.Data': Perhaps you haven't installed the profiling libraries for package `uniplate-1.5.1'? Use -v to see a list of the files searched for. And of course I do have uniplate installed, and everything works fine if I don't compile with profiling enabled. I guess I either need to install profiling libraries for uniplate, or disable profiling of those functions coming from uniplate. Any suggestion is welcome. Best, -- Ozgur Akgun

3 3

Fwd: [Haskell-cafe] Re: Simple game: a monad for each player
by Limestraël 01 May '10

01 May '10

Okay, I just understood that 'Prompt' was just a sort of view for 'Program'. >I'd like to make it very accessible, so please don't hesitate to report >any difficulties with finding and understanding documentation and examples! Then I think the name 'Prompt' may be misleading for those who doesn't know the MonadPrompt package. Maybe something like 'ProgramView' ? >> What have we achieved? Both players still can only access functions from >> whatever monad m turns out to be. But now each strategy can pile its own >> custom monad stack on the Player m monad! And of course, the use of >> the m Monad is completely optional. > >Of course, the custom monad stack has to provide a projection back to >the Player m a type > > runMyStackT :: MyStackT (Player m) a -> Player m a According to what Bertram said, "each strategy can pile its own custom monad stack ON the (Player m) monad". Here, you are stacking the (Player m) monad ON the custom monad stack. What is then the use of the 'm', in (Player *m*)? Is it not supposed to be a custom monad? (MonadIO for human, Identity for AI, etc.) But then, I don't see how the game function could work: > game :: Monad m => Player m () -> Player m () -> m () As it is written, it requires both players to run in the SAME monad. And if have a network player ( e.g.* Player (StateT Handle IO)* ) and an AI storing former opponent's moves ( e.g. *(Monad m) => Player (StateT [Move] m)* ), then they can't be in the same monad... * * 2010/4/14 Heinrich Apfelmus <apfelmus(a)quantentunnel.de> > Limestraël wrote: > > > I have some difficulties to see the use of PromptT, because in the > tutorial, > > this type is never mentioned, and its operations (Return and :>>=) are > > instead constructors of ProgramT... > > > > Would you have some concrete examples? Because there I'm a bit lost > (since > > the tutorial doesn't match the operational package as it is, because of > the > > type PromptT)... > > The project page > > http://projects.haskell.org/operational/ > > links to documentation that describes the differences to "The > Operational Monad Tutorial", in particular the new Prompt and PromptT > types. It also links to several examples. Two small examples are also > included in the Haddock documentation. > > > I'd like to make it very accessible, so please don't hesitate to report > any difficulties with finding and understanding documentation and examples! > > > Regards, > Heinrich Apfelmus > > -- > http://apfelmus.nfshost.com > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

3 9

Writing C libraries in Haskell
by Richard G. 01 May '10

01 May '10

Hello I have a few questions about using Haskell (GHC, in particular) to write self-contained libraries with a C interface. ---- If libraries foo and bar are compiled using the same version of GHC, is is possible to link the two libraries into the same executable? Does anything special need to be done? ---- If library foo was compiled using GHC 6.10.4 and library bar was compiled using 6.12.1, is it possible to link the two libraries into the same executable? ---- I'm using the following code to initialize the GHC runtime: hs_init(&argc, &argv); hs_add_root(__stginit_CabalProjectzm0zi0zi2zi0_SimpleModule); where SimpleModule is a module that exports functions via FFI. Is it safe to add more modules after I call a function exported by this module? ---- Also, once hs_exit() has been called, is it possible to reinitialize the GHC runtime? Thanks, Richard

2 1

Writing Mac OS X Plug-Ins in Haskell
by Richard G. 01 May '10

01 May '10

Hello I'm trying to write self-contained Mac OS X plug-ins (http://developer.apple.com/mac/library/documentation/CoreFoundation/Concept…) using GHC. I want the plug-in to be usable on a clean system (i.e., GHC and XCode are not installed). Does anyone have experience with this or does anyone know of some resources on this? I've gotten something working with GHC 6.10.4 and XCode but I found that it requires a lot of work to find and link all of the necessary libraries and the result in not portable. Is there a simple way to take care of all of the dependencies? Thanks, Richard

1 0

How to make way into a hadoop infrastructure
by C K Kashyap 01 May '10

01 May '10

Dear Haskellers, A big part of my new job requires tuning app's on Hadoop. I was wondering if there is a way to push some Haskell code in the mix. I did some googling on "Hadoop/Haskell" and came across Holumbus - but looks like that is parallel to Hadoop. I was thinking in the lines of doing a Haskell implementation that could run in a Hadoop cluster - has anyone tried anything like that? -- Regards, Kashyap

2 2

why does Data.Text.Lazy.IO.readFile return the internal type Data.Text.Lazy.Internal.Text, when Data.Text.IO.readFile returns plain IO Data.Text.Text?
by Thomas Hartman 30 Apr '10

30 Apr '10

*Main> :t Data.Text.IO.readFile Data.Text.IO.readFile :: FilePath -> IO T.Text but *Main> :t Data.Text.Lazy.IO.readFile Data.Text.Lazy.IO.readFile :: FilePath -> IO text-0.7.1.0:Data.Text.Lazy.Internal.Text why does the lazy version use the internal type, whereas the strict version of Text IO just using plain Data.Text type? and how can I get from internal type to regular type when using Data.Text? also the internal type doesn't appear to be reflected in the haddock: http://hackage.haskell.org/packages/archive/text/0.7.1.0/doc/html/Data-Text… ghc-pkg list | grep -i text text-0.7.1.0 thanks for any help!

5 9