January 2003 - Haskell-Cafe

``Cannot instantiate a type variable with a forall-type''
by Jon Cast 30 Jan '03

30 Jan '03

Why does GHC place this constraint? I would expect forall to be predicative, and a type variable to range over all types, but obviously I'm missing something. Jon Cast

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List comprehensions
by Rijk J. C. van Haaften 30 Jan '03

30 Jan '03

Hello, Recently, I came accross this expression: [ x + y | x <- xs | y <- ys ] As far as I can see (Haskell Report), this is not allowed by the haskell 98 standard. So I assume it to be an ex- tension. Where can I find information about this? Thanks, Rijk

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Problem with backtracking monad transformer
by Guest, Simon 30 Jan '03

30 Jan '03

I'm trying to make a backtracking state monad using Ralf Hinze's backtracking monad transformer. My problem is that it won't backtrack very far. Suppose I try ( a >> b ) `mplus` c. If b fails, it should try c, but it doesn't rewind past a. My sample code is below. GHCI> c [0,1] match_1 -- (1 or 0) then 1, OK GHCI> c [1,0] match_2 -- (1 then 0) or (1,1), OK GHCI> c [1,1] match_2 -- (1 then 0) or (1,1), fails What have I misunderstood? cheers, Simon (A disclaimer in an attachment? - it wasn't my idea.) -- backtracking state monad -- requires -fglasgow-exts import qualified Monad as M import qualified Control.Monad.Trans as MT -- turn tracing on and off by uncommenting just one of the following lines import Debug.Trace( trace ) --trace s x = x -- -- Ralf Hinze's efficient backtracking monad transformer -- newtype BACKTR m a = BACKTR { mkBACKTR :: (forall b. (a -> m b -> m b) -> m b -> m b) } instance (Monad m) => Monad (BACKTR m) where return a = BACKTR (\c -> c a) m >>= k = BACKTR (\c -> mkBACKTR m (\a -> mkBACKTR (k a) c)) -- We don't use a Backtr class, but do it with the MonadPlus class, -- mzero is false (fail), -- mplus is ¦ (orelse) instance (Monad m) => M.MonadPlus (BACKTR m) where mzero = BACKTR (\c -> id) m1 `mplus` m2 = BACKTR (\c -> mkBACKTR m1 c . mkBACKTR m2 c) -- standard MonadTrans class has lift for promote, and doesn't have observe instance MT.MonadTrans BACKTR where lift m = BACKTR (\c f -> m >>= \a -> c a f) observe :: (Monad m) => BACKTR m a -> m a observe m = mkBACKTR m (\a f -> return a) (fail "false") -- -- State Monad -- data SM st a = SM (st -> (a,st)) -- The monadic type instance Monad (SM st) where -- defines state propagation SM c1 >>= fc2 = SM (\s0 -> let (r,s1) = c1 s0 SM c2 = fc2 r in c2 s1) return k = SM (\s -> (k,s)) -- extracts the state from the monad readSM :: SM st st readSM = SM (\s -> (s,s)) -- updates the state of the monad updateSM :: (st -> st) -> SM st () -- alters the state updateSM f = SM (\s -> ((), f s)) -- run a computation in the SM monad runSM :: st -> SM st a -> (a,st) runSM s0 (SM c) = c s0 -- backtracking state monad -- type NDSM st a = BACKTR (SM st) a readNDSM :: NDSM st st readNDSM = MT.lift readSM updateNDSM :: (st -> st) -> NDSM st () updateNDSM f = MT.lift (updateSM f) --run a computation in the NDSM monad runNDSM :: st -> NDSM st a -> (a,st) runNDSM s0 m = runSM s0 (observe m) -- -- the state -- type Bit = Int data CState = CState { ok :: Bool, remaining_data :: [Bit], history :: [String] -- log, kept in reverse } deriving Show initState xs = CState True xs [] -- prepend a message in the log logit :: CState -> String -> CState logit s logmsg = s { history = logmsg : (history s) } -- -- matching action -- match_bits :: [Bit] -> NDSM CState () match_bits xs = do s <- readNDSM let s' = logit s ("attempt match_bits " ++ show xs ++ " remaining: " ++ show (remaining_data s)) s'' = if xs == take (length xs) (remaining_data s') then s' { remaining_data = drop (length xs) (remaining_data s') } else s' { ok = False } if ok s'' then updateNDSM (\s -> s'') else trace (unlines $ "MATCH FAILED":(reverse $ history s'')) M.mzero -- -- test routines -- -- just fine match_1 = (match_bits [1] `M.mplus` match_bits [0]) >> match_bits [1] -- this one only rewinds past the [0] attempt, not the [1] attempt match_2 = ( (match_bits [1] >> match_bits [0]) `M.mplus` match_bits [1, 1] ) c :: [Bit] -> NDSM CState () -> ([Bit], [String]) c h hspec = let (v, s) = runNDSM (initState h) hspec in case (ok s) of True -> ([], "ok":(reverse $ history s)) _ -> ([(negate)1], ["fail"])

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RE: List comprehensions
by Jan de Wit 30 Jan '03

30 Jan '03

> Hello, > > Recently, I came accross this > expression: > [ x + y | x <- xs | y <- ys ] > > As far as I can see (Haskell Report), > this is not allowed by the haskell 98 > standard. So I assume it to be an ex- > tension. Where can I find information > about this? This is a parallel list comprehension, a GHC extension. See http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#PA RALLEL-LIST-COMPREHENSIONS for more info. The code above does the same as zipWith (+) xs ys, basically. Cheers, Jan ########################################### This message has been scanned by F-Secure Anti-Virus for Microsoft Exchange. For more information, connect to http://www.F-Secure.com/ ###########################################

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Haskell and professional aspirations.
by guilhermelopes＠netcabo.pt 28 Jan '03

28 Jan '03

Hello there I'm a computer science undergraduate student, and would like to continue (or rather start) working with Haskell after graduation. Which universities welcome postgraduate work using functional programming languages? That is, which groups/departments of which universities should I keep an eye on? Which companies use functional programming for medium-large scale projects? What should be in my curriculum vitae if I wanted to have a good array (hmmm... list!) of offers to choose from? If Haskell is indeed being used professionally, it is responsible for which part of the job? I am aware of some of the issues concerned with "Haskell in the real-world", through haskell.org, and search for an account on the subject more shifted towards personal experience. If you do have experience on working with Haskell, I would love to hear about it. Although this question is essentially personal in nature, I consider it to be (and tried to make it) broad enough to justify its presence here, propped up by Haskell being used around the world for teaching purposes. If you think otherwise, reply directly to my address: tretalhada(a)netcabo.pt . Guilherme Lopes

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RE: Exception handling in GHC - ThrowCatch.hs (1/1)
by Simon Marlow 27 Jan '03

27 Jan '03

> You might be interested in this program to investigate what catches > what. Pinched for GHC's testsuite - thanks very much :) Cheers, Simon

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Creating COM objects and passing out pointers to them via a COM interface
by Sarah Thompson 24 Jan '03

24 Jan '03

Hi again, A slightly harder problem this time. I want to implement something that looks something like the Microsoft XML parser. This is a very COM heavy contraption that provides an interface that looks to client applications like a tree of COM objects that reflects the internal structure of an XML document. The idea is, you start with the document object, make a method call requesting the root node, and have a COM object that wraps that node returned to you. You can then request the children of that node, and their children and siblings, all which involves being passed back interface pointers to COM objects. Effectively, there is one COM object per syntactic item in the XML document. I already have my document object working nicely. This is where the actual database resides. I want to be able to spawn views of this database as COM objects. When I've done this in the past (using ATL mostly, euuuugh), I've used an IDL definition something like HRESULT CreateView([in,string] BSTR query, [out,retval] IDbView **); What's the correct way to implement the necessary Haskell code to create the coclass and return a pointer to it? I fiddled around for ages, but either couldn't get GHC to like the types I was using, or when I finally got it to compile, I found that my test harness was being passed an invalid COM object. This should be simple stuff, so I'm sure someone will have done it! Secondly, it is a nasty, though fairly common trick, if you know your objects are going to be created in the same process space, to allow them some 'behind the scenes' communication. In my case, I'd like to avoid needing to serialise and deserialse the data in a view during creation of these child objects (of which there may be a great many, and which may be created on the fy rather often). Ideally, I'd like to be able to do something like: newView :: String -> State -> IO State newView db (State st) = do db <- readIORef st r <- newIORef db -- Insert some kind of COM magic here return (State r) so the database content gets copied within the Haskell environment (lazily, I'd hope), without needing to be serialised and deserialised. Is this feasible? One (messy) approach might be to create a bunch of extra interfaces on the main database object, then create 'thin wrapper' child objects that simply make calls back into the parent object. I'm not so keen on that for various reasons, however. Sarah

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Exeption handling in GHC
by Sarah Thompson 24 Jan '03

24 Jan '03

Hi all, I'm trying to get exception handling working in GHC, but don't seem to be able to make it work. Am I likely to be missing a compiler switch, or something? Cutting things down to basics, I'd have thought that the following expression (typed at the ghci command prompt) should work: try (error "x") but the result is *** Exception: x I seem to get the same problem with compiled code. Any ideas anyone? TIA, Sarah

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Re: Lazy evaluation alternative
by Kevin S. Millikin 24 Jan '03

24 Jan '03

>>>>> "Chris" == Chris Clearwater <chris(a)sharkzone.com> writes: Chris> Sarcasm, Kevin, sarcasm. :) Why is it that those who most need to point out that they were joking are least able to see it in others? Chris> The difference is with the CPS way it is implicit. For Chris> example take the Y combinator written using both methods Chris> (again this is pseduo code, not intended to be real haskell Chris> code or run in hugs) That's a funny definition of "implicit". All you need to delay evaluation is lambda. You're performing a CPS transformation as a way to insert a lambda in the proper place. BUT THAT'S NOT NECESSARY. Just write the lambda where you want it. You don't need to abstract over a continuation to delay evaluation, you can abstract over anything at all (it doesn't matter what). And you only ever apply the most recent continuation (notice how you could use the identifier "c" for every one of your continuations?). You don't have to apply c to pass a value to the current continuation, there's alread a way to do that: just return the value. Chris> Using the lambda way: Y f = f (lazy Y f) Then writing ones Chris> would be something like: ones_fix f = 1:(force f) Chris> Now the CPS way: Y f c = c (f (Y f)) Now writing ones: Chris> ones_fix f c = c 1:f Your translation is wrong. Here's the correct "lambda way" (in the same pseudo functional language): Y f = \ () -> f (Y f) where I didn't presume any "delay" or "lazy" macro. Here's ones_fix: ones_fix f = \ () -> 1 : f Compare to: Y f = \ c -> c (f (Y f)) ones_fix f = \ c -> c (1 : f) Which one seems to be more "implicit"? Where you abstract over a continuation to delay evaluation, I abstract over anything I feel like. Where you apply a continuation to a value, I just return a value. Where you would pass in a continuation to a CPSed function, I would just apply the result to some suitable token (unless I had zero argument functions) (that is, I would "force" an expression wherever you would pass a continuation to a CPSed function). You've performed a CPS transformation, but you never do anything *interesting* with the continuation. That might suggest that, whatever you seem to have achieved, it didn't come from the CPS transformation. Chris> Notice how the CPS way behaves as if it were actually Chris> passed itself, while the lambda way behaves as if it were Chris> passed itself quoted with lambda. The lambda way seems Chris> like a hack while the CPS way feels consistant. I guess I don't understand. Change "\ c ->" to "\ () ->". Remove the application of every "c". Change occurrences of "c" in argument position to "()". -- Kevin S. Millikin Architecture Technology Corporation Research Scientist Specialists in Computer Architecture (952)829-5864 x. 162 http://www.atcorp.com

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Handling error in production code
by Sarah Thompson 24 Jan '03

24 Jan '03

What is the best way to handle error in production code? I have successfully built a COM object (many thanks Sigbjorn!!!), which works fine. The only hassle is that an error being thrown causes the program using the COM object to hard terminate. This is clearly not desirable, especially since parse errors can cause this. What I'd *ideally* like would be for a call to return an error code as an HRESULT, but leave the internal state of the object unchanged. Is this feasible? Do I need to think carefully about using something other than 'error'? If so, how do I handle library code's use of error here and there? TIA, Sarah

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