- Haskell-Cafe - Haskell.org

Pattern match failure, then inconsistent function
by Adam Wyner 25 Aug '05

25 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 24 Aug '05

24 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 24 Aug '05

24 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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RE: [Haskell-cafe] Functional dependencies and type inference
by Simon Peyton-Jones 23 Aug '05

23 Aug '05

Thomas Strange and interesting example! If I had a pound for every time functional dependencies had messed up my brain, I'd be a rich man. Apart from anything else, your example is another case of the "critical example" http://www.haskell.org/pipermail/haskell-cafe/2005-August/010974.html as you mention. It's also jolly strange that GHC infers the (un-writable) type bar :: (forall r. (Dep Int r) => Int -> r) -> Bar -> Char but you can write neither that type nor bar :: (Int -> Bool) -> Bar -> Char so in fact GHC won't type the bogus type-cast. Nevertheless, those are artefacts of GHC's not-yet-right implementation of fundeps. I think the real problem here is that the "newtype-deriving" mechanism isn't conservative enough. Given class C a where { op :: ty } newtype T = MkT S the idea is that if S is an instance of a class C, then T can be, witnessed by the same functions. That amounts to saying that ty[S/a] is interchangeable with ty[T/a] and that isn't really true when fundeps get in on the act. I can't say I really understand it fully, but that's as far as I can take it today. Question: what restrictions on the newtype-deriving mechanism would make it sound? Simon | -----Original Message----- | From: Thomas Jäger [mailto:thjaeger@gmail.com] | Sent: 23 August 2005 02:02 | To: Simon Peyton-Jones | Cc: Einar Karttunen; haskell-cafe(a)haskell.org | Subject: Re: [Haskell-cafe] Functional dependencies and type inference | | Simon, | | I believe there may be some nasty interactions with generalized | newtype-deriving, since we can construct two Leibniz-equal types which | are mapped to different types using fundeps: | | class Foo a where | foo :: forall f. f Int -> f a | | instance Foo Int where | foo = id | | newtype Bar = Bar Int deriving Foo | | -- 'Equality' of Int and Bar | eq :: forall f. f Int -> f Bar | eq = foo | | class Dep a b | a -> b | | instance Dep Int Bool | instance Dep Bar Char | | newtype Baz a = Baz { runBaz :: forall r. Dep a r => a -> r } | | conv :: (forall f. f a -> f b) -> | (forall r. Dep a r => a -> r) -> (forall r. Dep b r => b -> r) | conv f g = runBaz $ f (Baz g) | | bar = conv eq | | Here, after type erasure, 'bar' is the identity function . Ghc infers | | bar :: (forall r. (Dep Int r) => Int -> r) -> Bar -> Char | | This is not valid as an explicit type signature, but presumably the | proposal implies that we could type bar as | | bar :: (Int -> Bool) -> Bar -> Char | | instead. Now | | \x -> bar' (const x) (Bar 0) :: Bool -> Char | | would become an unsafe coercion function from Bool to Char. | | | Thomas | | On 8/11/05, Simon Peyton-Jones <simonpj(a)microsoft.com> wrote: | > Einar | > | > Good question. This is a more subtle form of the same problem as I | > described in my last message. In fact, it's what Martin Sulzmann calls | > "the critical example". Here is a boiled down version, much simpler to | > understand. | > | > module Proxy where | > | > class Dep a b | a -> b | > instance Dep Char Bool | > | > foo :: forall a. a -> (forall b. Dep a b => a -> b) -> Int | > foo x f = error "urk" | > | > bar :: (Char -> Bool) -> Int | > bar g = foo 'c' g | > | > | > You would think this should be legal, since bar is just an instantation | > of foo, with a=Char and b=Bool. But GHC rejects it. Why?

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RE: [Haskell-cafe] Functional dependencies and type inference
by Simon Peyton-Jones 23 Aug '05

23 Aug '05

Einar Good question. This is a more subtle form of the same problem as I described in my last message. In fact, it's what Martin Sulzmann calls "the critical example". Here is a boiled down version, much simpler to understand. module Proxy where class Dep a b | a -> b instance Dep Char Bool foo :: forall a. a -> (forall b. Dep a b => a -> b) -> Int foo x f = error "urk" bar :: (Char -> Bool) -> Int bar g = foo 'c' g You would think this should be legal, since bar is just an instantation of foo, with a=Char and b=Bool. But GHC rejects it. Why? GHC looks at the call to foo (in bar's RHS), and instantiates it with a=Char. That tells it that the second argument should have type (forall b. Dep Char b => Char -> b) But it doesn't! It has type (Char -> Bool). And to GHC these are not the same types. You may say that they *should* be the same types. The crispest way I know to explain why it arguably should be rejected is to try translating the program into System F (which is what GHC actually does). Then the RHS of bar looks like bar (g::Char->Bool) = foo {Char} 'c' (...???...) The {Char} is the type argument to foo. But what can we pass for (...???...)?. We must pass a polymorphic function, with type forall b. Dep Char b -> Char -> b But all we have is g::Char->Bool. And System F has no way to connect the two. Well, that's the problem anyway. I can think of three "solutions": - Reject such programs (GHC's current solution) - Abandon compilation via System F - Extend System F I'm working on the third of these, with Martin, Manuel, and Stephanie. Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of | Einar Karttunen | Sent: 15 July 2005 13:48 | To: haskell-cafe(a)haskell.org | Subject: [Haskell-cafe] Functional dependencies and type inference | | Hello | | I am having problems with GHC infering functional dependencies related | types in a too conservative fashion. | | > class Imp2 a b | a -> b | > instance Imp2 (Foo a) (Wrap a) | > | > | > newtype Wrap a = Wrap { unWrap :: a } | > data Foo a = Foo | > data Proxy (cxt :: * -> *) | > | > foo :: Imp2 (ctx c) d => Proxy ctx -> (forall a b. (Imp2 (ctx a) b) => a -> b) -> c -> d | > foo p f x = f x | | The type of "foo (undefined :: Proxy Foo)" is inferred as | "forall c. (forall a b. (Imp2 (Foo a) b) => a -> b) -> c -> Wrap c" | which shows the outmost functional dependence is working fine. ctx | is carried to the inner Imp2. | | However "foo (undefined :: Proxy Foo) Wrap" will fail complaining that | | Couldn't match the rigid variable `b' against `Wrap a' | `b' is bound by the polymorphic type `forall a b. (Imp2 (ctx a) b) => a -> b' | at <interactive>:1:0-32 | Expected type: a -> b | Inferred type: a -> Wrap a | In the second argument of `foo', namely `Wrap' | | My guess is that GHC cannot see that the functional dependency | guarantees that there are no instances which make the inferred | type invalid. Any solutions to this problem? | | | - Einar Karttunen

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Re: [Haskell-cafe] Transactional in-memory model
by vlcakm＠tiscali.cz 23 Aug '05

23 Aug '05

> would the STM constructs in GHC 6.4 be suitable for your project? > jake Hi Jake, as far as I understand STM is does not exactly match my requirements - it is meant to support concurrent programming. But it is a great reading - thanks for pointing that out to me! I'll get to use it later when I learn more of Haskell in general. vlcak --------------------------------------------------------------------------------- Nokia 3120 - stribrny design a barevny displej jiz od 77 Kc http://ad.tiscali.com/adclick/CID=00027edc0000000000000000/SITE=CZ.TISCALI/…

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RE: Random matrices
by Scherrer, Chad 23 Aug '05

23 Aug '05

> Ah ha. That'll do. > Lesson: avoid hidden space leaks in monads. Hmm, I'm still missing something. It seems a good lesson, but practically speaking, it doesn't help me any more than saying "write efficient programs". What could I have looked for in the original code to predict it may be leaky? Is there a way of thinking about this that can help me stop hitting space leaks like this? BTW, I really do appreciate the help. I've been amazed at the level of effort put forth by the Haskell community as a whole to help out newcomers. -Chad

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RE: [Haskell-cafe] FFI and callbacks
by Simon Marlow 23 Aug '05

23 Aug '05

On 20 July 2005 14:35, John Goerzen wrote: > I'm looking at packaging an event-driven console widget set (CDK) for > Haskell using FFI. I know that other event-driven widget sets have > Haskell bindings, but I'm not quite sure how to make everything play > nice with forkIO. > > These systems generally have some sort of an opaque main loop, > implemented in C. This loop would usually never return, or perhaps > only return once the UI is destroyed. Is the library thread-safe or not? I mean OS-thread safe. If it is, then you're home and dry: just compile your program with GHC's -threaded option, and any foreign calls that need to run concurrently with Haskell threads must be declared "safe" (which is the default in fact, so instead I should really advise you to mark all calls that don't need to run concurrently as "unsafe", because that's good for performance). If the library isn't thread-safe, then you're in the same boat as Gtk and wxHaskell (I believe) where there are known problems with having a multithreaded Haskell GUI app. You can only have one OS thread (and hence only one Haskell thread) doing GUI operations, and that is the thread in the main loop. You have to somehow set up a communication between your other Haskell threads and the thread running the main loop - perhaps you can send requests of type (IO ()) down a channel to the main loop thread which wakes up occasionally to run them, for example. Cheers, Simon

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RE: [Haskell-cafe] Re: Oracle + Haskell advice?
by Bayley, Alistair 22 Aug '05

22 Aug '05

> From: Keean Schupke [mailto:k.schupke@imperial.ac.uk] > > Still I now know that HaskellDB has significant limitations, and the > relational algebra approach I took is far more robust and flexable... > > ... there's no documentation. If anyone were interested in > using/contributing I could give CVS access to the code. Me too. Please. I'm quite interested in your relational algebra approach. 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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