- Haskell-Cafe - Haskell.org

STM and `orElse` on a few thousand TMVars
by Joel Reymont 06 Dec '05

06 Dec '05

Is it efficient to wait on a few thousand TMVars? How would I write something like that, assuming that my TMVars were in a list. Thanks, Joel -- http://wagerlabs.com/

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RE: [Haskell-cafe] Re: :t main
by Scherrer, Chad 06 Dec '05

06 Dec '05

> From: Cale Gibbard [mailto:cgibbard@gmail.com] > See: > http://haskell.org/pipermail/haskell-cafe/2003-January/003794.html > > The OI comonad as previously envisioned breaks referential > transparency. I/O just doesn't seem to be something which one > can easily do comonadically, since once coeval/extract is > applied, you're back to plain values, and there's no > imposition of sequencing. > > - Cale Hmm, I hadn't seen that. The asymmetry is pretty frustrating in that case. After poking around a bit more, I (re)discovered some discussion of this on Hawiki: http://www.haskell.org/hawiki/CoMonad After reading Dave Menendez's comments, I'm wondering... If we consider IO a = Realworld -> (Realworld, a) then wouldn't we dually have something like OI a = (Realworld, Realworld -> a)? Could this be what screws things up? Right now it seems like OI is acting like it has the same type as IO, with a different name. Chad Scherrer Computational Mathematics Group Pacific Northwest National Laboratory "Time flies like an arrow; fruit flies like a banana." -- Groucho Marx

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Re: [Haskell-cafe] Learning Haskell
by Donn Cave 06 Dec '05

06 Dec '05

Quoth Jimmie Houchin <jhouchin(a)cableone.net>: ... | Haskell looks like a very interesting language. I am only so-so with | Python and I thought that maybe if instead of spending sufficient time | to get proficient with Python, I could invest a similar time (more or | less) and get reasonably (pragmatically speaking) proficient with | Haskell. I know I may never understand the theory and maths behind the | design, but I believe I can appreciate the design and be a decent user | of the tools it provides. I'm coming from a similar background, Python early adopter (well, they do have something in common! Just something sort of trivial.) and no great math background. I find Haskell MUCH more mentally rigorous. I don't think that's just because I have previously been living in "imperative" land, it's because Haskell gets far more abstract. I have lately been thinking I might see if I can achieve a clear understanding of StateT. Not in any theoretical or mathematical sense, just want to understand how it goes together where honestly right now it seems kind of like magic. Not complaining, there could be worse things than magic like that, but I'm just saying our mathematical ignorance doesn't excuse us from some mental heavy lifting that doesn't compare to anything in Python. Then there are performance issues with Haskell that don't come up in other languages. I have tried to do things like analyze big text files with Haskell and given up, because I could not figure out how to get within an order of magnitude of reasonable completion time. I still think it's possible, though maybe not with standard String type, but anyway it's something to look out for. | I am also hoping to use it for web development. Wonder how `links' is coming along. (Hope they figured out a better name, anyway.) Donn

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GADT question
by oleg＠pobox.com 06 Dec '05

06 Dec '05

John Meacham wrote: > data Type = ForAll [TyVar] Rho -- Forall type > | Fun Type Type -- Function type > | TyCon TyCon -- Type constants > | TyVar TyVar -- Always bound by a ForAll > with the three synonyms and their constraints being > type Sigma = Type > type Rho = Type -- No top-level ForAll > type Tau = Type -- No ForAlls anywhere > I would like the type system to distinguish the three types and enforce > their properties. Perhaps the enclosed code might be of help. Tests: test1 = TyCon "Int" *Forall> :t test1 test1 :: Type FANil test2 = Forall ["a"] (Fun (TyVar "a") (TyVar "a")) *Forall> :t test2 test2 :: Type FATop -- Inferred type! test3 = Fun test2 test1 *Forall> :t test3 test3 :: Type FASomewhere test3' = Fun test1 test1 *Forall> :t test3' test3' :: Type FANil test4 = Forall ["b"] test3 *Forall> :t test4 test4 :: Type FATop test4' = Forall ["b"] test2 -- Error! No instance for (RhoC FATop) arising from use of `Forall' at /tmp/m.hs:43:9-14 That is, in test4, the second argument of Forall is not of the type rho! Typeclasses would work too. I'm lacking a good example to show off this machinery... {-# OPTIONS -fglasgow-exts #-} module Forall where data FANil data FATop data FASomewhere type Id = String type Tau = Type FANil data Type a where Forall :: (RhoC a) => [Id] -> Type a -> Type FATop Fun :: (FAMerge a b c) => Type a -> Type b -> Type c TyCon :: Id -> Type FANil TyVar :: Id -> Type FANil type Sigma a = Type a --type (RhoC a) => Rho a = Type a -- I wish that worked... class RhoC a instance RhoC FANil instance RhoC FASomewhere class FAMerge a b c | a b -> c instance FAMerge FANil FANil FANil instance FAMerge FANil FASomewhere FASomewhere instance FAMerge FANil FATop FASomewhere instance FAMerge FATop a FASomewhere instance FAMerge FASomewhere a FASomewhere test1 = TyCon "Int" test2 = Forall ["a"] (Fun (TyVar "a") (TyVar "a")) test3 = Fun test2 test1 test3' = Fun test1 test1 -- I guess we need to hide the data constructors of Type and use -- functions like rho, fun, tycon, etc. to construct the values of -- Type rho :: (RhoC a) => Type a -> Type a rho = id test4 = Forall ["b"] test3 -- The following is error! --test4' = Forall ["b"] test2

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GADT question
by John Meacham 06 Dec '05

06 Dec '05

I have been trying to learn GADTs but feel I don't quite grok them yet. I am playing with an implementation of SPJ's algorithm from http://research.microsoft.com/Users/simonpj/papers/higher-rank/index.htm in it, he has a data type that represents a type, but with several special cases that can't easily be expressed in haskell proper. I was thinking GADTs might help but havn't been able to come up with something good and was hoping someone could point me in the right direction or let me know what I am trying to do is impossible/improbable. right now, it looks like so data Type = ForAll [TyVar] Rho -- Forall type | Fun Type Type -- Function type | TyCon TyCon -- Type constants | TyVar TyVar -- Always bound by a ForAll with the three synonyms and their constraints being type Sigma = Type type Rho = Type -- No top-level ForAll type Tau = Type -- No ForAlls anywhere I would like the type system to distinguish the three types and enforce their properties. so far, I have come up with the following: data Sigma data Tau data Rho data Type a where ForAll :: [Id] -> Type Rho -> Type Sigma Fun :: Type a -> Type a -> Type a TyCon :: TyCon -> Type Tau TyVar :: TyVar -> Type Tau but this is obviously not right, it can't create Rho types at all, Fun should change a Sigma into a Rho but leave Tau the same, Fun should be able to be applied to different types infering the correct one. and a function that accepts a Rho should be able to accept a Tau... (though explicit upconversions would be okay, meaning I need to create TyCons for example with type 'Type Rho' in the upconverter) anyone have any ideas on how to make this work? is this even possible to express with GADTs? John -- John Meacham - ⑆repetae.net⑆john⑈

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RE: [Haskell-cafe] Re: Invalid object in isRetainer
by Simon Marlow 05 Dec '05

05 Dec '05

On 05 December 2005 15:29, Joel Reymont wrote: > So how do you profile for memory leaks then? Ordinary heap profiling (-hc) should work fine with STM. The more exotic kinds of heap profiling haven't been updated yet, so unfortunately they're broken with STM right now. Cheers, Simon

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RE: Invalid object in isRetainer
by Simon Marlow 05 Dec '05

05 Dec '05

On 04 December 2005 17:02, Joel Reymont wrote: > ./randomplay +RTS -hr > ... <- some time passes and then > > randomplay: internal error: Invalid object in isRetainer(): 67 > Please report this as a bug to glasgow-haskell-bugs(a)haskell.org, > or http://www.sourceforge.net/projects/ghc/ > > The combination of this and the failure with -hc -hbdrag,void is > preventing me from optimizing the program and it has a huge space > leak somewhere. Retainer profiling (-hr) doesn't currently work with STM. It is a bug, but I briefly looked into it and it's not a trivial fix so I left it. I'll file a ticket for it. Cheers, Simon

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RE: Wierd activation record found on stack
by Simon Marlow 05 Dec '05

05 Dec '05

On 04 December 2005 16:50, Joel Reymont wrote: > To add a little bit of detail (thanks to dons for the tips), the > signal is being caught in one of the threads launched by the main > thread. Simply calling exitWith (ExitFailure 1) from this thread > causes the error below. I changed my code to send messages to running > threads asking them to quit and this made the error go away. > > On Dec 4, 2005, at 12:27 PM, Joel Reymont wrote: > >> randomplay: internal error: scavenge_stack: weird activation record >> found on stack: 0 Please report this as a bug to >> glasgow-haskell-bugs(a)haskell.org, or >> http://www.sourceforge.net/projects/ghc/ >> >> I installed my own signal handlers with the intent of having my >> profile reports written out. The program runs forever and otherwise >> generates no profile reports when I interrupt it. This was fixed recently, I believe: revision 1.276 date: 2005-11-08 10:44:22 +0000; author: simonmar; state: Exp; lines: +1 -2 raiseAsync: fix bug that can cause a scavenge_stack panic for a thread that has just been killed. The fix will be in 6.4.2, or you can grab a STABLE snapshot from the haskell.org. Cheers, Simon

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RE: [Haskell-cafe] STM, orElse and timed read from a channel
by Simon Peyton-Jones 05 Dec '05

05 Dec '05

| > I think you are really asking for some way to create top-level | > transactional variables? | | Yes. Perhaps it would be easier to do it cleanly than arbitrary | top-level IO bindings? It turns out to be easy to provide newTVarIO :: a -> IO (TVar a) which you can call from inside 'unsafePerformIO'. That means you can allocate top-level TVars without fuss. The whole question of top-level IO remains open, but this will address the immediate question very nicely. I've also added a SourceForge bug to remind us to fail more tidily if you do atomically inside atomically. Simon

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RE: [Haskell-cafe] STM, orElse and timed read from a channel
by Simon Peyton-Jones 05 Dec '05

05 Dec '05

Can you provide a small test case that demonstrates the crash? I think it's pretty unlikely that we'll support nested atomically (via unsafePerformIO) anytime soon. UnsafePerformIO is, well, unsafe. I think you are really asking for some way to create top-level transactional variables? Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of | Tomasz Zielonka | Sent: 04 December 2005 18:32 | To: Simon Marlow | Cc: Haskell Cafe | Subject: Re: [Haskell-cafe] STM, orElse and timed read from a channel | | On Sat, Dec 03, 2005 at 10:35:54PM +0100, Tomasz Zielonka wrote: | > so I could export a waitUntil function with type | > | > waitUntil :: Time -> STM () | > | > but I tripped on something that was reported before, namely that STM | > transactions can't be nested (as a result of unsafePerformIO or | > unsafeInterleaveIO). Is there a plan to support such scenario? | | OK, it can be worked around by running the atomically block in a new | transaction. But still it would be nice if the program didn't segfault, | but cause an exception for example. | | Updated modules attached. | | Best regards | Tomasz | | -- | I am searching for a programmer who is good at least in some of | [Haskell, ML, C++, Linux, FreeBSD, math] for work in Warsaw, Poland

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