December 2005 - Haskell-Cafe

RE: [Haskell-cafe] Re: TChan implementation: Why TVarList
by Simon Peyton-Jones 06 Dec '05

06 Dec '05

sounds as if you need a priority queue, so you can say "give me the message with the earliest time" but otherwise yes. Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Joel | Reymont | Sent: 06 December 2005 11:46 | To: Simon Peyton-Jones | Cc: Haskell Cafe | Subject: [Haskell-cafe] Re: TChan implementation: Why TVarList | | Well, I meant more like TVar [a] but I see that you are pulling from | the front and appending to the rear. | | I need to implement a mailbox where messages can be pulled out based | on a predicate or in order of arrival. I'm thinking of using a Map | keyed on ClockTime. | | Do you have any suggestions? | | Thanks, Joel | | On Dec 6, 2005, at 11:31 AM, Simon Peyton-Jones wrote: | | > The mutable cell is in the tail. A [TVar a] would be quite different. | > You can read about a very similar impl (based on MVars) in the | > original | > "Concurrent Haskell" paper (on my papers page) | | -- | http://wagerlabs.com/ | | | | | | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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TChan implementation: Why TVarList
by Joel Reymont 06 Dec '05

06 Dec '05

Simon, Why did you guys implement TChan on top of your own TVarList instead of a regular list? -- | 'TChan' is an abstract type representing an unbounded FIFO channel. data TChan a = TChan (TVar (TVarList a)) (TVar (TVarList a)) type TVarList a = TVar (TList a) data TList a = TNil | TCons a (TVarList a) Thanks, Joel -- http://wagerlabs.com/

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Signal 2 and -threaded
by Joel Reymont 06 Dec '05

06 Dec '05

I'm seeing some funky behavior in my program compiled with -threaded. It runs for a little while and then catches signal 2 and quits. This does not happen when -threaded is not used. I'm compiling my library with -threaded so that is always constant. The above applies to my main module and another module that is part of it. Any ideas? I decided to compile with -threaded because otherwise killThread seems to block when a thread is in a foreign call. Thanks, Joel -- http://wagerlabs.com/

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RE: [Haskell-cafe] GADT question
by Simon Peyton-Jones 06 Dec '05

06 Dec '05

| 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 It looks as if you want a kind of subtyping relationship, so that Tau < Rho < Sigma. The standard way to achieve that is using polymorphism (see, for example, papers about FFI to object-oriented languages) data Sigma a = S a data Rho a = R a data Tau = Tau data Type a where | ForAll :: [Id] -> Type (Sigma (Rho a)) -> Type (Sigma b) | Fun :: Type a -> Type a -> Type a | TyCon :: TyCon -> Type (Sigma (Rho Tau)) | TyVar :: TyVar -> Type (Sigma (Rho Tau)) I'm not sure if this would work. Simon

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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 05 Dec '05

05 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 05 Dec '05

05 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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