March 2003 - Haskell - Haskell.org

RE: flock and sockets
by Simon Marlow 20 Mar '03

20 Mar '03

> I am planning a Haskell project and I need to access files. Since the > program will be automatically started whenever a mail comes > in I will need > to be able to lock the access to files. Is there any support > for this in some library? Yes, the Posix library distributed with GHC has support for file locking. See http://www.haskell.org/ghc/docs/latest/html/hslibs/sec-posix.html This library is currently undergoing a rewrite - the functionality will be available under System.Posix in future releases of GHC (and possibly Hugs & NHC). > The second option that I have is to use a daemon and let the > programs that > get started by mail connect via some sort of socket. Is there > any socket support in Haskell? Yes: http://www.haskell.org/ghc/docs/latest/html/network/index.html (currently only available with GHC). Cheers, Simon

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Web server code in Haskell by Simon Marlow?
by Ahn Ki-yung 20 Mar '03

20 Mar '03

I am very interested in writing server programs in haskell. There is a link to a paper and code of haskell web server in the following page. http://www.haskell.org/practice.html I can see the case study paper, but there is no code at the link location. I think inspecting through code might be very helpful, even if it does not compiling in the recent version of GHC. Please notice me where I can see the code if it is available. Also I hope the link at the haskell hompage be updated as well :-) Thanks. -- Ahn Ki-yung

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History of Haskell, first draft
by Steffen Mazanek 19 Mar '03

19 Mar '03

Hello, a first draft of my little Haskell-history collection is online now: http://pseiko.gmxhome.de/pseiko/Haskell-History.html Please note, that one part is still just copied from http://www.idt.mdh.se/kurser/cd5100/ht02/history.html but I hope it will evolve quickly. Moreover some parts are still missing. So please understand it as a framework and base where all kinds of additional contributions are pluggable. A big thank you to all helpers. Ciao, Steffen

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RE: recursive modules in Haskell
by Simon Peyton-Jones 19 Mar '03

19 Mar '03

| i am curious however, what is difficult about implementing recursive | modules (that is, if it can be explained without getting into the | technical details of GHC). Nothing deep. GHC is just a fairly big thing and one of its assumptions is that it is compiling one module at a time. There'd be quite a bit of chuffing around to remove this assumption. Nothing fundamental, but real work. Simon

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RE: recursive modules in Haskell
by Simon Peyton-Jones 19 Mar '03

19 Mar '03

| there is no need to use such hacks. it is not dificult to add suport | for mutually recursive modules to an implementation directly. | unfortunatley none of the working haskell implementations support | recursive modules, Simple in principle, not so simple in practice. If it was easy to make GHC do mutually recursive modules we'd do it right away. It's certainly possible but it's real work, and .hi-boot files, while clunky, do the job. S

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flock and sockets
by Daniel Luna 19 Mar '03

19 Mar '03

I am planning a Haskell project and I need to access files. Since the program will be automatically started whenever a mail comes in I will need to be able to lock the access to files. Is there any support for this in some library? The second option that I have is to use a daemon and let the programs that get started by mail connect via some sort of socket. Is there any socket support in Haskell? #Luna -- Daniel Luna | Top reasons that I have a beard: luna(a)update.uu.se | a) Laziness. http://www.update.uu.se/~luna/ | b) I can. Don't look at my homepage (it stinks).| c) I can get away with it.

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data vs. newtype, abstractly
by Dean Herington 19 Mar '03

19 Mar '03

If a type that could be defined either by `data` or `newtype` (i.e., a single-constructor type) is exported abstractly (without its constructor), can the user of the type tell how the type was declared? Dean

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Context not part of a function definition?
by Graham Klyne 18 Mar '03

18 Mar '03

I'm trying to use the following idiom to selectively import functions from the List module: import qualified List nub = List.nub but I'm finding that HUGS complains about "unresolved top level overloading" with "outstanding context: "Eq b". If I duplicate the type signature thus: import qualified List nub :: Eq a => [a] -> [a] nub = List.nub All seems to be well. I find it counter-intuitive that I cannot simply define a = b such that any occurrence of a is equivalent to an occurrence of b. I thought that was the point of referential transparency in functional languages. I don't know where to look in the Haskell documents to decide whether the above is a bug or truly according to the language spec. #g ------------------- Graham Klyne <GK(a)NineByNine.org> PGP: 0FAA 69FF C083 000B A2E9 A131 01B9 1C7A DBCA CB5E

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Question about scope of 'let' and 'where'
by Graham Klyne 18 Mar '03

18 Mar '03

In the function body (rhs): let { a = (e1) } in (e2) where { b = f a } Does the Haskell specification indicate that the definition of 'a' is in-scope for the definition of 'b'? Practical experience using HUGS suggests the answer is no, but my intuition is that the answer should be yes. I was unable to find anything in either the report or the "gentle introduction" that made the correct answer clear to me (which is not to say it's not there, just that I didn't find it). I think the tutorial might benefit from a discussion of what is in-scope for where clauses. <comment> FWIW, my intuition was that e where defs was a form of expression (like let ... in), in which some sub-expressions were factored out as subsidiary definitions; i.e. that I'd expect to be able to replace each occurrence of a name defined by 'where' with the body of the corresponding definition. Hence I'd expect the let definitions to be in-scope. I now see that use of 'where' is restricted to specific contexts. I wonder if such restriction is needed? The differences between let and where in Haskell are something I find to be confusing. </comment> #g ------------------- Graham Klyne <GK(a)NineByNine.org> PGP: 0FAA 69FF C083 000B A2E9 A131 01B9 1C7A DBCA CB5E

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Haskell History
by Steffen Mazanek 17 Mar '03

17 Mar '03

Hello, I want to create a source of historic information to delight the Haskell community and therefore I need a lot of material relating the following topics: - from lambda calculus to functional programming - other roots of Haskell (e.g. Gofer, ...) - genesis of the well known and beloved standard Haskell98 This website should act as an archive of old releases of popular Haskell interpreters and compilers, too (seemingly there is nothing online at present). Sometimes you need an old release of a particular software, e.g. for comparisons and you become really bored with following quit a lot of dead links in the internet, wasting time and staying without results. Help me to close this existing gap. All suggestions are welcome. Steffen Mazanek

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