August 2003 - Haskell-Cafe

why are you waiting
by a_tatarsky2＠saksinc.com 07 Aug '03

07 Aug '03

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Re: Another typing question
by Jon Cast 06 Aug '03

06 Aug '03

I wrote: > Wolfgang Jeltsch <wolfgang(a)jeltsch.net> wrote: > > On Tuesday, 2003-08-05, 15:22, CEST, Nick Name wrote: > > <snip> > > > > This is called "dependent types" and is not a feature of haskell > > > (nor of any language that I know); there was "cayenne" (try a google > > > search) but I don't believe it is still mantained. > > <snip> > > > > BTW, why is there no general interest for such a thing as dependent > > > types? > > > What negative consequences does their implementation have? > > It depends on whether you just want to write functions to take in > values of such types, or whether you want to write functions to return > values of such types. Doing the former uses dependant products, which > would require a significant extension to GHC's guts, but not (AIUI) > inherently unpleasant. Doing the latter (AIUI) essentially requires > dependant sums. Basically, a family of types A_i parameterized by a > value i :: I is written, for long-honored traditional reasons too > complex to get into here, Sigma i::I.A_i. Now, if you have such > dependant sums, there's a bad interaction with polymorphism: either > you must have a type alpha which cannot appear in a family of types > A_i in a dependant sum, or, for any function f :: alpha -> alpha, > there exists a type beta such that f cannot be aplied at either beta > or Sigma i::Int.beta. Clear? Actually, on second thought, I think this could be simplified to: if we have dependant types, then either there is a type which cannot be used in a dependant type, or for every polymorphic function there is a type it cannot be applied at. Jon Cast

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Re: Another typing question
by Jon Cast 05 Aug '03

05 Aug '03

> On Tuesday, 2003-08-05, 15:22, CEST, Nick Name wrote: <snip> > > This is called "dependent types" and is not a feature of haskell > > (nor of any language that I know); there was "cayenne" (try a google > > search) but I don't believe it is still mantained. <snip> > > BTW, why is there no general interest for such a thing as dependent > > types? > What negative consequences does their implementation have? It depends on whether you just want to write functions to take in values of such types, or whether you want to write functions to return values of such types. Doing the former uses dependant products, which would require a significant extension to GHC's guts, but not (AIUI) inherently unpleasant. Doing the latter (AIUI) essentially requires dependant sums. Basically, a family of types A_i parameterized by a value i :: I is written, for long-honored traditional reasons too complex to get into here, Sigma i::I.A_i. Now, if you have such dependant sums, there's a bad interaction with polymorphism: either you must have a type alpha which cannot appear in a family of types A_i in a dependant sum, or, for any function f :: alpha -> alpha, there exists a type beta such that f cannot be aplied at either beta or Sigma i::Int.beta. Clear? The alternative, btw., is to have both dependant sums and an undecidable type-checking problem. > I think, sometimes they could be quite handy. Sure. <Sarcastic comment censored>. Jon Cast

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Another typing question
by Konrad Hinsen 05 Aug '03

05 Aug '03

Is there any way to parametrize a type by a value, rather than another type? What I would like to do is to define "list of length 3" and "list of length 4" as separate parametrization of the same type, such that I could write functions that accept lists (under my new typename) of any length as long as parameters have the same length. The goal is to remove the need for runtime checks all over the code. Konrad.

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Show with Strings
by Thomas L. Bevan 05 Aug '03

05 Aug '03

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Is there any reason why show :: String -> String is not equivalent to 'id' ? At the moment, show "one" = "\"one\"" which leads to problems because it often requires String to be treated as a special case, rather than just a member of Show. Tom -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.2 (GNU/Linux) iD8DBQE/Li8QYha8TWXIQwoRApCgAJ49iJcOuf88WTIKVJWBkHF/QyugOwCfS2Gu jcJFn48T4uc+ooeSAaCkAhs= =pIel -----END PGP SIGNATURE-----

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Newbie Design Question
by Tanton Gibbs 05 Aug '03

05 Aug '03

Haskellers, I'm currently working on my first Haskell program. I really like the language so far, though it has been hard to break the OO frame of mind. Normally, I'm a C++/Perl programmer, but I have really enjoyed the type-safety of Haskell over Perl as well as the Hugs interpreter over a make cycle. Since execution time is not important, I haven't found any issues with using Hugs. Now, on to my problem. Our company uses XML to describe our processes. I'm writing a DSL in Haskell that will print that XML out. I've done most of it, but now I'm describing the I/O in the XML. Basically, we have files come in in various formats from various file contributors and we use a mapping program to map the fields to the right spots in the XML. So, for example, one contributor could have the following layout: Field Name Start Pos Length First Name, 1, 10 Last Name, 11, 10 and another contributor could have them in exactly the opposite order. Furthermore, some contributors may have fields that others don't (for example: DOB). I'm now trying to come up with a way to map this information into Haskell dynamically. It would be trivial to create a module for each Contributor and then import all the modules. However, we have hundreds of contributors and it would be a pain to have to change and re-release the program every time we added a new contributor. Therefore, I need someway to take this information and map it into my Haskell structures. I have come up with two options. One would be to let Haskell dynamically import a contributor module (but I don't think that is currently possible); the other would be to use a configuration file and have the user specify the information through it. Is there a standard Config file module? Is there any other way to do this that I missed? Thanks in advance, Tanton Gibbs, Ph.D. Technical Architect Abilitec Knowledge Base Acxiom Corporation Conway, AR 72034

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Persistent storage
by Nick Name 04 Aug '03

04 Aug '03

Is there someone who has implemented some limited form of persistency in haskell? I don't mean the longly-debated persistence of functional values, but something rough, like a persistent MVar with a thread saving modified values every n seconds or so. Vincenzo

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Parsec allocating a lot of memory
by Nick Name 04 Aug '03

04 Aug '03

Hi all, I am using "parsec" to parse the output from "xmame -listinfo" wich is a list of records of the form game ( attr1 value1 ... attrN valueN ) and for approx. 3500 records I got ~250 mb of RSS memory during parsing, wich takes 20 seconds on my athlon 1400. I think that I must have done something wrong (this is the first time I use parsec), here is my parser: --- Begin games = many game game = do openGame x <- manyTill attribute closeGame return (mkGameInfo x) attribute = do whitespaces x <- identifier whitespaces y <- tillEOL return (x,y) openGame = do string "game (" newline closeGame = do string ")" newline newline whitespace = "\v\f\t\r " whitespaces = skipMany (oneOf whitespace) tillEOL = manyTill anyChar newline identifier = many alphaNum --- End Thanks for any advice Vincenzo

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PROMOTION
by SANDER SMITH 04 Aug '03

04 Aug '03

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by webmaster 02 Aug '03

02 Aug '03

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