mutable records
Hi, Is it possible to define oject types in Haskell and what does one propose to do it? Is it possible to define parts of a record with the help of the ST s monad mutable during the whole program? (As is possible in Ocamel)? Thx Scott
Re: mutable records
Scott J. [EMAIL PROTECTED] wrote, Is it possible to define oject types in Haskell and what does one propose to do it? Depends on what you mean by object types. You can surely define a record with funcions dubbing as methods and non-functional values dubbing as object data. Is it possible to define parts of a record with the help of the ST s monad mutable during the whole program? (As is possible in Ocamel)? Just make the fields that you want to update be values of `STRef s a'. (You can also do the same with the IO monad and `IORef's.) Having said this, there are not that many situations where you need to do this (and in consequence ST-monadify your program). Purely functional updates (using the record syntax) where the system effectively copies the whole record (not all data in it, just the references to that data) is perfectly suitable in most cases. If you are concerned about efficiency first profile the code to see whether the performance bottleneck is really where you think it is. Cheers, Manuel ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
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Does anyone know what happened to the wiki on haskell.org? There was lots of stuff there last year (mostly to do with design patterns) which seems to have been lost. Some of the links are still there but they don't go anywhere. Cheers Chris
Wiki problems
We're aware of the wiki problems. We've got out wiki guy working on it and I hope everything will be working again soon. We won't lose anything that's been posted to the wiki. Please be patient! John ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: mutable records
Sorry for my late reply, But is it possible to be more specific (little examples) In Haskell one can use existential lists but I doubt about the efficiency. It is not my aim to make of everything an object like in Jave e.g. . The objects I have in mind are showable ojects: windows, scrollbars, messageboxes, etc. . Of such objects I demand fast response with respect to key input or mouse clicks. I am sure Ocamel can do that. Regards Scott - Original Message - From: Manuel M T Chakravarty [EMAIL PROTECTED] To: [EMAIL PROTECTED] Cc: [EMAIL PROTECTED] Sent: Tuesday, September 03, 2002 10:37 AM Subject: Re: mutable records Scott J. [EMAIL PROTECTED] wrote, Is it possible to define oject types in Haskell and what does one propose to do it? Depends on what you mean by object types. You can surely define a record with funcions dubbing as methods and non-functional values dubbing as object data. Is it possible to define parts of a record with the help of the ST s monad mutable during the whole program? (As is possible in Ocamel)? Just make the fields that you want to update be values of `STRef s a'. (You can also do the same with the IO monad and `IORef's.) Having said this, there are not that many situations where you need to do this (and in consequence ST-monadify your program). Purely functional updates (using the record syntax) where the system effectively copies the whole record (not all data in it, just the references to that data) is perfectly suitable in most cases. If you are concerned about efficiency first profile the code to see whether the performance bottleneck is really where you think it is. Cheers, Manuel ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: mutable records
In Haskell one can use existential lists but I doubt about the efficiency. Existential lists don't have any special time overhead. All you're doing is making the typechecker happy about what you're doing. Of course, there's a small overhead in that any function you invoke on that object will almost certainly be an indirect function call rather than a direct function call so you get the same (small) overhead as in C++ virtual function calls, Haskell method calls (when not optimized away), etc. It is not my aim to make of everything an object like in Jave e.g. . The objects I have in mind are showable ojects: windows, scrollbars, messageboxes, etc. . Of such objects I demand fast response with respect to key input or mouse clicks. I am sure Ocamel can do that. We're sure Haskell can too. You can probably translate your OCaml code fairly directly into Haskell but directly translating code which uses mutable state tends to result in unidiomatic Haskell code. If you want nice Haskell code (i.e., which exploits Haskell's strengths, plays well with other libraries, etc.) it's often best to think about the problem you're trying to solve and find a way to express that in Haskell instead of trying to express the standard solution in Haskell. For example, C/C++/Java GUIs are usually implemented by creating a bunch of objects and then connecting the objects together by invoking methods on them to add event handlers, etc. Such programs almost never change the object interconnections after this initialization phase. In effect, we have a bunch of single-assignment variables: they get initialized but never change after that. Haskell has single-assignment variables too (i.e., the normal, non-mutable variables you learn about on page 2 of any Haskell textbook) and you can often use them to express the connections between the objects. Doing this has an enormous benefit that you don't get in C, C++, Java, etc. which is that Haskell's typechecker can catch some of the errors you might make when making these object interconnections. This example probably doesn't apply to your code but it's an example of how you can eliminate gratuitious use of mutable variables from your code. The key is to ask us about the problem you're trying to solve not ask how to express the standard (imperative) solution in Haskell. -- Alastair Reid [EMAIL PROTECTED] Reid Consulting (UK) Limited http://www.reid-consulting-uk.ltd.uk/alastair/ ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
