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Question abt Num
by Pratik Bhadra 23 Oct '03

23 Oct '03

Hi I have a data type Expr for handling Expressions data Expr = Lit1 Int | Lit2 Bool | Var String | BinOp Op Expr Expr As you can see, I am trying to have an Expression have both Int and Bool values so that I can work with both arithmetic (+,-,*,/) and logical(and,or,<,<=,>,>=) operators. What I am aiming at is to parse a line which contains an expression, evaluate it and return the result. I am facing some problems in this regard. When I write the evaluate function, the Haskell compiler refuses to compile my code as it says, instance of Num Expr required. Any ideas what I am supposed to do to fix that? My evaluate code is as follows... evaluate :: Expr -> Store -> Expr evaluate ( Lit1 n ) st = n evaluate ( Lit2 n ) st = n evaluate ( Var v ) st = value st v -- returns the value of v evaluate ( BinOp op e1 e2 ) st = opValue op v1 v2 where v1 = evaluate e1 st v2 = evaluate e2 st opValue :: Op -> Expr -> Expr -> Expr opValue Add e1 e2 = e1 + e2 opValue Sub e1 e2 = e1 - e2 opValue Mul e1 e2 = e1 * e2 opValue Div e1 e2 = e1 / e2 opValue LT e1 e2 = e1 < e2 opValue LE e1 e2 = e1 <= e2 opValue GT e1 e2 = e1 > e2 opValue GE e1 e2 = e1 >= e2 opValue And e1 e2 = e1 && e2 opValue Or e1 e2 = e1 || e2 Now I can't return an Int as I return a Bool at times and hence I return an Expr. What does the error...instance of Num Expr or instance of Fractional Expr required mean? Thanks in advance! Pratik -------------------------------- Pratik Bhadra Undergraduate Section Leader The University of Texas at Austin College of Natural Sciences Junior BS Computer Sciences ---------------------------------

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Enum on Float/Double
by George Russell 23 Oct '03

23 Oct '03

The situation with Enum on Ratio is pretty bad but at least it's not hopeless, since rational numbers are at least exact. But for Float/Double it seems to be a total disaster area. Since it seems to be impossible to abolish Enum Float/Double altogether, would it at least be possible to make it rather more useful, by making succ x return the next representable float after x (or x if x is non-finite) and pred x the previous representable float. This would (a) be useful; (b) make mathematical sense; (c) is tricky to implement with the existing primitives. Before someone else points it out, this is of course unimplementable should someone in the future implement infinite-precision reals in Haskell, but as (barring a solution to the halting problem) Eq would be as well, that does not bother me. My preference would be for succ (+-0) to return the smallest positive real, since then you could define succ x to be the unique y with x < y and forall z . z < y => not (x < z), where such a y exists, and I'm not sure if the Haskell standard knows about signed zeros. Feri wrote > Everybody working with floats should know their quirks While in an ideal world, everyone who represented a mouse position or anything else as a float should have a PhD in numerical analysis, in practice they do not. Indeed I think the Haskell Library Report contains quite a few examples of floating point code which a numerical analyst would have written rather better. So I don't think it's good enough to treat every Float/Double operation as if it had an implicit "UNSAFE" flag indicating that the compiler was entitled to wierd behaviour only comprehensible to experts, as seems to be the case with floating enumerations now.

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Re: Question abt num
by Pratik Bhadra 23 Oct '03

23 Oct '03

Thanks, Artie and thanks to Dr Richards! I figured it out. I was not correlating the Exprs properly. Now that I broke it up into Lit1 and Lit2 and am returning a Lit1 or a Lit2 which will return an Expr, it works! :) Pratik -------------------------------- Pratik Bhadra Undergraduate Section Leader The University of Texas at Austin College of Natural Sciences Junior BS Computer Sciences ---------------------------------

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Graduate Student
by htrego＠lancasterlabs.com 22 Oct '03

22 Oct '03

I am a graduate student who is interested in Haskell. I wanted to take a look at the Haskell Wish List but I get the error Warning: MySQL Connection Failed: Can't connect to MySQL server on 'marutea.pms.informatik.uni-muenchen.de' (113) in /usr/local/apache/pms/htdocs/forschung/haskell-wish-list/items.php3 on line 5 Is there anyone who has a copy of the Wish List? Holly Trego Senior Computer Specialist CONFIDENTIAL MATERIAL: This message is intended only for the use of the individual or entity to which it is addressed and may contain information that is privileged, confidential, and exempt from disclosure under applicable law. If received in error, please notify sender by return e-mail and destroy all copies of the original transmission and any attachments. Thank you. If you wish to view information about Lancaster Laboratories, Inc., please visit our website at www.lancasterlabs.com

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RE: Graduate Student
by Simon Peyton-Jones 22 Oct '03

22 Oct '03

So far as GHC is concerned, we're now keeping our "wish list" in SourceForge ,under "Tasks" http://sourceforge.net/pm/?group_id=8032 We've not been very assiduous in maintaining these lists, though. If more people used them, we might be more so. Did you have anything in mind? What sort of project are you looking for? Simon | -----Original Message----- | From: haskell-bounces(a)haskell.org [mailto:haskell-bounces@haskell.org] On Behalf Of | htrego(a)lancasterlabs.com | Sent: 20 October 2003 18:50 | To: haskell(a)haskell.org | Subject: Graduate Student | | I am a graduate student who is interested in Haskell. I wanted to take a | look at the Haskell Wish List but I get the error | | Warning: MySQL Connection Failed: Can't connect to MySQL server on | 'marutea.pms.informatik.uni-muenchen.de' (113) in | /usr/local/apache/pms/htdocs/forschung/haskell-wish-list/items.php3 on line | 5 | | Is there anyone who has a copy of the Wish List? | | Holly Trego | Senior Computer Specialist | | | | CONFIDENTIAL MATERIAL: This message is intended only for the use of the | individual or entity to which it is addressed and may contain information | that is privileged, confidential, and exempt from disclosure under | applicable law. If received in error, please notify sender by return e-mail | and destroy all copies of the original transmission and any attachments. | Thank you. If you wish to view information about Lancaster Laboratories, | Inc., please visit our website at www.lancasterlabs.com | _______________________________________________ | Haskell mailing list | Haskell(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell

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ANNOUNCE: HToolkit 1.2 released
by Krasimir Angelov 21 Oct '03

21 Oct '03

Dear Haskellers, I am pleased to announce HToolkit 1.2 for Haskell. The HToolkit is a platform independent package for Graphical User Interface. The package is split into two libraries GIO and Port. The Port is a low-level Haskell 98+FFI compatible API, while GIO is a high-level user friendly interface to Port. The current release supports all Windows based platforms and Linux/Unix platforms with GTK/GNOME toolkit. There are many new features and many bugfixes. The 1.2 release is a source incompatible with 1.0 release. Home page ~~~~~~~~~ http://htoolkit.sourceforge.net Incompatible changes: ~~~~~~~~~~~~~~~~~~~~~ 1. There is a new API for creating: - radio menu items (MenuRadioItem) - check boxes (CheckBox) - radio buttons (RadioBox) In the new API each widget is created idependently from the other widgets and after that they are linked together to form a group of related items. 2. The 'drawInWidget' function is renamed to 'paintIn' and the 'drawInBitmap' is renamed to 'paintInBitmap'. 3. The 'dialogFont' function is removed. Both windows and dialogs now use the same font (returned from the 'defaultFont' function). 4. The 'defaultFont', 'defaultFontDef', 'dialogColor', 'windowColor' and 'textColor' functions now return values which are system dependent. The returned values are given from the current theme selected in Windows/GNOME. 5. The 'itemCount' attribute of Menu is removed. There is a new 'count' attribute which is defined in the type class and instances are defined for Popup, ListBox, Notebook, Menu and ToolBar types. 6. The 'visible' attribute of Entry is renamed to 'password'. 7. The windows and the dialogs are created invisible by default and can be shown with the 'showWindow' or 'runDialog' (for dialogs) functions. New features ~~~~~~~~~~~~ 1. The implementation for the ToolBar is completed. Unfortunatelly in this release the toolbars for Linux don't work. The reason is that the CVS version of Port uses GTK 2.3. 2. Each window and dialog can be shown/hidden with the 'showWindow' and 'hideWindow' functions or by changing the value of the new attribute 'visible'. The attribute is read/write and its current value determines whether the window is visible. The windows and the dialogs are created invisible by default and can be shown with the 'showWindow' or 'runDialog' (for dialogs) functions. 4. In HToolkit 1.0 when the dialog size becomes smaller than the minimal size required to layout the controls, the dialog automatically shows scrollers. In HToolkit 1.2 the dialogs cannot be shrinked to a size smaller than the minimal. Under Windows a size gripper is drawn at the righthand bottom corner of the resizable dialogs just like the modern resizable dialogs under Windows. 5. The controls are created invisible by default and are shown only if they are included in the layout expression of their parent. In this way it is possible to dynamically hide/show and rearrange the controls. 6.Three new controls are added: Notebook, GroupBox and CompoundControl. The CompoundControl is a container for other controls with automatic scrollbars. The control supports 'mouse', 'keyboard' and 'paint' events just like the toplevel windows and dialogs. The primary purpose of CompoundControl is to create new custom controls. 7. The layout algorithm is extended with 'stretch', 'hstretch' and 'vstretch' functions in addition to 'fill', 'hfill' and 'vfill'. The new functions stretch only the specified control to fit the entire space without affecting the rest controls in the table. Cheers, Krasimir __________________________________ Do you Yahoo!? The New Yahoo! Shopping - with improved product search http://shopping.yahoo.com

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Re: Calling an external command from a Haskell program
by C.Reinke 21 Oct '03

21 Oct '03

> The function system works fine in Hugs except on windows where DOS limitations > cause the function to always return ExitSuccess. (ghc suffers from the same > problem on Windows.) Actually, that is not quite correct: ghc seems to suffer from this problem only on older Windows versions (such as Windows 98), whereas Hugs seems to have the bug also on newer Windows versions (such as XP), at least in its current binary release.. I reported the ghc/win98 problem earlier, below is what happens on Windows XP. So the bug seems even worse in Hugs than in GHC? Claus PS. why the differences in default access to standard modules? should GHC be more restrictive there? --------------------------------------------------- ___ ___ _ / _ \ /\ /\/ __(_) / /_\// /_/ / / | | GHC Interactive, version 6.0.1, for Haskell 98. / /_\\/ __ / /___| | http://www.haskell.org/ghc/ \____/\/ /_/\____/|_| Type :? for help. Loading package base ... linking ... done. Prelude> System.system "fail" >>= print 'fail' is not recognized as an internal or external command, operable program or batch file. ExitFailure 1 Prelude> --------------------------------------------------- || || || || || || ||__ Hugs 98: Based on the Haskell 98 standard ||___|| ||__|| ||__|| __|| Copyright (c) 1994-2002 ||---|| ___|| World Wide Web: http://haskell.org/hugs || || Report bugs to: hugs-bugs(a)haskell.org || || Version: Nov 2002 _________________________________________ Haskell 98 mode: Restart with command line option -98 to enable extensions Reading file "C:\Program Files\Hugs98\lib\Prelude.hs": Hugs session for: C:\Program Files\Hugs98\lib\Prelude.hs Type :? for help Prelude> :a System Reading file "C:\Program Files\Hugs98\lib\System.hs": Hugs session for: C:\Program Files\Hugs98\lib\Prelude.hs C:\Program Files\Hugs98\lib\System.hs System> system "fail" >>= print 'fail' is not recognized as an internal or external command, operable program or batch file. ExitSuccess

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RE: About Haskell Thread Model
by Simon Marlow 21 Oct '03

21 Oct '03

> On Mon, Oct 13, 2003 at 11:13:56AM +0200, Wolfgang Thaller wrote: > > The reason why we currently do not take advantage of SMP is > that the > > Haskell Heap is a shared data structure which is modified > whenever a > > thunk (an unevaluated expression) is evaluated. Using > synchronisation > > primitives like pthread_mutex_lock for every evaluation of a thunk > > would be deadly for performance. > > There is some old (1999) code in the GHC RTS that attempts > this (using > > intel cmpxchg instructions for synchronisation), but it's currently > > "bitrotted" and I don't know how successful it was. > > cmpxchg and then taking a blocking lock sounds like the 2-tier locking > supported with Linux' new futex system calls. I wonder how they chose > to block in the older GHC RTS. It was a spinlock. There was also an attempt to avoid having to lock every thunk entry and update by partitioning the allocation area per-thread, taking advantage of the fact that most thunk entries and updates are to objects created recently by the current thread. This is about where the experiment stalled - it was interesting, but even if the heap partitioning turned out to be effective, the gains weren't really going to be there without tackling multithreaded GC too (as Jan-Willem already pointed out). Cheers, Simon

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Re: Calling an external command from a Haskell program
by C.Reinke 21 Oct '03

21 Oct '03

> If you want access to its I/O streams as well, you can use > Posix.popen, which is not standard Haskell 98, I think, but it's > in GHC. Even worse, it is not portable (AFAICT)! I'm worried by the tendency towards Posix, at a time when, e.g., GHC by default no longer supports this on Windows (went missing in the move from cygwin to mingw). Some also thought Posix overkill, and wanted a portable replacement for the most-commonly-wanted functionality subset.. Could someone in the know please summarise the state of Posix-support in Haskell implementations in general? I've cc-ed to the libraries list - perhaps someone there could comment on the state of portable replacements for Posix functionality, especially for popen and friends? I seem to recall a very promising discussion about portable and flexible ways to start subprocesses a while ago - has that led to any concrete libraries? Cheers, Claus

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Calling an external command from a Haskell program
by Salvador Lucas 21 Oct '03

21 Oct '03

Dear all, I think this question is not new, but I am not able to find the answer. How to proceed to execute an external Linux command (e.g., 'ls') from a Haskell program? Thanks in advance, Salvador.

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