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What's wrong with my Haskell code?
by Meihui Fan 13 Apr '04

13 Apr '04

when loading the following code, hugs escaped and reported that ERROR "cal24.hs":10 - Instance of Integral (Ratio Integer) required for definition of Main.eval I don't know why and how to solve it, anyone help me? data ETree = Add ETree ETree | Sub ETree ETree | Mul ETree ETree | Div ETree ETree | Node Integer deriving Show eval :: ETree->Maybe Rational eval (Node x) = Just (fromInteger x) eval (Add t1 t2) = do { x<-eval t1; y<-eval t2; if x>=y then return (x+y) else Nothing } eval (Sub t1 t2) = do { x<-eval t1; y<-eval t2; return (x-y) } eval (Mul t1 t2) = do { x<-eval t1; y<-eval t2; if x>=y then return (x*y) else Nothing } eval (Div t1 t2) = do { x<-eval t1; y<-eval t2; if y/=0 then return (x `div` y) else Nothing }

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RE: [Haskell-cafe] Building Haddock on Windows
by Simon Marlow 13 Apr '04

13 Apr '04

Yes, I suspect you have a mismatch between the version of Haddock on your system and the version of Haddock used to produce the interfaces. Try upgrading GHC. Cheers, Simon > -----Original Message----- > From: Bayley, Alistair [mailto:Alistair_Bayley@ldn.invesco.com] > Sent: 05 April 2004 16:19 > To: Simon Marlow > Cc: haskell-cafe(a)haskell.org > Subject: RE: [Haskell-cafe] Building Haddock on Windows > > Trying to invoke Haddock with > --read-interface=c:\ghc\ghc-6.0\doc\html\base\base.haddock > > Initially I get a stack overflow, so I added: +RTS -K2M -RTS ... > > and now I get: > Fail: end of file > Action: Data.Binary.getWord8 > > Is the --read-interface option specified correctly? This is > where my GHC > installation is, and the base.haddock file exists in there. I > assume this is > the interface file. > > Is this a compatibility problem? i.e. haddock file > distributed with GHC 6.0 > too old for Haddock 0.6? > > > > -----Original Message----- > > From: Simon Marlow [mailto:simonmar@microsoft.com] > > Sent: 17 March 2004 15:29 > > To: Bayley, Alistair > > Cc: haskell-cafe(a)haskell.org > > Subject: RE: [Haskell-cafe] Building Haddock on Windows > > > > > > > - why doesn't it know what Int, String, Float, IO, Monad, > > > Show, etc are? > > > What invocation option do I need to ensure it links to > > > Prelude/hierarchical-library stuff? (I think it's > > > --read-interface; must I > > > generate interface files for the Prelude and libraries though?) > > > > You get the interfaces for the libraries with a GHC > > installation. Just > > use the --read-interface flag to tell Haddock about them. > > > > Cheers, > > Simon > > ----------------------------------------- > ***************************************************************** > Confidentiality Note: The information contained in this > message, and any attachments, may contain confidential > and/or privileged material. It is intended solely for the > person(s) or entity to which it is addressed. Any review, > retransmission, dissemination, or taking of any action in > reliance upon this information by persons or entities other > than the intended recipient(s) is prohibited. If you received > this in error, please contact the sender and delete the > material from any computer. > ***************************************************************** > >

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handling read exceptions
by S. Alexander Jacobson 13 Apr '04

13 Apr '04

I want to read strings that look like "2" or "hello" into values of type Integer or String. The problem is that read requires that strings be read as "\"hello\"". Is there a way either to convince read to not require wrapping quotation marks or, alternetively, to catch a read exception, and do something sane? -Alex- _________________________________________________________________ S. Alexander Jacobson mailto:me@alexjacobson.com tel:917-770-6565 http://alexjacobson.com

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[Haskell-cafe] A useful trick for inclusion and exclusion of commented code
by Sunil KOTHARI 12 Apr '04

12 Apr '04

Here's a little trick that I often use for inclusion and exclusion of a chunk of code. Imagine we have a function foo defined below. The three lines of code can be included or excluded just by a changing a single character --- that's the trick. So, the idea is to comment our code in a slightly different way like this to exclude the code from executing: 1 {- 2 myand :: Bool -> Bool -> Bool 3 myand a b = if a == b then True 4 else False 5 --} Notice how line 5 is commented '--}' instead of '-}'. Now, if we change the first character in line 11 from '{' to '-' the entire chunk of code from line 12-14 no longer remains comment. 11 -- 12 myand :: Bool -> Bool -> Bool 13 myand a b = if a == b then True 14 else False 15 --} This trick can also be applied to C++ comments. Best wishes, Sunil

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Re: [Haskell-cafe] Newbie: Is it possible to catch _|_ ?
by Jon Cast 10 Apr '04

10 Apr '04

Russ Lewis <spamhole-2001-07-16(a)deming-os.org> wrote: > Another newbie question: Is it possible to catch _|_ - that is, to > encounter it, handle it and continue? I'm assuming that you all are > going to say no, since I can't figure out any way to do this and retain > the functional nature of Haskell. This isn't possible in a deterministic language, for several reasons. The IO monad, however, is non-deterministic, and its `catch' function can be used to catch any _|_ value that can be caught, specifically those arising from calls to throw, error, and certain infinite loops. It is non-deterministic, though, so it won't catch all _|_s, nor will it give any guarantee as to which _|_ it will catch (for example, in error 3 + throw (DynException (toDynamic False)) it's indeterminate whether error's return value or throw's return value will ultimately be caught. Jon Cast

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Re: [Haskell-cafe] How to implement `amb' operator?
by Andy Moran 07 Apr '04

07 Apr '04

[ Was sent only to participants, but I meant to send it to the list ] On Wednesday 07 April 2004 06:04 am, Brandon Michael Moore wrote: > Keith is talking about a "comitted choice" style of nondeterminism, where > one of the arguments is picked and the computation continues from there. > > If you want a computation with backtracking, or a list of all possibly > results then you should use the list monad, or another monad that > supports nondetermanism. An important feature of amb is known as "bottom-avoidance", and you can get that with Keith's scheme (although it won't be terribly fair; see mailing list discussions about the fairness or lack thereof of the GHC RTS scheduler), but you can't get it with backtracking (indeed, backtracking is entirely deterministic). Of course, Keith's scheme forces you to work within the IO monad, as well it should. If we allowed amb to have the type amb :: a -> a -> a then all of the transformations carried out within GHC's mid-stage would need to be re-examined for their correctness (in the context of a semantics that understood sharing, as non-determinism makes sharing observable). It's possible that many would still be valid, but it's by no means clear. To my knowledge, no-one has developed a correct semantics for sharing and amb that enables proof of simple transformations like let-floating or lambda-floating or even call-by-need beta reduction, let alone some of the more complex ones like deforestation (which requires a theory that can handle recursion well). Cheers, Andy -- Andy Moran Ph. (503) 626 6616, x113 Galois Connections Inc. Fax. (503) 350 0833 12725 SW Millikan Way, Suite #290 http://www.galois.com Beaverton, OR 97005 moran(a)galois.com

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RE: [Haskell-cafe] How to implement `amb' operator?
by Bayley, Alistair 07 Apr '04

07 Apr '04

Not wanting to appear glib, but... SICP 4.3.3 Implementing the Amb Evaluator http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-28.html#%_sec_4.3.3 Ralf Hinze (2000): Deriving Backtracking Monad Transformers http://www.informatik.uni-bonn.de/~ralf/publications.html#P12 > -----Original Message----- > From: Meihui Fan [mailto:mhfan@ustc.edu] > Sent: 07 April 2004 21:20 > To: haskell-cafe(a)haskell.org > Subject: [Haskell-cafe] How to implement `amb' operator? > > > Hi, > I think it's good idea to compute non-deterministic > problems with the `amb' > operator, just as in LISP/scheme. But how to implement it in haskell? > > noclouds ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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How to implement `amb' operator?
by Meihui Fan 07 Apr '04

07 Apr '04

Hi, I think it's good idea to compute non-deterministic problems with the `amb' operator, just as in LISP/scheme. But how to implement it in haskell? noclouds

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Newbie: Is it possible to catch _|_ ?
by Russ Lewis 07 Apr '04

07 Apr '04

Another newbie question: Is it possible to catch _|_ - that is, to encounter it, handle it and continue? I'm assuming that you all are going to say no, since I can't figure out any way to do this and retain the functional nature of Haskell.

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Data structure for filesystem representation
by Gerd M 06 Apr '04

06 Apr '04

Hello! For an university project i need to design a data structure (in haskell) that's capable of representing a unix filesystem and therefore fast insertion/deletion. Support for persistent storage is also required. Principally i was thinking to use a Binary Search Tree or an AVL Tree but since this is my first real world application in Haskell I'm not so sure if this is a good (performant) solution. Any advice is welcome! Regards, Gerd. _________________________________________________________________ MSN 8 with e-mail virus protection service: 2 months FREE* http://join.msn.com/?page=features/virus

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