March 2001 - Haskell - Haskell.org

fold and unfold
by Graham Hutton 13 Mar '01

13 Mar '01

Dear all, The following new paper may be of interest to some readers of this group: When is a Function a Fold or an Unfold? Jeremy Gibbons, Graham Hutton, and Thorsten Altenkirch http://www.cs.nott.ac.uk/~gmh/when.pdf http://www.cs.nott.ac.uk/~gmh/when.ps We give a necessary and sufficient condition for when a set-theoretic function can be written using the recursion operator fold, and a dual condition for the recursion operator unfold. The conditions are simple, practically useful, and generic in the underlying datatype. (To appear in the Proceedings of the 4th International Workshop on Coalgebraic Methods in Computer Science, Genova, Italy, April 2001.) Best wishes, Graham +-----------------------------------------------------------------------------+ | Dr Graham Hutton Email : gmh(a)cs.nott.ac.uk | | Languages and Programming Group Web : http://www.cs.nott.ac.uk/~gmh/ | | School of Computer Science and IT | | The University of Nottingham | | Jubilee Campus, Wollaton Road | | Nottingham NG8 1BB | | United Kingdom | +-----------------------------------------------------------------------------+

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Re: newbie
by edrx＠inx.com.br 13 Mar '01

13 Mar '01

Frank Atanassow wrote: > G Murali wrote (on 09-03-01 00:43 +0000): > > I'm new to this monads stuff.. can you tell me what it is simply ? > > an example would be highly appreciated.. i want it is very very > > simple terms please.. > > A monad on category C is a monoid in the category of endofunctors on C. > > Is that simple enough? ;) > > No? Then see "Using Monads" at http://haskell.org/bookshelf/ > > (Sorry, I just couldn't resist!) Uh-oh. I'm a junior categorist and toposopher and I confess that all my few attempts to understand what Haskell's monads have to do with the categorical notion of a monad have failed more or less miserably. Can someone point me to a relevant paper, or give a quick explanation? Thanks in advance, and sorry for the dumb question, Eduardo Ochs http://angg.twu.net/ edrx(a)inx.com.br

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Any in Haskell Report
by Simon Peyton-Jones 12 Mar '01

12 Mar '01

Folks, In the Haskell Report, Feliks points out: | On pp. 5 and 120 the rule for "ANYseq" has a very strange | structure: why the | "{ANY}" at the beginning and end of the subtracted stuff? It | seems it could | be simplified, or at least explained. I have no idea why it can't be just ANYseq -> {ANY}_<opencomment | closecomment> Can anyone see why this simpler form would not work? I propose to adopt this simplification. Simon

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Re: newbie
by Frank Atanassow 12 Mar '01

12 Mar '01

> On Fri, 9 Mar 2001, Frank Atanassow wrote: > > A monad on category C is a monoid in the category of endofunctors on C. Surprisingly, my facetious little remark about monads elicited the following response (whose author I will not disclose, as I haven't obtained permission yet): Somebody wrote (on 09-03-01 15:38 +0000): > Hi Frank --- Believe it or not your one-liner actually *is* helpful to me. > >From dim & distant algebra days I remember the definition of monoid (set, > binary operation, associative, left & right unit) but being a Physics > major/Maths minor, a long time ago, I was deprived of any introduction to > Category Theory. I've read some of the papers Haskell monads section & > also had a look at some category theory books --- but I can't remember > seeing your nice one-line definition --- do you have a reference to the > paper or book(s) that do it this way? I think the monoidal characterization is well known. I first saw it in Asperti & Longo [1] (p.75), but I'm sure it can be found in other texts. The connection with monoids is not very evident in Haskell, but is worth keeping in mind if you know a bit of category theory. BTW, part of the reason I'm posting this to the Haskell list is to plug a little project I have started on my home page. It's a small collection of useful reference texts which deal with programming languages and semantics, all of which have become available on the web. [1] is among these. <start violins for impassioned plea> I occasionally see posts on this list from people who are not researchers or educators (and consequently do not have ready access to such published material) expressing some interest in exploring PL theory and other, related formal topics further, if only the ``barrier-to-entry'' were not so high. Indeed, there is a wealth of technical articles which are available online, and could prove useful or at least interesting to many Haskell and ML programmers, but even the ones directed at a relatively wide audience of PL researchers can be hard to grasp for non-researchers. Personally, I think this is one reason that Haskell and ML are still fairly marginalized languages. My hope is that this list can grow into a useful resource for go-getters and self-starters from other fields and communities, as well as the average PL researcher. <end violins for impassioned plea> OK, I am only a stupid PhD student with no publications under my belt, but anyhow that is how I felt before I became a researcher. Over the space of a few years, while I was still hacking C++ in the dungeons of our vast corporate megacomplex, I laid out several hundred dollars of my own money to buy enough texts to jumpstart my own education, and what started with one book soon exploded into a mini-library of hardbacks and articles (because you know the dependencies in any non-trivial bibliography resemble a large bowl of sticky spaghetti), and I would have been deliriously happy to learn that, for example, Barendregt's encyclopaedic book on lambda-calculus was actually readable from the web. So here's to you, my comrades-in-arms. As it states on my page, if any of you know of any other relevant texts which might be added to the list, please contact me and I will augment the list forthwith. The list is available at: http://www.cs.uu.nl/people/franka/ and here is the reference I promised: [1] Andrea Asperti and Giussepe Longo. Categories, Types and Structures. An introduction to Category Theory for the working computer scientist. MIT Press, 1991. URL: http://www.dmi.ens.fr/users/longo/download.html Regards, -- Frank Atanassow, Information & Computing Sciences, Utrecht University Padualaan 14, PO Box 80.089, 3508 TB Utrecht, Netherlands Tel +31 (030) 253-3261 Fax +31 (030) 251-379

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list of results monad
by G Murali 12 Mar '01

12 Mar '01

hi, I'm trying to build a monad that will give a list of results. this is what i want to be able to do I have data M a = I [a] instance Monad M where return x = I [x] I (x:xs) >>= f = ?? The type of >>= is m a -> (a -> m b) -> m b but here the input is [a], now if I apply the input individually on f then i can get a [m b] and if i extract at each step then I can get [b] and then if i say return ([b]) i get m b.. right ?? Is this how I go about it .. if so can you give me a small code snippet of how to do that. Thanks, gmu _________________________________________________________________________ Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com.

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RE: exception handling..
by Simon Peyton-Jones 12 Mar '01

12 Mar '01

http://research.microsoft.com/~simonpj/papers/marktoberdorf.ps.gz http://research.microsoft.com/~simonpj/Papers/except.ps.gz http://research.microsoft.com/~simonpj/papers/asynch-exns.ps.gz | -----Original Message----- | From: G Murali [mailto:gmu@hotmail.com] | Sent: 09 March 2001 23:15 | To: haskell(a)haskell.org | Subject: exception handling.. | | | Hi, | | Is it possible to do/mimic exception handling in Haskell | using monads.. if | so can you throw in some pointers on this please.. | | gmu | ______________________________________________________________ | ___________ | Get Your Private, Free E-mail from MSN Hotmail at | http://www.hotmail.com. | | | _______________________________________________ | Haskell mailing list | Haskell(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell |

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Re: newbie
by Steinitz, Dominic J 12 Mar '01

12 Mar '01

Monads in Haskell use the Kleisli triple definition which is equivalent to the two natural transformations and functor definition (with the appropriate laws) - see Algebraic Theories by Manes. When using the do notation you are effectively working in the Kleisli category generated by the monad. Dominic. edrx(a)inx.com.br on 10/03/2001 10:13:00 To: haskell cc: bcc: Dominic Steinitz Subject: Re: newbie Frank Atanassow wrote: > G Murali wrote (on 09-03-01 00:43 +0000): > > I'm new to this monads stuff.. can you tell me what it is simply ? > > an example would be highly appreciated.. i want it is very very > > simple terms please.. > > A monad on category C is a monoid in the category of endofunctors on C. > > Is that simple enough? ;) > > No? Then see "Using Monads" at http://haskell.org/bookshelf/ > > (Sorry, I just couldn't resist!) Uh-oh. I'm a junior categorist and toposopher and I confess that all my few attempts to understand what Haskell's monads have to do with the categorical notion of a monad have failed more or less miserably. Can someone point me to a relevant paper, or give a quick explanation? Thanks in advance, and sorry for the dumb question, Eduardo Ochs http://angg.twu.net/ edrx(a)inx.com.br _______________________________________________ Haskell mailing list Haskell(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell ------------------------------------------------------------------------------------------------- 21st century air travel http://www.britishairways.com

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Re: newbie
by Leonid Bouriakovsky 10 Mar '01

10 Mar '01

Perhaps, this paper will help: http://citeseer.nj.nec.com/62964.html >Eduardo Ochs wrote: > >>Frank Atanassow wrote: >> >> A monad on category C is a monoid in the category of endofunctors on C. >> >> Is that simple enough? ;) > >Uh-oh. I'm a junior categorist and toposopher and I confess that all >my few attempts to understand what Haskell's monads have to do with >the categorical notion of a monad have failed more or less miserably. >Can someone point me to a relevant paper, or give a quick explanation?

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Re: newbie
by edrx＠inx.com.br 10 Mar '01

10 Mar '01

Frank Atanassow wrote: > G Murali wrote (on 09-03-01 00:43 +0000): > > I'm new to this monads stuff.. can you tell me what it is simply ? > > an example would be highly appreciated.. i want it is very very > > simple terms please.. > > A monad on category C is a monoid in the category of endofunctors on C. > > Is that simple enough? ;) > > No? Then see "Using Monads" at http://haskell.org/bookshelf/ > > (Sorry, I just couldn't resist!) Uh-oh. I'm a junior categorist and toposopher and I confess that all my few attempts to understand what Haskell's monads have to do with the categorical notion of a monad have failed more or less miserably. Can someone point me to a relevant paper, or give a quick explanation? Thanks in advance, and sorry for the dumb question, Eduardo Ochs http://angg.twu.net/ edrx(a)inx.com.br

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exception handling..
by G Murali 09 Mar '01

09 Mar '01

Hi, Is it possible to do/mimic exception handling in Haskell using monads.. if so can you throw in some pointers on this please.. gmu _________________________________________________________________________ Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com.

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