November 2000 - Haskell

Re: Union Types for Haskell!?
by Christian Lescher 27 Nov '00

27 Nov '00

Hi Bernd, > data B = ... > data C = ... > type A = B | C | D -- A accepts values of either B or C or D (cf. the > "Either a" type in the Prelude) What about the construction of a union type in Haskell like this?: data B = B1 | B2 data C = C1 | C2 | C3 data D = D1 | D2 data A = UB B | UC C | UD D -- here's the union type For instance a list of type [A] may look like this: [UB B1,UC C2,UC C1,UD D1] Christian

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Union Types for Haskell!?
by Bernd Holzmüller 27 Nov '00

27 Nov '00

I am about to finish my PhD, where I used the Vienna Definition Language (VDL) to specify the semantics of some programming language constructs. In order to verify this semantics description using a type checker and by dynamic execution, I found and used Haskell as the language of choice to map the VDL description to Haskell. I was quite impressed about the expressive power of Haskell (e.g. the list comprehension syntax and the use of the functions "any" and "all" together with ad-hoc definable lambda-abstractions) and the flexible type system, which together allowed me to almost literally transcribe the VDL description into Haskell. Since then, I have used Haskell to build several parsers and compilers for simple script languages as well as for other purposes. There is one thing I was really missing in all these projects: the existence of union types (i.e. the union of value sets of two existing data types), for two reasons: - Firstly, to map VDL union types to Haskell, I needed to introduce one further indirection, using another data type with one special type constructor for each former variant. This was actually the biggest inconvenience in transforming VDL to Haskell. - Secondly, after having built abstract tree representations of languages while parsing, I did want to traverse and manipulate such trees (consisting of nodes belonging to several different data types) for semantic analysis and code generation several times. Unfortunately, I was not able to use a single function to map over the different nodes of the abstract syntax tree. The only viable solution I found was to use a set of functions, one for each node type, and to pass the whole set down the tree, extracting and applying the appropriate function for each node (type). To define one Haskell data type that defines each of the different nodes as one variant would of course lead to much less type safety, because each reference to a node would be much less precise/less informative. When I got to know Haskell, I was expecting a construct for union types like: data B = ... data C = ... type A = B | C | D -- A accepts values of either B or C or D (cf. the "Either a" type in the Prelude) but this is not valid Haskell. Is there any reason for this restriction in the Haskell type system? Does this lead to losing the principal type property? And, if so, would you please give me an example where the principal type does not exist any more when introducing union types in the form above? Do any papers exist about this topic? Is there any Haskell compiler supporting union types? Are there any considerations of integrating union types into the Haskell type system, at least for explicit type annotations (if type inference using union types should in any way be problematic)? I find the existence of union types very attractive. Apart from enhanced flexibility in modelling, type error messages would possibly be more traceable, because different branches in if- or case-expressions would have the *same* relevance, rather than the first branch being type-checked becoming normative for all other branches. Thanks in advance, Bernd Holzmüller ICS AG Sonnenbergstraße 13 70184 Stuttgart ++49 711 21037 0 holzmueller(a)ics-ag.de

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MBR'01 Deadline extension
by Lorenzo Magnani 26 Nov '00

26 Nov '00

********** DEADLINE EXTENSION TO DECEMBER 15, 2000 *************** We have extended the submission deadline to December 15. Please send electronically (Microsoft Word, RTF, PDF, or Postcript) the extended abstract (about 1000 words) - No hard copies - to the program chair at the address lmagnani(a)cc.gatech.edu NEW Web Site Address of MBR'01 http://www.unipv.it/webphilos_lab/courses/progra1.html Last updated November 25, 2000 ********************************************************************** MODEL-BASED REASONING: SCIENTIFIC DISCOVERY, TECHNOLOGICAL INNOVATION, VALUES (MBR'01), Pavia, Italy, May 17-19, 2001. ********************************************************************** Up-to date information on the conference will be found at http://www.unipv.it/webphilos_lab/courses/progra1.html ********************************************************************** GENERAL INFORMATION >From Thursday 17 to Saturday 19 May 2001 (three days) the International Conference "MODEL-BASED REASONING. SCIENTIFIC DISCOVERY, TECNOLOGICAL INNOVATION, VALUES" will be held at the University of Pavia (near Milan, Italy). PROGRAM The conference will deal with the logical, epistemological, and cognitive aspects of modeling practices employed in scientific discovery and technological innovation, including computational models of such practices. Abduction is widely recognized as a significant reasoning process in discovery whose features are in need of explication. We will solicit papers that examine various forms of model-based reasoning, such as analogical and visual modeling, from philosophical, historical, sociological, psychological, or computational perspectives. We also plan to address the problem of model-based reasoning in ethics reasoning, especially pertaining to science and technology. RELEVANT RESEARCH AREAS We shall call for papers that cover topics from the following list: - abduction - analogical reasoning - causal and counterfactual reasoning in model construction - computational models of model-based reasoning and scientific reasoning - conceptual combination and theory formation - hypothetical and explanatory reasoning - logical analyses that may contribute to our understanding of the issues in model-based reasoning - model-based reasoning in ethics - models and manipulative reasoning - models and technological innovation - thought experimenting - visual, spatial, imagistic modeling, reasoning, and simulation SUBMISSIONS OF PAPERS All submitted papers will be carefully refereed. The precise format of the conference will be fixed after we have an idea of the number of accepted papers. We are thinking in terms of presentations of 40 and 20 minutes. The funding is Italian and US, but we are also looking elsewhere for further financing (and would appreciate any suggestions). A selected subset will be invited for inclusion (subject to refereeing) in a book which will constitute an advanced handbook for scientists and researchers. The book will be published by an international publishing house. Moreover another selected subset will be invited for inclusion (subject to refereeing) in special issues of suitable international Journals. FORMAT Authors must submit three printed copies and an electronic version - formatted in Microsoft Word, RTF, PDF, or Postcript format - of an extended abstract (about 1000 words) not later than December 18, 2000. Please send electronically the extended abstract to the program chair at the address lmagnani(a)cc.gatech.edu in case of problem with the above address please use lorenzo(a)philos.unipv.it or lmagnaniusa(a)netscape.net REGISTRATION AND FURTHER INFORMATION Registration Fees: Before 15 March 2001: Normal: ITL. 300.000 = appr. US$ 155 (EUR 154.93) (to participate in all the activities of the Conference) Students: Free After 15 March 2001: Normal: ITL. 350.000 = appr. US$ 175 (EUR 180.75) (to participate in all the activities of the Conference) Students: Free METHOD OF PAYMENT AND REGISTRATION DEADLINE: Bank (Swift) Transfer to BANCA REGIONALE EUROPEA S.p.A BRANCH PAVIA - Sede SWIFT BREUITM2 301 Bank Code 06906.11301 Acc. n. 520 Dipartimento di Filosofia indicating CONVEGNO INTERNAZIONALE MBR'01 PLEASE REGISTER by email, fax or air mail (before March 15, 2001) by sending PROGRAM CHAIR first and last name, function, institution, full address, phone, fax and email. For information about paper submission and the program that is not available on the web site, please contact the program chair. IMPORTANT DATES Registration deadline............................15 March 2001 Submission deadline..............................15 Dec 2000 Notification of acceptance.....................28 Feb 2001 Final papers (from those selected for publication) due........30 June 2001 Conference....................................17-19 May 2001 PROGRAM CHAIR Lorenzo MAGNANI School of Public Policy and College of Computing Program in Philosophy, Science, & Technology Georgia Institute of Technology, 685 Cherry Street Atlanta, GA, 30332 - 0345, USA Office: 404-894-0950 & 404-385-0884, Home: 404-875-3566 Fax: 404-385-0504 & 404-894-2970 Email: lorenzo.magnani(a)cc.gatech.edu Address in Italy: Department of Philosophy and Computational Philosophy Laboratory University of Pavia, Piazza Botta 6, 27100 Pavia, Italy Office: +39-0382-506283, Home: +39-0383-371067 Fax: +39-0382-23215 Email: lorenzo(a)philos.unipv.it PROGRAM CO-CHAIR Nancy J. NERSESSIAN (Program Co-Chair) Program in Cognitive Science School of Public Policy and College of Computing, Georgia Institute of Technology, Atlanta, USA Email: nancyn(a)cc.gatech.edu PROGRAM CO-CHAIR Kenneth J. KNOESPEL (Program Co-Chair) School of History, Technology, and Society, and Program in Cognitive Science Georgia Institute of Technology, Atlanta, USA Email: kenneth.knoespel(a)hts.gatech.edu PROGRAM COMMITTEE - Ann Bostrom, School of Public Policy, Georgia Institute of Technology, Atlanta, GA, USA - Elena Gagliasso, Department of Philosophical and Epistemological Studies, University of Rome La Sapienza, Rome, ITALY - Dedre Gentner, Psychology Department, Northwestern University, Evanston, IL 60208, USA - Ronald N. Giere, Department of Philosophy, University of Minnesota, MN, USA - Mark L. Johnson, Department of Philosophy, 1295 University of Oregon, Eugene, OR, USA - Kenneth Knoespel, School of History, Technology, and Society, Georgia Institute of Technology, Atlanta, GA, USA - Lorenzo Magnani, Department of Philosophy, University of Pavia, Pavia, ITALY and School of Public Policy and College of Computing, Georgia Institute of Technology, Atlanta, GA, USA - Pat Langley, Adaptive Systems Group, DaimlerChrysler Research & Technology Center, Palo Alto, CA, USA - Nancy J. Nersessian, School of Public Policy and College of Computing, Georgia Institute of Technology, Atlanta, GA, USA - Brian Norton, School of Public Policy, Georgia Institute of Technology, Atlanta, GA, USA - Claudio Pizzi, Department of Philosophy and Social Sciences, University of Siena, Siena, ITALY - Mario Stefanelli, Department of Computer Science, University of Pavia, Pavia, ITALY - Paul Thagard, Department of Philosophy, University of Waterloo, Waterloo, CANADA - Ryan D. Tweney, Bowling Green State University, Bowling Green, OH, USA - Stella Vosniadou, Department of Philosophy and History of Science, Brain and Cognitive Science Division, National and Capodistrian University of Athens, Athens, GREECE. LOCAL ORGANIZING COMMITTEE Riccardo Dossena (riki.dox@libero), Elena Gandini (elegand(a)yahoo.com) Rosella Gennari (gennari(a)hum.uva.nl) Lorenzo Magnani (lmagnani(a)cc.gatech.edu) Massimo Manganaro (triskel(a)worldonline.it) Stefania Pernice (stepernice(a)libero.it) Matteo Piazza (pimat(a)yahoo.com) Giulio Poletti (philosophia(a)libero.it) Stefano Rini (s.rini(a)philos.unipv.it) Andrea Venturi (aventuri@philos.unipv.,it) (Department of Philosophy, University of Pavia, Pavia, Italy), Mario Stefanelli (mstefa(a)ipvstefa.unipv.it) (Department of Computer Science, University of Pavia, Pavia, Italy). IMPORTANT ADDRESSES LORENZO MAGNANI (Conference Chair) School of Public Policy and College of Computing Program in Philosophy, Science, & Technology Georgia Institute of Technology, 685 Cherry Street Atlanta, GA, 30332 - 0345, USA Office: 404-894-9050 & 404-385-0884, Home: 404-875-3566 Fax: 404-385-0504 & 404-894-2970 Email: lorenzo.magnani(a)cc.gatech.edu Address in Italy: Department of Philosophy and Computational Philosophy Laboratory University of Pavia, Piazza Botta 6, 27100 Pavia, Italy Office: +39-0382-506283, Home: +39-0383-371067 Fax: +39-0382-23215 Email: lorenzo(a)philos.unipv.it CONFERENCE SITE: Collegio Ghislieri, Piazza Ghislieri, 27100 PAVIA, Italy, phone +39 0382 22044. The Conference is sponsored by UNIVERSITY OF PAVIA, ITALY GEORGIA INSTITUTE OF TECHNOLOGY, ATLANTA, GA, USA UNIVERSITY OF SIENA, ITALY UNIVERSITY OF ROME "LA SAPIENZA", ITALY, MURST (Ministero dell'Università e della Ricerca Scientifica e Tecnologica), ITALY, CARIPLO (CASSA DI RISPARMIO DELLE PROVINCIE LOMBARDE, MILAN, ITALY) HOW TO REACH PAVIA LINATE Airport: People arriving by plane at LINATE should take the bus to the CENTRAL STATION of Milan (cf below fron this Station to Pavia). In LINATE it could be convenient to take a Taxi because the airport is close to the center of Milan. Moreover, The bus company SGEA offers six runs from LINATE to Pavia at 9.00, 10.00, 12.00 AM and 2.00, 5.00, 8.30 PM. The last stop is Pavia, near the station (see again our updated web page for possible alterations of this time-table) (from Pavia to LINATE six runs at 5,00, 7.45, 10.00 AM, 1.00, 4.00, 6.00 PM) (one hour trip). In Pavia there is only one station. The easiest way to reach the center of the town is to get off at the station and than take the bus n. 3. MALPENSA 2000 and OLD MALPENSA Airports (usually people arrive to Malpensa 2000 and not to OLD MALPENSA): People arriving by plane at MALPENSA 2000 (also called MALPENSA 2000 Terminal 1) or at "old" MALPENSA (now called MALPENSA NORTH but also called Malpensa 2000 Terminal 2) should take the bus to the CENTRAL STATION of Milan. There is also a bus AND A TRAIN from Malpensa 2000 to the NORTH STATION (Piazzale Cadorna) of Milan, in this case from NORTH Station you will have to take the underground MM1 to the Central Station: trains to Pavia leave from Central station). Moreover, the bus company SGEA offers four runs from MALPENSA 2000 to Pavia at 9.00 AM, 1:30 PM, 5.00 PM, and 9:30 PM - from Malpensa North (OLD Malpensa or Malpensa 2000 Terminal 2 5 munutes later) (from Pavia to MALPENSA 2000 and to OLD MALPENSA four runs at 7.00 AM, 11:00 AM, 3.15 PM, and 7:00 PM) (one hour and half trip). The last stop is Pavia, near the station (see again our updated web page for possible alterations of this time-table) In Pavia there is only one station. The easiest way to reach the center of the town is to get off at the station and than take the bus n. 3. There are trains from MILAN (Central Station) to PAVIA and vice Versa about every an hour (routes: MILAN-GENOVA; MILAN-VENTIMIGLIA; MILAN-LA SPEZIA; MILAN-SAVONA; MILAN-SESTRI LEVANTE; MILAN-IMPERIA; MILAN-ALBENGA; Pavia is the first stop only if the train is not slow, that is, if it is not, in ITALIAN, "L", locale). In Pavia there is only one rail station. The easiest way to reach the center of the town is to get off at the station and than take the bus n. 3. ACCOMMODATION The WEB site of the Tourist Office is http://www.systemy.it/pavia/home.html (new! sorry, only in Italian). The email address is info(a)apt.pv.it. When available you will find the whole list of hotels and other information concerning Pavia and its history. See also http://www.itwg.com/ct_00036.asp. In case of accommodation problems remember we will have at our disposal some rooms at special "conference rates" in the Colleges of the University. For further information please contact the Program Chair. As the the conferences dates are very close to summer holidays we recommend making your reservations as early as possible and before March 31, 2000 at the latest. ALL ACCOMMODATIONS (EXCEPT FOR INVITED SPEAKERS) WILL BE PROCESSED BY: Agenzia Viaggi ALOHATOUR Corso Cairoli 11 I - 27100 PAVIA Italy Phone: +39-0382-539565 Fax: +39-0382-539572 +39-0382-539504 email (only to request information): aloha(a)buonviaggio.it (cut here) ********************************************************************** ACCOMMODATION FORM - MBR'01 ---------------------------------------------------------------------- TO BE FAXED: +39-0382-539572 +39-0382-539504 OR MAILED: Agenzia Viaggi ALOHATOUR Corso Cairoli 11 I - 27100 PAVIA Italy email (only to request information): aloha(a)buonviaggio.it ---------------------------------------------------------------------- FILL IN CAPITAL LETTERS, PLEASE LAST NAME:___________________FIRST NAME:_____________Prof./Dr./Mr./Ms. AFFILIATION/UNIVERSITY/DEPT.__________________________________________ STREET:_______________________________________________________________ TOWN:___________________________CODE:_____________COUNTRY:____________ PHONE:__________________FAX:__________________E-MAIL:_________________ TYPE OF TRAVELLING:__________________DATE OF ARRIVAL:_________________ DATE OF DEPARTURE:___________________NUMBER OF NIGHTS:________________ CREDIT CARD NUMBER AND TYPE__________________________ EXPIRATION_____________ (you can also pay by bank transfer and postal order, please see below) ACCOMMODATION INCL. BREAKFAST SINGLE ROOM+BATH. / DOUBLE ROOM+BATH. ***HOTEL EXCELSIOR, Piazza Stazione, PAVIA LIT. 100.000 / LIT. 150.000 ______________ (EUR. 51.64 / EUR. 77.46) ****HOTEL ARISTON, Via Scopoli, PAVIA LIT. 130.000 / LIT. 190.000 +_____________ (EUR. 67.13 / EUR. 98.12) ****HOTEL MODERNO, Viale V. Emanuele, PAVIA LIT. 170.000 / LIT. 230.000 +_____________ (EUR. 87.79 / EUR. 118.78) RESERVATION CHARGE LIT. 25.000 +_______ (EUR. 12.91) TOTAL AMOUNT: =_____________ ACCOMMODATION DEPOSIT: ONE NIGHT LIT......... - _____________ (EUR) ACCOMMODATION BALANCE: LIT......... =_____________ (EUR)........ Hotel Excelsior (from the station walk east) Hotel Moderno (from the station walk north) To reach Hotel Ariston take the bus n. 3 or taxi. ____________________________________________________________________ PLEASE FAX OR MAIL THIS FORM AND PAY BY *CREDIT CARD* BEFORE *15 MARCH 2001* TO: FAX: +39-0382-539572 +39-0382-539504 MAIL ADDRESS: Agenzia Viaggi ALOHATOUR Corso Cairoli 11 I - 27100 PAVIA Italy email (only to request information): aloha(a)buonviaggio.it DATE OF PAYMENT____________YOUR SIGNATURE____________________ ________________________________________________________________________ PLEASE FAX OR MAIL THIS FORM AND PAY BY *BANK TRANSFER* BEFORE *15 MARCH 2001* (fax or mail also a copy of the bank transfer) TO: BANCA REGIONALE EUROPEA S.p.A.BRANCH PAVIA - SedeSWIFT BREUITM2 301Bank Code 6906.11301 Agenzia Viaggi ALOHATOUR S.r.l. Acc.n 19952/4 DATE OF PAYMENT____________YOUR SIGNATURE___________________ ________________________________________________________________________ PLEASE FAX OR MAIL THIS FORM AND PAY BY *POSTAL ORDER* BEFORE* 15 MARCH 2001* (fax or mail also a copy of the postal receipt) TO: Agenzia Viaggi ALOHATOUR Corso Cairoli 11 I - 27100 PAVIA Italy DATE OF PAYMENT______________________YOUR SIGNATURE______________________ _________________________________________________________________________ ALHOATOUR WILL MAIL OR FAX YOU THE RESERVATION VOUCHER ALOHATOUR WILL SATISFY THE REQUESTS AS FAR AS POSSIBLE. IF NOT POSSIBLE, ANOTHER SIMILAR ACCOMMODATION WILL BE ARRANGED.

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Memory usage
by Dusan Kolar 25 Nov '00

25 Nov '00

Hello, working on a small utility for de/compression of special files using special algorithm (useless in other cases and not important for the problem) I've encountered I cannot decompress even small files because I always run out of the memory. After some time I've found the problem. It can be simplified to the following function (computing nothing reasonable): tst l [] = id tst l (x:xs) = if null l then ((++) [x]) . (tst (l++[x]) xs) else if null (tail l) then ((++) [x,last l]) . (tst (tail l++[last l]++[x]) xs) else ((++) [x,last l]) . (tst (tail (tail l)++[last l]++[x]) xs) Trying to run it in hugs (default settings) this way: tst [] [1..20000] [] the program stops writing almost all the lines and running out of memory. Nevertheless, just a minor change to the function ([last l] replaced with [x]): tst' l [] = id tst' l (x:xs) = if null l then ((++) [x]) . (tst' (l++[x]) xs) else if null (tail l) then ((++) [x,last l]) . (tst' (tail l++[x]) xs) else ((++) [x,last l]) . (tst' (tail (tail l)++[x]++[x]) xs)tst' l [] = id and we can run the: tst' [] [1..1000000] [] easily, till it writes out the complete list. Could you point me somewhere to see where is the error and how it can be solved? Thanx Dusan Kolar

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"Semantic Knowledge Acquisition and Categorisation" - Workshop at ESSLLI 2001
by Alessandro Lenci 25 Nov '00

25 Nov '00

************************* CALL FOR PAPERS ****************************** SEMANTIC KNOWLEDGE ACQUISITION AND CATEGORISATION Workshop at ESSLLI XIII (Helsinki) Helsinki, August 13th - 17th 2001 http://www.ilc.pi.cnr.it/~esslli --------------------------------------------------------------------- The sheer amount of knowledge necessary to shed light on the way word meanings mutually relate in context or distribute in lexico-semantic classes appears to exceed the limits of human conscious awareness and descriptive capability. Particularly at this level of linguistic analysis, then, we seem to be in need of automatic ways of filtering, structuring and classifying semantic evidence through inspection of a large number of word uses in context. Totally or partially unsupervised inductive methods of knowledge acquisition from corpus data are credited with being able to provide such ways. Yet, it remains to be seen how acquired information can best be represented in current formal models for knowledge representation, for it to be made available to mainstream NLP applications. There are reasons to believe that this integration will require much more than a simple extension of off-the-shelf machine learning technology. At the same time, any major breakthrough in this area is bound to have significant repercussions on the way word meanings and lexico-semantic classes in general are formally represented and used for applications. With these purposes in mind, the workshop intends to focus on the issue of interaction between techniques for inducing semantic information from corpus data and formal methods of linguistic knowledge representation. In particular, we encourage in-depth analysis of underlying assumptions of the proposed techniques and methods and discussion of possible relevant connections with cognitive, linguistic,logical and philosophical issues. TOPICS OF INTEREST Possible themes for contributions are: - development of 'data-driven' semantic type systems - dynamic update and tuning of formal ontologies with text data - 'hybrid models' of knowledge extraction, whereby machine learning methods are integrated with formal structures of knowledge representation - creation of dynamic lexical knowledge-bases - formal representation and structuring of the flow of information extracted from texts. WORKSHOP ORGANISATION The workshop will be held on five subsequent days, August 13th-17th 2001. It will take place during the ESSLLI-Summer School in Helsinki, and will be open to all members of the LLI-community, computational linguistics and cognitive science as well. Each session will consist of two talks plus discussion (30mins + 15mins each). The workshop language will be English. ---------------------------------------------------------------------------- GUIDELINES FOR SUBMISSIONS The organizers welcome contributions from different fields of computational linguistics, logic and cognitive science. 1. Submissions will consist of full papers reporting on original, unpublished works. Papers should not exceed 5000 words (including references), must contain a 5-10 lines abstract, and must be submitted in postscript format 2. The reviewing of papers will be blind. Hence the paper should not include the authors' names and affiliations. Furthermore, self-references that reveal the author's identity should be avoided. 3. Identification information will be put in a separate file consisting of: Title of the paper: Author(s): Affiliation(s): E-mail(s): Abstract: 4. Submitted papers and identification information must be sent to the following address by MARCH 15th 2001: esslli2001(a)ilc.pi.cnr.it Accepted papers will be notified by APRIL 30th, together with the comments of the reviewers. The final version of the papers will be prepared in LaTeX by MAY 31st, for publication in the Workshop proceedings. The stylesheet will be circulated in due time. Note that all workshop participants must register as participants of ESSLLI. The early registration fees apply to authors of accepted papers. --------------------------------------------------------------------------- IMPORTANT DATES March 15th 2001 - Deadline for submitting papers April 30th 2001 - Notification of acceptance May 31st 2001 - Preparation of the final version of the paper June 15th 2001 - Final program of the workshop August 13th 2001 - The workshop begins --------------------------------------------------------------------------- PROGRAM COMMITEE Ted Briscoe (Cambridge University, Cambridge, United Kingdom) Nicoletta Calzolari (Istituto di Linguistica Computazionale, CNR, Pisa, Italy) Christiane Fellbaum (Princeton University, Princeton, USA) Ed Hovy (USC, Marina del Rey CA, USA) Dekang Lin (University of Alberta, Edmonton, Canada) Antonio Sanfilippo (LingoMotors Inc., Cambridge MA, USA) Piek Vossen (SAILLABS, Antwerpen, The Netherlands) ORGANISING COMMITEE Alessandro Lenci Simonetta Montemagni Vito Pirrelli Istituto di Linguistica Computazionale (ILC) - CNR Area della Ricerca CNR Via Alfieri 1 (San Cataldo) I-56010 PISA Italy CONTACT ADDRESS esslli2001(a)ilc.pi.cnr.it WEB: http://www.ilc.pi.cnr.it/~esslli --------------------------------------------------------------------------

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unsubscribe haskell
by Wooseok Yang 25 Nov '00

25 Nov '00

unsubscribe haskell

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Re: Union Types for Haskell!?
by Ashley Yakeley 25 Nov '00

25 Nov '00

At 2000-11-24 02:02, Bernd =?iso-8859-2?Q?Holzm=FCller?= wrote: >When I got to know Haskell, I was expecting a construct for union types >like: > > data B = ... > data C = ... > type A = B | C | D -- A accepts values of either B or C or D (cf. the >"Either a" type in the Prelude) > >but this is not valid Haskell. Is there any reason for this restriction >in the Haskell type system? My understanding is that every value in Haskell is a member of exactly one type. Well, that's not exactly true: it might be better to say that every Haskell value represents a set of 'items', each of which has exactly one fully-specified type (i.e. a type with no free type-variables). For instance, in such definitions as data Maybe a = Nothing | Just a the value that the constructor 'Nothing' gives has type 'Maybe a', which represents a set of 'items' that have such types as 'Maybe Boolean', 'Maybe String' etc. This rules out union types and OOP-style polymorphism. It also makes it difficult to make arrays of values of mixed type. >Does this lead to losing the principal type property? Yes. >And, if so, would you please give me an example where the >principal type does not exist any more when introducing union types in >the form above? Certainly. Consider, in your extended Haskell: data B = B1 | B2 data C = ... type A = B | C | D -- A accepts values of either B or C or D Both A and B are types for B1, but neither is a substitution-instance of the other. This violates the principal type property whereby for every term x, either x is not typable or there exists a type T such that every type assignable to x is a substitution-instance of T. A better question might be, having extended the type system (and indeed the notion of 'type') in this way, how do I need to modify the concept of 'principal type'? >Do any papers exist about this topic? Is there any >Haskell compiler supporting union types? You might look at O'Haskell, which I understand has some kind of OOP-style polymorphism. I don't know if it has union types though. -- Ashley Yakeley, Seattle WA

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Re: Union Types for Haskell!?
by Pixel 24 Nov '00

24 Nov '00

Fergus Henderson <fjh(a)cs.mu.oz.au> writes: [...] > (2) Allow pattern matching on values of particular types, > using `case'. [...] > If we allow (2), then some programming mistakes that previously > were type errors would instead become just inexhaustive pattern > matches for which you get a run-time error. > > For example: > > foo :: Maybe a -> b -> Maybe b > foo x y = case list of > [] -> Nothing > (x:xs) -> Just x > where list = case x of > Nothing -> 0 -- oops, meant [] instead of 0 > Just foo -> [y] > > If union types were allowed, with (2) above, this example would be > legal, with `list' having the union type { [a], Integer }. if you disallow undeclared union types, you'll catch easily this kind of errors. > But `foo Nothing 42' would evaluate to bottom, since the `case list' > expression has no case with a pattern of type Integer. > > So allowing union types in this way would reduce static type safety, > at least unless we also forbid inexhaustive pattern matches. well allowing inexhaustive pattern matching always leads to less type safetyness, no news :) And of course subtyping also reduce type safetyness. But it is the same for type classes. No one is more safe than using '+' for integers and '+.' for floats... > > If instead you only allow (1), then probably union types would > not be suitable for expressing the kinds of things that you want > to express with them. sure, you don't win anything. > > > Is there any reason for this restriction > > in the Haskell type system? Does this lead to losing the principal type > > property? > > If you allow (2) above, there may be serious problems for > principal types. For example, consider > > f x = case x of > Nothing -> False > Just _ -> True > > What's the most general type for `f'? > The type `f :: Maybe a -> Bool' is less general than > e.g. `f :: Union { Maybe a, ... } -> Bool', > but you certainly don't want to infer the latter type. not necessarily. If you disallow undeclared union types, the only occurence of Union { Maybe a, ... } will be Maybe a, unless one really wants to extend Maybe... cu Pixel.

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Re: Union Types for Haskell!?
by Ashley Yakeley 24 Nov '00

24 Nov '00

At 2000-11-24 03:14, Fergus Henderson wrote: >> Is there any reason for this restriction >> in the Haskell type system? Does this lead to losing the principal type >> property? > >If you allow (2) above, there may be serious problems for >principal types. For example, consider > > f x = case x of > Nothing -> False > Just _ -> True > >What's the most general type for `f'? >The type `f :: Maybe a -> Bool' is less general than >e.g. `f :: Union { Maybe a, ... } -> Bool', >but you certainly don't want to infer the latter type. There are two different kinds of 'general' here. Informally, you want the type most general in the type-substitution sense, but probably most specific in the subtype sense. -- Ashley Yakeley, Seattle WA

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Four lectureships in Nottingham
by gmh＠marian.cs.nott.ac.uk 24 Nov '00

24 Nov '00

Dear all, We are currently advertising four new lectureships in Nottingham. There are no particular research areas specified for these positions, but applications in the area of the Languages and Programming research group (http://www.cs.nott.ac.uk/Research/lap/) would be most welcome. Graham Hutton ---------------------------------------------------------------------- THE UNIVERSITY OF NOTTINGHAM School of Computer Science and Information Technology Four Lectureships Applications are invited for the above posts in a rapidly-expanding School based at one of the UK's strongest research-led universities. The School was graded 4 in the 1996 Research Assessment Exercise and is now building up its research strengths for a further improvement in grade. These posts are part of a steady increase in the size of the School accompanied by a move to new purpose-built accommodation in September 1999. Although the successful candidates will have the opportunity of teaching within their specialist area at undergraduate and postgraduate level, there will also be a requirement to teach general computer science and information technology modules outside of any particular specialisation. Candidates should already have a PhD in computer science, or another closely related discipline, together with strong evidence of existing research work and the potential for future research at Grade 5 level. Outstanding candidates will be welcomed from any area of computer science. Salary will be within the range #18,731 - #30,967 per annum, depending on qualifications and experience. Informal enquiries may be addressed to Professor P H Ford, Head of School, tel: +44 (0)115 951 4251 or Email: Peter.Ford(a)Nottingham.ac.uk. Further information about the School is available on the WWW at: http://www.cs.nott.ac.uk. Further details and application forms are available from the Personnel Office, Highfield House, The University of Nottingham, University Park, Nottingham, NG7 2RD. Tel: +44 (0)115 951 5927. Fax: +44 (0)115 951 5205. Email: Carole.Matthews(a)Nottingham.ac.uk. Please quote ref. LEG/537. Closing date: 5 January 2001. ----------------------------------------------------------------------

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