Re: Existential types
The Hope+C existential type is not unsound. With some help from Evan I decoded your Haskell example, please excuse the reply being in Hope+C :-) Heterogenous list of windows in Hope+C (examples here are actually Massey Hope+C, Hope+C is a moving target - but the type system is as in Hope+C). data Win == Win(alpha); ! useless, but is only an example ! type WinList == list(Win); Index function: let index : WinList # int - Win == fun (nil, _) = error(...) | (h::_, 1) = h | (_::t, n) = index(t, n-1) end; so the two applications: Index(wl, 1) Index(wl, 2) have the same type, but that is correct as the existential types havn't been unpacked. However: let same : Win # Win - truval == fun (Win(a), Win(b)) = a == b end; is INCORRECT as the type of a b are DIFFERENT - each seperate unpack introduces a new unique type constant. The removal of Mitchell restriction on the "escaping" of unpackaged values is fine; as each one has a DIFFERENT unique type. [Note: Hope+C is not totally sound - but this is not due to the type system itself but rather to the existance of the unsafe (and untypeable) polymorphic equality function (alpha # alpha - truval). As in many (most?) functional languages this function is a bit-level compare... you can apply it to functions... infinite objects and get true... It should really be removed, but everybody likes it! The auto-generation of equality functions as in Haskell solves some of the problems (type unsoundnes) and not others (equality of user-defined abstract data types where structural equality is not correct), but is a step in the right direction.] Cheers, Nigel --- Dr Nigel PerryEmail: [EMAIL PROTECTED] Department of Computer ScienceTel: +64 6 356 9099 ext 8900 Massey University Fax: +64 6 350 5611 Palmerston North New Zealand
Haskell report 1.2 now available
Announcing == The Haskell Report Version 1.2 1 March 1992 The Haskell Committee, formed in September 1987 to design a "common" non-strict purely functional language, has released an updated version of the Haskell Report. Version 1.2 of the report was prepared for publication in SIGPLAN Notices in the May 1992 issue. It corrects some typographical errors in the 1.1 report, and clarifies the presentation in places, sometimes by giving new examples. A few small changes have also been made to the syntax and standard prelude. In addition to the new report, a Haskell tutorial by Paul Hudak and Joe Fasel is also now available. This will also be published in SIGPLAN Notices as a companion to the report. --- Both the new report and the tutorial are available via anonymous internet ftp. Three Haskell ftp sites have been established: nebula.cs.yale.edu (128.36.13.1) ftp.dcs.glasgow.ac.uk (130.209.240.50) animal.cs.chalmers.se (129.16.225.66) Each of these sites has an area devoted to Haskell-related material in pub/haskell. The file pub/haskell/FILES_ORG contains a complete list of the available material. The report can be found in: pub/haskell/report/report-1.2.dvi.Z -- A compressed dvi file pub/haskell/report/report-1.2.ps.Z -- A compressed postscript file The tutorial can be found in: pub/haskell/tutorial/tutorial.ps.Z -- A compressed postscript file (No dvi version available) An online supplement to the tutorial for users of Yale haskell is also available -- look in pub/haskell/tutorial/README for more information. These sites also contain current Haskell implementations, papers related to Haskell, the Gofer system, and archives of the Haskell mailing list. --- The report is also available from Yale as a technical report; however, we recomend waiting for the SIGPLAN publication instead. To get a hardcopy version of the report, send $10 to: The Haskell Project Department of Computer Science Yale University Box 2158 Yale Station New Haven, CT 06520 USA --- A mailing list for general discussions about Haskell is reachable in one of two ways: [EMAIL PROTECTED] or [EMAIL PROTECTED] To be added to the above mailing list, send mail to either [EMAIL PROTECTED] or [EMAIL PROTECTED], whichever is the nearer site to you. Please do not post administrative requests direct to the mailing list. To inquire about implementation status, send mail to one of the addresses mentioned in the preface of the report.
