[L7] ================================================ AMAST Links 02 08

                Typing in object-oriented languages: 
                 Achieving expressibility and safety
                          Kim B. Bruce
                        Williams College

  While simple static-typing disciplines exist for object-oriented
languages like C++, Object Pascal, and Modula-3, they are often so 
restrictive that programmers are forced to by-pass the type system with
type casts.  Other languages allow more freedom, but require run-time 
checking to pick up the type errors that their more permissive systems 
missed.

  This paper consists of a survey of problems (illustrated by a series
of sample programs) with existing type systems, and suggests ways of 
improving the expressibility of these systems while retaining static 
type safety.  In particular we will discuss the motivations behind 
introducing "MyType", "matching", and "bounded matching" into these 
type systems.

  We also suggest a way of simplifying the resulting type system by 
replacing subtyping by a type system with a new type construct based 
on matching.  Both systems provide support for binary methods, which 
are often difficult to support in statically-typed languages.

  The intent is to avoid pages of type-checking rules and formal 
proofs, but instead explain why the problems are interesting via the 
series of sample programs.  The technical details (including proofs 
of subject reduction and type safety) are available elsewhere.

  The paper is available by following links from my _home page_
URL: http://www.cs.williams.edu/~kim/ 
or by anonymous ftp from URL: ftp://cs.williams.edu/pub/kim/ 
as file Static.{dvi,ps}. Comments are welcomed.  

  (An earlier draft version of this paper available on the net was 
entitled: "Problems with static typing in object-oriented languages".  
Please discard any copies of the earlier version.)