Path: utzoo!attcan!uunet!dlogics!dsa
From: dsa@dlogics.COM (David Angulo)
Newsgroups: comp.object
Subject: Re: Do we really need types in OOPL's?
Summary: Your statements show the REASON that typed-checked languages are better
Message-ID: <668@dlogics.COM>
Date: 22 Oct 90 15:54:40 GMT
References: <1990Oct9.190813.23402@ux1.cso.uiuc.edu> <2444@runxtsa.runx.oz.au> <1990Oct19.220747.5536@Neon.Stanford.EDU>
Organization: Datalogics Inc., Chicago
Lines: 44

In article <1990Oct19.220747.5536@Neon.Stanford.EDU>, craig@Neon.Stanford.EDU (Craig D. Chambers) writes:
| In article <1990Oct19.180646.8649@ux1.cso.uiuc.edu> render@cs.uiuc.edu (Hal Render) writes:
| 
| Consider the perform: primitives in Smalltalk.  These are very hard to
| type-check without doing analysis of the possible *values* of the
| run-time selector.  No static type system I know of can statically
| type check a perform: primitive in any but the most trivial cases.
| The user interface in Smalltalk uses perform: all the time to execute
| arbitrary user actions when a menu entry is selected. Become: is
| another hard-to-type-check primitive in Smalltalk.

The perform: is, in fact, one case where checking is NEEDED.  I have been
using c++ for nearly two years now and have just (the last two months) started
using Smalltalk.  I had a bug using the perform: message that took me two
days to find because the compiler wasn't smart enough to catch it for me.
(The cause: I had capitalized the symbol that was being used in the perform:
message.  The result: the compiler did not complain, the run time program
did not complain, but the application did not work - highly unsatisfactory).

| 
| Another case in Smalltalk is the implementation of growable arrays (I
| think; some collection in Smalltalk has this problem).  The
| representation of a growable array is a vector of indexable fields, a
| lower bound, and an upper bound (or maybe a length, I don't remember).
| The elements of the growable array are in the fields indexed between
| the lower bound and the upper bound; the contents of the extra padding
| fields are nil.  This poses problems for a static type checker, since
| the best static type systems that I've seen will treat the type of the
| indexable fields as "T | Nil" (where "T" is the parameterized type of
| the elements of the growable array).  So fetches out of the indexed
| fields return objects that the type checker thinks are of type "T |
| Nil", while the programmer knows that they really should be just "T".
| 

Well, I have made growable arrays in c++ for a long time with satisfactory
results so your objections must be implementation dependant and not due
to the methodology of static type checking in itself.


-- 
David S. Angulo                  (312) 266-3134
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