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From: hal@cornell.UUCP (Hal Perkins)
Newsgroups: net.lang
Subject: Re: Treating Data Abstractly
Message-ID: <5221@cornell.UUCP>
Date: Fri, 2-Sep-83 16:04:04 EDT
Article-I.D.: cornell.5221
Posted: Fri Sep  2 16:04:04 1983
Date-Received: Sat, 3-Sep-83 14:06:27 EDT
References: <5215@cornell.UUCP>
Organization: Cornell Computer Science
Lines: 40

Yes, I do know about Smalltalk, Mesa, CLU, Modula-2, and whatnot.  These
don't seem to do what I want, but I must tread lightly here.  I am not a
certified expert in any of these languages and still have some reading to
do, so I will speak in vague generalities and try to avoid putting my foot
too far down my throat.

All of these languages do a decent job of separating the specification of
a module from the implementation.  What they don't do (CLU might, but as
I said, I still need to look into it) is handle the problem of multiple
representations of the same abstract type simultaniously.  For example,
suppose I have two implementations of complex variables:  one using polar
coordinates the other using rectangular coordinates.  And suppose I want
to have some of my data use one representation and other data use the
other one.  While the languages Pavel mentions make it easy for me to
write a program in terms of a single abstract type (complex) and later
specify an implementation (rectangular or polar), I don't think it is easy
or transparent to use several different implementations of the same abstract
type at once.  This is the same problem as having some arrays in main
storage and others in the Unix or OS/360 file system.

As for Smalltalk, the paradigm of sending messages to objects does indeed
seem to solve the problem at a conceptual level.  But there are two things
I worry about here:  One is efficiency--doesn't it require a fair amount
of overhead to access an array element by sending a message to an array
object?  It would seem that no matter how cleverly this is implemented,
it wouldn't be as fast as executing a half-dozen instructions to compute
a memory location and load the desired array element.  I realize that it
is not fashionable to worry about efficiency (particularly when you have
a dorado on your desk), but I don't want to exclude it from consideration.
Secondly, Smalltalk lives in a world of its own.  I'm hoping that the
ideas in it can be applied in other settings and other languages.


Anyway, I'm getting a little beyond my depth.  I think I'll avoid any
further comments on Smalltalk, Mesa, and CLU until I've learned more
about them.


Hal Perkins                         UUCP: {decvax|vax135|...}!cornell!hal
Cornell Computer Science            ARPA: hal@cornell  BITNET:  hal@crnlcs