Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!sun-barr!cs.utexas.edu!uunet!mcvax!ukc!etive!aiai!ken
From: ken@aiai.ed.ac.uk (Ken Johnson)
Newsgroups: comp.lang.misc
Subject: Re: Expressive Power - What is it ?
Message-ID: <494@skye.ed.ac.uk>
Date: 30 May 89 16:55:35 GMT
References: <769@cf-cm.UUCP>
Reply-To: ken@aiai.UUCP (Ken Johnson)
Organization: AIAI, University of Edinburgh, Scotland
Lines: 23

In article <769@cf-cm.UUCP> ted@computing-maths.cardiff.ac.uk (Ted Lawson) writes:
>
>	Can anyone provide or (preferably) point-to a definition of,
>	or discussion about, the oft-heard term "Expressive Power" ?
> ... If, for every
>	type, A's type includes B's corresponding type, then A
>	is more expressive than B (or equally expressive)."

No, this doesn't seem to hold water as a complete definition.  Suppose
that in language A I can form statements of the type S1, S2 and S3.  In
language B I can form statements of the type S2, S3, S4,S5, ... 
S100000. 

Then language B can be said to be more expressive than language A,
but language A is handy if I want to execute statements of the
form S1.

The definition you suggest does not allow for this sort of case.
-- 
Ken Johnson, AI Applications Institute, 80 South Bridge, Edinburgh EH1 1HN
E-mail ken@aiai.ed.ac.uk, phone 031-225 4464 extension 212
`I have read your article, Mr.  Johnson, and I am no wiser than when I
started.' -- `Possibly not, sir, but far better informed.'