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From: roelof@idca.tds.PHILIPS.nl (R. Vuurboom)
Newsgroups: comp.lang.misc,comp.lang.c
Subject: Re: The Fundamental Concept of Programming language X
Keywords: programming languages, abstractions
Message-ID: <585@ssp11.idca.tds.philips.nl>
Date: 3 Jan 90 14:50:57 GMT
References: <1470@mdbs.UUCP> <1782@aipna.ed.ac.uk>
Organization: Philips Telecommunication and Data Systems, The Netherlands
Lines: 98

In article <1782@aipna.ed.ac.uk> sean@aipna.ed.ac.uk (Sean Matthews) writes:

[A number of interesting points a few of which I would like to look at.] 
|
|Take first some of the the sorts of models that we have for computers:
|
|1. the Von Neumann stored program/ processor machine.
|2. the lambda calculus.
|2a. combinatory logic
|3. communicating processes of various sorts.
|4. proof search (e.g. sequent calculus)
|
1 is obviously a computer model, 3 could be used as a computer model. 
But 2 2a and 4? Looks a lot more like language models to me.

|
|The Von Neumann machine, which is a sort of sophisticated Turing
|machine, comes to us in all sorts of forms, and has had a decisive
|influence (necessarily, probably) on the way computers have been built
|up to now, and (unfortunately, definitely) on the way computer
|scientists have been educated up to now.
|
Imo the von Neumann machine is a sort of sophisticated Turing machine
in the same way an integer variable is a sort of sophisticated bit
box. You can emulate a computer with a Turing machine. You can emulate
an integer with a bit box. What you don't do is conceptualize an integer
as a bit box and you (at least I don't) _conceptualize_ a computer as
a Turing machine - the difference in levels of sophistication and 
complexity are just too great (at least for me :-). As I see it,
a Turing machine is no longer used by computer scientists as a computer
paradigm (except for some admitedly interesting theoretical results).
 
|It has produced languages such as Fortran, Algol{58,60,68,W}, COBOL,
|C, PL/1 Sumula, Pascal, Modula{2}, C, BCPL, Ada, Mesa, BASIC etc.
|

Agreed:
The von Neumann machine has produced assignment (imperative programming)
and this is a backbone of all the above languages. A futher separation
into stack-based and non-stack based is possible as you noted. 

|
|The second model that I have listed is the Lambda calculus. Which is a
|much nicer theoretical (and practical) model than load and store.  

As always it depends what your practice is as to whether or not its 
practical...

|In fact, ML is an interesting case, since it started out with pure
|typed lambda calulus and then added as much as was necessary of
|imperative structures as was needed (it turns out to be very little),
|and got a programming language, that, for complex tasks, does not need
|to be much less efficient than C.
|
Maybe this is a little too simplistic, if you're using C to develop
a model which can be naturally implemented in ML you're probably
right. Vice verse you could lose out.
 
|
|So what are the advantages that Lambda calculus gives us?  Higher
|order functions, powerful typing systems, no side effects (except when
|we want them) compact code, more abstraction.  The elimination of the
|Von Neumann bottleneck, both in the programmer, and in the machine; we
|no longer have to think in terms of one thing at a time.
|
And let's not forget the disadvantages: its next to impossible to
model any number of real world situations with it.

|What about paradigm three?  Well here there is the notion of Hoare's
|CSP (yes and Milner's CCS, but let us not complicate things) which is
|best seen in occam (no capitals), which is a load store language that
|has been heavily modified to remove the Von Neumann bottleneck
|mentioned above.  

Agreed.
|
|paradigm four is maybe the least obvious to the typical programmer.
|Prolog is the most obvious implimentation of this sort of thing.  I am
|not sure what we learn from it (I think it---i.e. prolog, and more
|generally, logic programming---is overrated as a programming
|technique) but it does at least give some good ideas about what can be
|done with pattern matching and non-determinism, and it does give a new
|set of intellectual tools with which to approach any problem.
|
I would consider pattern matching and non-determinism to be more 
mechanisms than policies (viewpoints,paradigms). The paradigm is not
pattern matching and non-determinism, the paradigm is descriptive
programming vs procedural programming. I find prolog a little
clumsy in its form of expression (syntax) but where relevant it
offers in my view a higher (and better) form of abstraction than
does lamda calculus (functional programming) or procedural programming.
 
|
|the important thing is to realise that the real differences are not of
|the sorts of data that programming languages deal with, but the way
|that they allow you to approach what you are doing.
|
Can't agree more.