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From: matthias@titan.rice.edu (Matthias Felleisen)
Newsgroups: comp.lang.scheme
Subject: Re: what makes programming?
Message-ID: <1990Aug21.041045.8425@rice.edu>
Date: 21 Aug 90 04:10:45 GMT
References: <9008201428.AA01042@samsung.com> <9008202136.AA02170@mozart.think.com>
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To: gls@THINK.COM
Subject: Re: what makes programming?
Newsgroups: comp.lang.scheme
In-Reply-To: <9008202136.AA02170@mozart.think.com>
References: <9008201428.AA01042@samsung.com>
Organization: Rice University, Houston
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In article <9008202136.AA02170@mozart.think.com> Guy Steele writes
that some definition of Scheme in terms of CALL/CC "reminds [him] of
the definition of a human being as a 'featherless biped': succinct
but not necessarily to the point."

That's true, so let's try something better:

I. Syntax: Scheme's syntax is simple (minus a few glitches, which the
committee will eventually eliminate) and resembles a mathematical
language of algebraic expressions.

II. Type structure: Scheme is dynamically typed. 

III. Core:  
1. Scheme has first-class procedural values, described by a
   Lambda-calculus-like syntax.
2. Scheme has assignments that can affect all (lexical) variables in
   an unrestricted manner.
3. Scheme provides access to a first-class procedural abstraction 
   of the control state of the underlying evaluation machinery. 
[4. Scheme implementations provide a modest form of a control delimiter
    in the form of a read-eval-print loop. (Private Opinion)]

IV. Primitive Constants and Functions: 
1. Scheme provides a fixed number of built-in sets of constants (domains),
   symbols (an arbitrary set of elements that are in 1-1 correspondence
   with identifiers), boolean values, several forms of numbers, strings,
   vectors, and perhaps a few others.
2. Scheme provides a domain predicate for each domain of constants. 
   It also provides a domain predicate for procedural values (which is
   the negation of all other domain predicates because Scheme has a
   FIXED number of domains).
3. Scheme provides several ways for constructing values in each
   domain, selecting components and destructively altering
   components (if there are any).

V. Pragmatics: Scheme comes with a programming philosophy: Use the
least expressive fragment (starting with the functional core
progressing to continuations and side-effects), in which a problem can
be formulated naturally.

--- This is all there is. The rest is noise. -- Also, Standard ML
satisfies all of the above modulo I, II and IV(2) [crucial!], III(4)
[replaceable by reference cells as in Chez Scheme].

The above classification borrows some elements from Landin's [1]
informal attempt at classifying languages and my own first attempt [2] at
formalizing the ideas that were behind Landin's stuff and the
original Scheme design [3].

[1] {Landin, P.J}. The next 700 programming languages.  {\it Commun.
ACM \bf9}(3), 1966, 157--166.

[2] {Felleisen, M}.  On the expressive power of programming languages.
In {\it Proc. 1990 European Symposium on Programming}.  Neil Jones,
Ed. Lecture Notes in Computer Science, 432. 1990, 134--151.

[3] {Steele, G.L., Jr. and G.J. Sussman}. Lambda: The ultimate
imperative. Memo 353, MIT AI Lab, 1976.

-- Matthias