SCOUG-Programming Mailing List Archives

Date:	Sat, 25 Nov 2006 08:01:59 -0800
From:	"Lynn H. Maxson" <lmaxson@pacbell.net >
Reply-To:	scoug-programming@scoug.com
To:	"SCOUG Programming SIG" <scoug-programming@scoug.com >
Subject:	SCOUG-Programming: A PL/I--SL/I Primer

Content Type: text/plain

SL/I shares PL/I's syntax. Every program element is a
statement. Every statement ends in a semi-colon (;). No
exceptions.

SL/I shares PL/I's respect for element variables. All
operations, including those involving aggregates, in PL/I occur
on an element by element basis. No exceptions.

SL/I shares PL/I's support of operations on aggregates, i.e.
arrays (homogeneous) and structures (heterogeneous). SL/I
adds "native" support for list aggregates a la LISP. SL/I adds
"native" support for APL operators beyond those present in
PL/I.

SL/I like PL/I does not rely on a separate, possibly third-party,
library. Instead both rely on a set of built-in functions, all of
which are known to the interpreter/compiler essentially as
"source code". Beyond this SL/I as an interpreter/compiler
uses source code input only contained within a single library.

As SL/I has a "native" list aggregate it can extend the
"assignment" statement with the addition of an "assertion"
statement. This results in taking PL/I from its third generation
HLL status to SL/I's fourth generation HLL. Where an
"assignment" statement may return one and only one "true"
result an "assertion" may return zero, one or more. Thus an
"assertion" may return a "false" (zero "true" results) or one
or more "true" results: a list aggregate of zero (null list), one
or more entries.

Note that if an assignment statement cannot return one and
only one true result the interpreter/compiler and the entire
program fails: no executable or execution possible. In the
compilation stage this is represented as a "severe" error. Note
that in SQL, a fourth generation language in which we pose a
query as an assertion, a query can "fail", i.e. produce no
"true" results or a zero (null) result. Thus what is
"catastrophic" for an assignment is "normal" for an assertion.

Just as with an SQL query (assertion) imbedded within a
program the programmer has to add logic accounting for its
possible "failure": no true instances. Just as with an imbedded
SQL query (assertion) the programmer has to add logic to
account for all true instances (one or more).

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