II. Notes on Data Structuring * - Cornell University

9.2. EXAMPLE
NOTES ON DATA STRUCTURING 1,47
A source text for an expression in a programming language is presented
as a sequence of symbols defined:
type symbol = (constant:(value: real), variable:(identifier:ident),
op" operator, leftbracket, rightbracket);
Write a program operating on an input variable
source:sequence symbol,
which reads from its beginning the longest possible legitimate expression,
delivers the corresponding abstract expression as a result, and exits to the
label error if this is impossible. The structure of the result and the syntax of
the source are as specified earlier in this chapter.
The structure of the program closely follows that of the desired result.
There are three functions:
compile expression
compile term (sign)
compile primary
each of which removes from the source the longest expression in its syntactic
category, and delivers the corresponding abstract structure as a resulf. The
main irregularity of the process is that the first term of an expression may be
unsigned; this is why the sign is provided as a parameter for compile term,
instead of being read from source by compile term itself. Each function has
the side-effect of shortening the source sequence if successful, and jumping
to error if not.
function compile expression: expression;
begin sign" operator;
end;
if source, first = plus v source, first = minus then sign from source
compile expression:= [compile term (sign)];
else sign: = plus;
while source, first = plus v source.first = minus do
begin sign from source;
end
function compile term (s: operator): term;
compile expression :''[compile term (sign)]
begin p" primary; sign" operator; fs" sequence factor;