Dicte

E. Parsing and Execution


A sentence is evaluated by executing its phrases in a sequence
determined by the parsing rules of the language.
For example, in the sentence 10%3+2 ,
the phrase 3+2 is evaluated first to obtain a result
that is then used to divide 10. In summary:

One important consequence of these rules is that in an
unparenthesized expression the right argument of any verb is
the result of the entire phrase to its right. The sentence 3*p%q^|r-5
can therefore be read from left to right: the overall result
is 3 times the result of the remaining phrase,
which is the quotient of p and the part following
the % , and so on.


Parsing proceeds by moving successive elements (or
their values except in the case of proverbs and names
immediately to the left of a copula) from the tail end of a queue
(initially the original sentence prefixed by a
marker §) to the top of a stack
(initially four markers),
and eventually executing some eligible portion of the stack and
replacing it by the result of the execution.
For example, if a=: 1 2 3, then b=:+/2*a
would be parsed and executed as follows. (The four markers at the bottom
of the stack are not shown.)

   § b =: + / 2 * a               
   § b =: + / 2 *            1 2 3      Move
   § b =: + / 2           *  1 2 3      Move
   § b =: + /          2  *  1 2 3      Move
   § b =: +         /  2  *  1 2 3      Move
   § b =: +               /  2 4 6    2 Dyad
   § b =:              +  /  2 4 6      Move
   § b             =:  +  /  2 4 6      Move
   § b               =:  +/  2 4 6    3 Adverb
   § b                      =:  12    0 Monad
   §                     b  =:  12      Move
   §                            12    7 Is
                              § 12

The foregoing illustrates two points:
1) Execution of the phrase 2 * 1 2 3 is deferred until
the next element (the /) is transferred;
had it been a conjunction, the 2 would have been
its argument, and the monad * would have applied
to 1 2 3; and
2) Whereas the value of the name a moves to the stack,
the name b (because it precedes a copula) moves unchanged,
and the pronoun b is assigned the value 12.


Parsing can be observed using the trace
facility in systempackagesmisctrace.ijs .



The executions in the stack are confined to the first four
elements only, and eligibility for execution is determined
only by the class of each element (noun, verb, etc.,
an unassigned name being treated as a verb),
as prescribed in the following parse table.
The classes of the first four elements of the stack are
compared with the first four columns of the table, and the
first row that agrees in all four columns is selected.
The bold italic elements in the row are then subjected
to the action shown in the final column, and are replaced
by its result. If no row is satisfied, the next element
is transferred from the queue.