2.2. Null clauses

Null clauses are clauses that consist of only whitespace, or
comments, or both; in addition to the terminating clause
delimiter.  These clauses are ignored when interpreting the code,
except for one situation: null clauses containing at least one
comment is traced when appropriate.  Null clauses not containing
any comments are ignored in every respect.

Example: Tracing comments

The tracing of comments may be a major problem, depending on the
context. There are basically two strategies for large comments:
either box multiple lines as a single comment, or make the text on
each line an independent comment, as shown below:

     trace all
     
     /*
       This is a single, large comment, which spans multiple lines.
       Such comments are often used at the start of a subroutine or
       similar, in order to describe both the interface to and the
       functionality of the function.
     */


     /* This is also a large comment, but it is written as multiple   */
     /* comments, each on its own line. Thus, this is several clauses */
     /* while the comment above is a single comment.*/

During tracing, the first of these will be displayed as one large
comment, and during interactive tracing, it will only pause once.
The second will be displayed as multiple lines, and will make
several pauses during interactive tracing. An interpreter may
solve this situation in several ways, the main objective must be
to display the comments nicely the to programmer debugging the
code.  Preferably, the code is shown in a fashion that resembles
how it is entered in the file.



If a label is multiple defined, the first definition is used and
the rest are ignored. Multiple defined labels is not an SYNTAX
condition.

A null clause is not a statement. In some situations, like after
the THEN subclause, only a statement come. If a null clause is
provided, then it is ignored, and the next statement is used
instead.

Consider the following code:

     parse pull foo
     
     if foo=2 then
         say ‘foo is not 2’
     else
         /* do nothing */
     say ‘that”s it’


This will not work the way indentation indicates, since the
comment in this example is not a statement. Thus, the ELSE reads
beyond the comment, and connects to the SAY instruction which
becomes the ELSE part. (That what probably not what the programmer
intended.)  This code will say that’s it, only when foo is
different from 2.  A separate instruction, NOP has been provided
in order to fill the need that was inadequately attempted filled
by the comment in the code fragment above.


Example: Trailing comments

The effect that comments are not statements can be exploited when
documenting the program, and simultaneously making the program
faster. Consider the following two loops:

     sum = 0
     do i=1 to 10
     /* sum 1 2 3 ... 8 9 10 */
          sum = sum + i
     end
     
     sum = 0
     do i=1 to 10
          sum = sum + i   /* sum 1 2 3 ... 8 9 10 */
     end

In the first loop, there are two clauses, while the second loop
contains only one clause, because the comment is appended to an
already existing clause. During execution, the interpreter has to
spend time ignoring the null clause in the first loop, while the
second loop avoids this problem (assuming tracing is unenabled).
Thus, the second loop is faster; although only insignificantly
faster for small loops. Of course, the comment could have been
taken out of the loop, which would be equally fast to the second
version above.



 PREV   NEXT