Chapter 16. Reserved Keywords

Keywords can be used as ordinary symbols in many unambiguous situations. The precise rules are given in this chapter.

The free syntax of Rexx implies that some symbols are reserved for use by the language processor in certain contexts.

Within instructions like IF, DO, or SELECT, some symbols can be reserved to separate the parts of the instruction. These symbols are referred to as keywords. Examples of Rexx keywords are the WHILE keyword in a DO instruction and the THEN keyword, which acts as a clause terminator in this case, following an IF or WHEN clause.

While typically these reserved keywords act as expression terminators, using them as message names in message terms, as Variable Reference terms, or with parentheses around or within the expression, can prevent these keywords from terminating the expression.

Here is an example how in a DO loop a variable named UNTIL can still be used in a WHILE expression by putting parentheses around it.

Example 16.1. Instructions — DO WHILE expression with parentheses

do i = 1 while (i < until)
   say i
end

Apart from these cases, only simple symbols that are the first token in a clause and that is not a label, not an assignment or extended assignment, and not a message instruction are checked to see if they are instruction keywords. The symbols can be freely used elsewhere in clauses without being understood as keywords.