The function calling mechanism is identical with that
for subroutines. The only difference between functions and subroutines
is that functions must return data, whereas subroutines need not.
The following types of routines can be called as functions:
- Internal
- If the routine name exists as a label in the program, the current
processing status is saved, so that it is later possible to return
to the point of invocation to resume execution. Control is then passed
to the first label in the program that matches the name. As with a
routine called by the CALL instruction, various other status information
(TRACE and NUMERIC settings and so forth) is saved too. See the CALL
instruction ( Purpose) for details
about this. You can use SIGNAL and CALL together to call an internal
routine whose name is determined at the time of execution; this is
known as a multi-way call (see SIGNAL).
If you are calling an internal routine as a function, you must specify an expression in any RETURN instruction to return from it.
This is not necessary if it is called as a subroutine.
- Example:
-
/* Recursive internal function execution... */
arg x
say x'! =' factorial(x)
exit
factorial: procedure /* Calculate factorial by */
arg n /* recursive invocation. */
if n=0 then return 1
return factorial(n-1) * n
While searching for an internal label, syntax checking
is performed and the exec is tokenized. See
Performance Considerations for more details. FACTORIAL is
unusual in that it calls itself (this is recursive invocation). The
PROCEDURE instruction ensures that a new variable
n is created for each invocation.
Note: When there is a search
for a routine, the language processor currently scans the statements
in the REXX program to locate the internal label. During the search,
the language processor may encounter a syntax error. As a result,
a syntax error may be raised on a statement different from the original
line being processed.
- Built-in
- These functions are always available and are defined in the next
section of this manual.
- External
- You can write or use functions that are external to your program
and to the language processor. External routines must be written in
REXX. You can call a REXX program as a function and, in this case,
pass more than one argument string. The ARG or PARSE ARG instructions
or the ARG built-in function can retrieve these argument strings.
When called as a function, a program must return data to the caller.
Note: - External REXX functions can easily perform an EXEC CICS LINK to
a program written in any CICS-supported language. Also, REXX/CICS
command routines may be written in assembler.
- Calling an external REXX program as a function is similar to calling
an internal routine. The external routine is, however, an implicit
PROCEDURE in that all the caller's variables are always hidden and
the status of internal values (NUMERIC settings and so forth) start
with their defaults (rather than inheriting those of the caller).
- Other REXX programs can be called as functions. You can use either
EXIT or RETURN to leave the called REXX program, and in either case
you must specify an expression.
- With care, you can use the INTERPRET instruction to process a
function with a variable function name. However, you should avoid
this if possible because it reduces the clarity of the program.