Compaq AA-PWCBD-TE Ctrl/C Handling, Return Statement, Example 4-12 Procedure That Returns a Value

Lexical Elements of the DEC Text Processing Utility Language

4.9Reserved Words

•If the error is not due to you pressing Ctrl/C, the error message is written to the message buffer and DECTPU performs the equivalent of the following sequence:

special_error_symbol := 0;

LEARN_ABORT; RETURN (FALSE);

In a procedure with a case-style error handler, an ABORT statement produces the same effect as the sequence Ctrl/C, with one exception: an ABORT statement in the TPU$_CONTROLC clause of a case-style error handler does not reinvoke the TPU$_CONTROLC clause, as is the case when Ctrl/C is pressed while TPU$_CONTROLC is executing. Instead, an ABORT statement causes DECTPU to exit from the error handler and look for a TPU$_CONTROLC selector in the procedures or program (if any) in which the current procedure is nested. If DECTPU does not find a TPU$_CONTROLC selector in the containing procedures or program, DECTPU places the message associated with TPU$_CONTROLC in the message buffer.

4.9.4.17 Ctrl/C Handling

The ability to trap a Ctrl/C in your DECTPU program is both powerful and dangerous. When you press Ctrl/C, you usually want the application that is running to prompt for a new command. The ability to trap the Ctrl/C is intended to allow a procedure to clean up and exit gracefully.

4.9.4.18 RETURN Statement

The RETURN statement causes a return to the procedure that called the current procedure or program. The return is to the statement that follows the statement that called the current procedure or program. You can specify an expression after the RETURN statement and the value of this expression is passed to the calling procedure.

Syntax

RETURN expression;

The expression is optional; if it is missing, DECTPU supplies a 0. Also, the RETURN statement itself is optional. That is, if DECTPU reaches the endprocedure of a procedure before encountering a RETURN statement, it will return 0.

Example 4–12 shows a sample procedure in which a value is returned to the calling procedure.

Example 4–12 Procedure That Returns a Value

PROCEDURE user_get_shift_key

LOCAL key_to_shift; ! Keyword for key pressed after shift key

SET (SHIFT_KEY, LAST_KEY);

key_to_shift := KEY_NAME (READ_KEY, SHIFT_KEY); RETURN key_to_shift;

ENDPROCEDURE;

In addition to using RETURN to pass a value, you can use a 1 (true) or a 0 (false) with the RETURN statement to indicate the status of a procedure. Example 4–13 shows this usage of the RETURN statement.

4–28Lexical Elements of the DEC Text Processing Utility Language