PL/I surprises
PHILIP G. BARKER · ITNOW · 1979
Few languages provide really good facilities for programming interactive dialogues through a visual display unit or teletype device. In contrast, PL/I provides a significant repertoire of input/output techniques [1–8]: of which the last mentioned is particularly useful as a tool to enable the programmer to implement interactive dialogues. However, potential users of this facility should be cautious since it is not featured in standard ANS PL/I [1]. Consequently, some manufacturers’ implementations of PL/I will not support this mode of input/output [7,8]. Where this is the case, analogous effects can be achieved through the invocation of suitable subroutine or function procedures. The discussion presented here relates to a somewhat old, shaky and creaky PL/I compiler—IBM’s F-level [9]—running under an inhouse operating system called MTS originating from the University of Michigan [10]. When the DISPLAY statement is used in a program it causes a message to be displayed to the machine operator. The operator may be required to reply to the program by typing a suitable message at the keyboard of his control console. The simplified dialogue in Figure 1 illustrates the type of interaction that might take place between a program and a machine operator.