Order of evaluation

PL/I statements often contain more than one expression or reference. Except as described for specific instances (for example, the assignment statement), evaluation can be in any order, or (conceptually) at the same time.

Consider the following example:

  dcl (X,Y,Z) entry returns(float), (F,G,H) float;
  F = X( Y(G,H), Z(G,H) );

The functions Y and Z can change the value of the arguments passed to them. Hence, the value returned by X might be different depending on which function is invoked first. You should not presume that the first parameter is evaluated first. In some situations, it is more optimal to evaluate the last first.

Assuming that the INC function increments the value of the argument passed to it and returns the updated value, the example that follows could put out B(1,2) or B(2,1) depending on which subscript is evaluated first. You should not presume which subscript is evaluated first.

  dcl B(2,2);
  I = 0;
  put list ( B( INC(I), INC(I) ) );