EWD249 - 28

if stopping was due to some kind of dynamic error, how can we identify the point of progress short of a complete memory dump?

For the sake of simplicity we assume our program text spread out in (linear) text space and assume an identifying mechanism for the program points corresponding to the discrete points of computation progress; let us call this identifying mechanism "the textual index". (If the discrete points of computation progress are situated in between successive statement executions, the textual index identifies, say, semicolons.) The textual index is a kind of generalized order counter, its value points to a place in the text.

If we restrict ourselves to decomposition by concatenation and selection, a single textual index is sufficient to identify the progress of the computation. With the inclusion of repetition clauses textual indices are no longer sufficient to describe the progress of the computation. With each entry into a repetition clauses, however, the system could introduce a so-called "dynamic index", inexorably counting the ordinal number of the corresponding current repetition; at termination of the repetition the system should again remove the corresponding dynamic index. As repetition clauses may occur nested inside each other, the appropriate mechanism is a stack (i.e. a last-in-first-out-memory). Initially the stack is empty; at entry of a repetition clause a new dynamic index (set to zero or one) is added on the top of the stack; whenever it is decided that the repetition is not terminated the top element of this stack is increased by 1; whenever it is decided that a repetition is terminated, the top element of the stack is removed. (This arrangement reflects very clearly that after termination of a repetition the number of times, even the fact that it was a repetition, is no longer relevant.)

As soon as the programming language admits procedures, then a single textual index is no longer sufficient. In the case that a textual index points to the interior of a procedure body, the dynamic progress of the computation is only characterized when we also describe to which call of the procedure we refer, but this can be done by giving the textual index pointing to the place of the call. With the inclusion of the procedure the textual index must be generalized to a stack of textual indices, increased by one element at procedure call and decreased by one element at procedure return.