The Efficient Administration of Blocks in ALGOL

The block structure of ALGOL, with its nested levels of nomenclature, is invaluable to the programmer but gives rise to difficulties in implementation. Briefly, the most serious troubles stem from the fact that reference must only be made to blocks with currently valid declarations. Within a procedure, and in particular a recursive procedure, it is not always easy to discover which blocks are active. As far as the procedure body is concerned the valid declarations are those valid at the time the procedure is declared, whereas the declarations valid for the actual parameters are those valid at the time of the procedure call. Particularly troublesome are go to statements leading out of the procedure, especially if used recursively or if there is recursion involving several procedures, because then it is necessary to reach the labelled statement in the correct activation of its block as well as restoring the declarations for the appropriate activations of the enclosing blocks.