Knotted list structures

• Authors: J. Weizenbaum
• Venue: ACM '61 Proceedings of the 1961 16th ACM national meeting
• Year: 1961

In (3) Perlis and Thornton describe a list structure language the central innovation of which is that the last word of each list specifies the location of the head of the list of which it is the terminal word. They call such list structures threaded lists. Beyond this, however, they hypothesize Sequencing Tables and associated sequencing modes (e, l, and w). The sequencing modes make possible forms of traversing list structures which leave the lists themselves undisturbed in memory. Since the Sequence Table can contain true names as well as aliases for particular list structures and since lists are undisturbed by sequencing, many programs may sequence through lists in different manners, at various rates, but still simultaneously. This partially mitigates against the need to store many copies of a list and becomes critical in multiple computer systems in which it may be expected that a number of sub-programs operate on a single list structure (under various aliases) independently. This note describes a generalization of the threaded list. The name given to such generalized threaded list structures is knotted list structures (KLS). The generalization consists essentially of replacing the thread link which Perlis and Thornton place in the R or link position of the last list cell of a list (or sub-list with a Control List (CL) which is attached to the list structure and which contains, among other information, a push down list capable of storing many thread links. The chief advantage of this scheme is that it permits a sub-list to be a sub-list of many lists, something which ordinary threaded lists do not permit. All other advantages of threaded lists are kept.