SLIP

What SLIP is

SLIP stands for Symmetric LIst Processor. Weizenbaum designed it around 1962 and described it in the Communications of the ACM in 1963, when the list-processing ideas of IPL and LISP were still new. It is not a standalone language but a set of routines bolted onto a host language, first FORTRAN and then MAD. The version ELIZA used ran on MIT’s CTSS on the IBM 7094: MAD for the procedures, SLIP for the lists.

Why “symmetric”

SLIP’s lists are doubly linked: every cell holds a pointer to the next cell and to the previous one. That two-way, symmetric linkage is where the name comes from. It lets a program walk a list forwards or backwards, splice cells in and out from either end, and treat any cell as a place to read from or write to. Free cells are kept on an Available Space List (AVSL); creating a list draws cells from it, deleting a list returns them.

A line of dialogue as a list

When you type to ELIZA, your words become a SLIP list, one word per cell. Here is the opening line of the 1966 conversation, Men are all alike, held the way ELIZA holds it:

What ELIZA added to SLIP

The original SLIP has two distinct modes. One handles organisation: the previous and next pointers and the sublists they string together. The other handles data, and in the original it was given short shrift, supporting only integers and real numbers. Weizenbaum extended the data half so a cell could also carry text, the words of a sentence, the keywords of a script, the templates of a reply. Most of the original SLIP is pointer maintenance; most of the SLIP that ELIZA needs is datum handling.

How ELIZA uses it

Almost every structure in ELIZA is a SLIP list. The user’s input is read into a list of words. The script’s keywords live in a hash table (KEY) of lists. Each keyword’s decomposition and reassembly rules are lists of lists. The memory of what you said is a list of transformed sentences. To produce a reply, ELIZA walks these lists with SLIP’s readers, matching and rebuilding as it goes.

Reading the idioms

Once you know they are SLIP calls, the dense lines of the recovered source begin to read. A few recur throughout:

• SEQRDR / SEQLR: make a sequence reader for a list, then read its next cell left to right. This is how ELIZA scans a list.
• POPTOP / POPBOT: remove and return the cell at the top (or bottom) of a list.
• NEWTOP / NEWBOT: add a new cell at the top (or bottom) of a list.
• HASH: turn a word into a table index, used both for the keyword table and for the memory mechanism.
• LSSCPY, SUBST, IRALST: copy a list, substitute a cell, and return a list’s cells to free space.

These are the verbs of ELIZA. The close reading of the program follows them through the main loop, and the step-by-step demo shows the result.