HOPL

The History of Programming Languages conference is held by SIGPLAN to discuss and record the history of programming language development. It is notable to APL historians because of the very detailed papers presented at the 1978 and 2020 conferences. ACM SIGPLAN is the Special Interest Group on Programming Languages; while SIGAPL is a chapter of SIGPLAN, it is not directly associated with HOPL.

To date, there have been four HOPL conferences: they are held infrequently, but with increasing frequency. The first, in 1978, featured a presentation by Ken Iverson and Adin Falkoff on APL. The fourth conference, in 2020, features an APL presentation again, this time by Morten Kromberg and Roger Hui.

Conference years appear to be given by the expression:



HOPL IV conference badge
One of the conference badges for HOPL IV features an APL expression $$\div+\!/\div(\mathcal E \ne 0)/\mathcal E$$ or  which computes multiple aspects of electrical circuits:

The equivalent mathematical formula would be quite involved:


 * $$1\over\displaystyle\sum_{i=1}^n\frac1{E_i}[E_i\neq 0]$$