What are S1 and S2 classes?

In R there are S3, S4 and with R 2.12, reference classes (unofficially called S5 classes).

Is there such a thing as a S1 (or S2) class? If so, what are they? If not, why start wi开发者_运维知识库th S3?

R is developed from the S-language, a statistical language developed at the Bell Labs. S3 refers to the third version of S. The syntax of R is largely based on this version of S, and hence the classes were called S3. In R, there have never been S1 and S2 classes. And as Gavin said, neither in S. Object-orientation only started in the third version.

A brief history by Chambers

a paper on the history by Ihaka