M-expression

In it he described a language of symbolic expressions (S-expressions) that could represent complex structures as lists.

Finally, he showed how the meta-language itself could be represented with S-expressions, resulting in a system that was potentially self-hosting.

Although McCarthy disapproved of the idea, Russell and colleague Daniel J. Edwards hand-coded an interpreter program that could execute S-expressions.

[2] This program was adopted by McCarthy's research group, establishing S-expressions as the dominant form of Lisp.

[8] A CGOL (1977) was implemented in MacLisp and follows a similar goal of introducing Algol-like syntax with infix operators.

[9] A more recent (circa 2003) variant is the I-expression, which use indentation to indicate parentheses implicitly, and are thus in some ways intermediate between S-expressions and M-expressions.

Like I-expressions, t-expressions are only a simple transformation away from S-expressions, so that theoretically they can be used on any Lisp dialect and not interfere with features like macros.