thumb|100px|John McCarthy
In Lisp programming language, M-expressions (or meta-expressions) were an early proposed syntax for the Lisp, inspired by contemporary languages such as Fortran and ALGOL. The notation was never implemented into the language and, as such, it was never finalized.
M-expressions are a syntax for LISP code and provide function notation, syntax for a form and for embedded literal data (via S-expressions) into programs. Thus M-Expressions used S-Expressions for literal data. The syntax for S-Expressions ("The Data Language") and M-Expressions ("The Meta Language") is defined on pages 8 and 9 of the Lisp 1.5 manual. The draft version of this paper is known as "AI Memo 8".
{| class="wikitable"
|+ Example M-expressions (LISP 1.5, 1965) This program was adopted by McCarthy's research group, establishing S-expressions as the dominant form of Lisp.
McCarthy reflected on the fate of M-expressions in 1979:
The book Anatomy of LISP by John Allen explains the definition of M-expressions and uses them throughout the book to explain Lisp and its implementation.
Examples
The definitions for the functions apply and eval from the Lisp 1.5 Manual, page 13.
apply[fn;x;a] =
[atom[fn] →
[eq[fn;CAR] → caar[x];
eq[fn;CDR] → cdar[x];
eq[fn;CONS] → cons[car[x];cadr[x]];
eq[fn;ATOM] → atom[car[x]];
eq[fn;EQ] → eq[car[x];cadr[x]];
T → apply[eval[fn;a];x;a]];
eq[car[fn];LAMBDA] → eval[caddr[fn];parlis[cadr[fn];x;a]];
eq[car[fn];LABEL] → apply[caddr[fn];x;cons[cons[cadr[fn];caddr[fn]];a]]]
eval[e;a] =
[atom[e] → cdr[assoc[e;a]];
atom[car[e]] →
[eq[car[e],QUOTE] → cadr[e];
eq[car[e];COND] → evcon[cdr[e];a];
T → apply[car[e];evlis[cdr[e];a];a]];
T → apply[car[e];evlis[cdr[e];a];a]]
Using the function eval on an s-expression.
eval[(EQ (QUOTE A) (CAR (CONS (QUOTE A) (QUOTE (B C D)))));NIL]
Implementations
For LISP
MLisp was a contemporary (1968–1973) project to implement an M-expression-like frontend for Lisp. A few extra features like hygienic macros, pattern matching, and backtracking were incorporated. It eventually evolved into an abandoned LISP70 draft. M-LISP (MetaLISP) from 1989 was another attempt to blend M-expressions with Scheme.
Further development
A CGOL (1977) was implemented in MacLisp and follows a similar goal of introducing Algol-like syntax with infix operators. It is known to work on Armed Bear Common Lisp.
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. I-expressions were introduced in Scheme Request For Implementation 49 as an auxiliary syntax for Scheme, but they have not been widely adopted.
A further development is the "sweet" t-expression, which has infix operators without precedence. 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.
Additional syntax-related include Apple's Dylan (Algol-like tokens) and Clojure's addition of other literal syntaxes.