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M-expression
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 cond 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.
M-Expressions also had a corresponding S-Expression representation. Code was manually translated from M-Expressions to S-Expressions. The in M-expressions embedded literal data, then had to be quoted in S-Expressions.
John McCarthy published the first paper on Lisp in 1960 while a research fellow at the Massachusetts Institute of Technology. In it he described a language of symbolic expressions (S-expressions) that could represent complex structures as lists. Then he defined a set of primitive operations on the S-expressions, and a language of meta-expressions (M-expressions) that could be used to define more complex operations. Finally, he showed how the meta-language itself could be represented with S-expressions, resulting in a system that was potentially self-hosting. The draft version of this paper is known as "AI Memo 8".
McCarthy had planned to develop an automatic Lisp compiler (LISP 2) using M-expressions as the language syntax and S-expressions to describe the compiler's internal processes. Stephen R. Russell read the paper and suggested to him that S-expressions were a more convenient syntax. Although McCarthy disapproved of the idea, Russell and colleague Daniel J. Edwards hand-coded an interpreter program that could execute S-expressions. 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 project of defining M-expressions precisely and compiling them or at least translating them into S-expressions was neither finalized nor explicitly abandoned. It just receded into the indefinite future, and a new generation of programmers appeared who preferred internal notation to any FORTRAN-like or ALGOL-like notation that could be devised.
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M-expression
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 cond 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.
M-Expressions also had a corresponding S-Expression representation. Code was manually translated from M-Expressions to S-Expressions. The in M-expressions embedded literal data, then had to be quoted in S-Expressions.
John McCarthy published the first paper on Lisp in 1960 while a research fellow at the Massachusetts Institute of Technology. In it he described a language of symbolic expressions (S-expressions) that could represent complex structures as lists. Then he defined a set of primitive operations on the S-expressions, and a language of meta-expressions (M-expressions) that could be used to define more complex operations. Finally, he showed how the meta-language itself could be represented with S-expressions, resulting in a system that was potentially self-hosting. The draft version of this paper is known as "AI Memo 8".
McCarthy had planned to develop an automatic Lisp compiler (LISP 2) using M-expressions as the language syntax and S-expressions to describe the compiler's internal processes. Stephen R. Russell read the paper and suggested to him that S-expressions were a more convenient syntax. Although McCarthy disapproved of the idea, Russell and colleague Daniel J. Edwards hand-coded an interpreter program that could execute S-expressions. 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 project of defining M-expressions precisely and compiling them or at least translating them into S-expressions was neither finalized nor explicitly abandoned. It just receded into the indefinite future, and a new generation of programmers appeared who preferred internal notation to any FORTRAN-like or ALGOL-like notation that could be devised.