McCarthy himself: https://en.wikipedia.org/wiki/M-expression https://en.wikipedia.org/wiki/M-expression The project of defining M-expressions precisely and co

McCarthy himself: https://en.wikipedia.org/wiki/M-expression https://en.wikipedia.org/wiki/M-expression 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.