M-expression

M-Expressionsso-called meta-expressions which were intendedbe usedLisp.

S-expressions intendedrepresent data structures or parsed mathematical expressions look like this:

(+ 4 (- 5 3))

whichsimply prefix notation(5-3) + 4 However,LISP, listsprogramming constructs such asconditional branchalso representedthis way, e.g.

 (if (>5) dothis orelsedothis)

A representation was developed so that this could be written down inmore user friendly way,example [1, 2, 3] forlist. These M-expressionsthen translatedS-expressionsbe executed, hencemeta designation.