McCarthy's definition of LISP 1.5 in itself

gistfile1.txt

; 20LOC to define a useful language
; However, no strings, IO, closure semantics or GC.

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];pairlis[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]]

evcon[c;a] = [eval[caar[c];a] -> eval[cadar[c];a];
              T -> evcon[cdr[c];a]]

evlis[m;a] = [null[m] -> NIL;
              T -> cons[eval[car[m];a];evlis[cdr[m];a]]]