Arthur's one-page interpreter fragment

typedef char C;
typedef long I;
typedef struct a { I t, r, d[3], p[2]; } * A;
#define P printf
#define R return
#define V1(f) A f(w) A w;
#define V2(f) A f(a, w) A a, w;
#define DO(n, x)                                                               \
  {                                                                            \
    I i = 0, _n = (n);                                                         \
    for (; i < _n; ++i) {                                                      \
      x;                                                                       \
    }                                                                          \
  }
I *ma(n) { R(I *) malloc(n * 4); }
mv(d, s, n) I *d, *s;
{ DO(n, d[i] = s[i]); }
tr(r, d) I *d;
{
  I z = 1;
  DO(r, z = z * d[i]);
  R z;
}
A ga(t, r, d) I *d;
{
  A z = (A)ma(5 + tr(r, d));
  z->t = t, z->r = r, mv(z->d, d, r);
  R z;
}
V1(iota) {
  I n = *w->p;
  A z = ga(0, 1, &n);
  DO(n, z->p[i] = i);
  R z;
}
V2(plus) {
  I r = w->r, *d = w->d, n = tr(r, d);
  A z = ga(0, r, d);
  DO(n, z->p[i] = a->p[i] + w->p[i]);
  R z;
}
V2(from) {
  I r = w->r - 1, *d = w->d + 1, n = tr(r, d);
  A z = ga(w->t, r, d);
  mv(z->p, w->p + (n * *a->p), n);
  R z;
}
V1(box) {
  A z = ga(1, 0, 0);
  *z->p = (I)w;
  R z;
}
V2(cat) {
  I an = tr(a->r, a->d), wn = tr(w->r, w->d), n = an + wn;
  A z = ga(w->t, 1, &n);
  mv(z->p, a->p, an);
  mv(z->p + an, w->p, wn);
  R z;
}
V2(find) {}
V2(rsh) {
  I r = a->r ? *a->d : 1, n = tr(r, a->p), wn = tr(w->r, w->d);
  A z = ga(w->t, r, a->p);
  mv(z->p, w->p, wn = n > wn ? wn : n);
  if (n -= wn)
    mv(z->p + wn, z->p, n);
  R z;
}
V1(sha) {
  A z = ga(0, 1, &w->r);
  mv(z->p, w->d, w->r);
  R z;
}
V1(id) { R w; }
V1(size) {
  A z = ga(0, 0, 0);
  *z->p = w->r ? *w->d : 1;
  R z;
}
pi(i) { P("%d ", i); }
nl() { P("\n"); }
pr(w) A w;
{
  I r = w->r, *d = w->d, n = tr(r, d);
  DO(r, pi(d[i]));
  nl();
  if (w->t)
    DO(n, P("< "); pr(w->p[i])) else DO(n, pi(w->p[i]));
  nl();
}

C vt[] = "+{~<#,";
A(*vd[])()={0,plus,from,find,0,rsh,cat},
(*vm[])()={0,id,size,iota,box,sha,0};
I st[26];
qp(a) { R a >= 'a' && a <= 'z'; }
qv(a) { R a < 'a'; }
A ex(e) I *e;
{
  I a = *e;
  if (qp(a)) {
    if (e[1] == '=')
      R st[a - 'a'] = ex(e + 2);
    a = st[a - 'a'];
  }
  R qv(a) ? (*vm[a])(ex(e + 1)) : e[1] ? (*vd[e[1]])(a, ex(e + 2)) : (A)a;
}
noun(c) {
  A z;
  if (c < '0' || c > '9')
    R 0;
  z = ga(0, 0, 0);
  *z->p = c - '0';
  R z;
}
verb(c) {
  I i = 0;
  for (; vt[i];)
    if (vt[i++] == c)
      R i;
  R 0;
}
I *wd(s) C *s;
{
  I a, n = strlen(s), *e = ma(n + 1);
  C c;
  DO(n, e[i] = (a = noun(c = s[i])) ? a : (a = verb(c)) ? a : c);
  e[n] = 0;
  R e;
}

main() {
  C s[99];
  while (gets(s))
    pr(ex(wd(s)));
}