[fexl]

OK, here's one more example of how to write a Fexl combinator. Back in 1924 the great Moses Schönfinkel demonstrated that all computable functions can be defined in terms of just two primitive functions: C (the constant function), and S (the fusion function). So here is the source code for S, the Mother of all functions:

  #include "node.h"
  #include "type_S.h"

  /*
  Fusion function (Verschmelzungfunktion)
  S x y z = ((x z) (y z))
  */
  static void step(void)
      {
      int f1 = pop();
      int f2 = pop();
      int f3 = pop();
      if (!f3) return;

      set_pair(f3, P(R(f1),R(f3)), P(R(f2),R(f3)));
      }

  int type_S(void)
      {
      static int node = 0;
      if (node == 0) node = new_combinator(step);
      return node;
      }