trampoline.c

#include <stdio.h>
#include <stdlib.h>

typedef int (*Fn) (int);

typedef struct _list {
  int n;
  Fn f;
} list;

int odd (int x)
{
  return x*2+1;
}

int fact (int x)
{
  if (x == 1 || x == 0)
    return 1;
  return x * fact(x - 1);
}

int head (void *l)
{
  list *lst = (list *)l;
  return lst->f(lst->n);
}

void* tail (void *l)
{
  list *lst = (list *)l;
  lst->n += 1;
  return l;
}

void* build(Fn f)
{
  list *lst = (list *) malloc(sizeof(list));
  lst->n = 0;
  lst->f = f;
  return lst;
}

int trampoline(Fn f, int n)
{
  void* lst = build(f);
  for (int i = 0; i < n; i++) {
    lst = tail(lst);
  }

  return head(lst);
}

int main()
{
  printf("Trampoline odd value: %d\n", trampoline(&odd, 5));
  printf("Factorial of 4 = %d", trampoline(&fact, 4));
  return 0;
}