C enumerator/generator; messing about with C macros - see macro expansion here: https://gist.github.com/jammy-dodgers/521e0e41ad8e042d19412a39484a2dcb

IEnumerable.c

#include <stdio.h>
#define foreach(GENERATOR_TYPE, GENERATOR, LOOP_VAL, BODY) { struct GENERATOR_TYPE foreach_loop_var; foreach_loop_var = GENERATOR; \
while (GENERATOR_TYPE ## _has_next(&foreach_loop_var)) { LOOP_VAL = foreach_loop_var.current; BODY }}
#define yield(NUM, VAR) vars->current = (VAR); vars->pos = NUM; return 1; case NUM:;
#define generator(GEN_TYPE, GEN_NAME, LOCALS, BODY) struct GEN_NAME { int pos; int current; LOCALS}; \
struct GEN_NAME new_ ## GEN_NAME() {struct GEN_NAME gener_temp_init_var; gener_temp_init_var.pos = 0; return gener_temp_init_var;} \
int GEN_NAME ## _has_next(struct GEN_NAME * vars) { switch(vars->pos) { case 0: BODY; return 0; } }


generator(int, Fibonacci, int n1; int n2;, { 
    vars->n1 = 0;
    vars->n2 = 1;
    while (vars->n1 < 10000) {
        int tmp = vars->n1;
        vars->n1 = vars->n2;
        vars->n2 = tmp + vars->n2;
        yield(1, tmp)
    }
})

int main() {
    struct Fibonacci fib = new_Fibonacci();
    foreach(Fibonacci, fib, int i, {
        printf("%d\n", i);
    })
}