From Dan Grossman course. How to use closures in C.

cl.c

#include <stdlib.h>
#include <stdint.h>
#include <stdbool.h>
#include <stdio.h>
typedef struct List list_t;
    struct List {
    void * head;
    list_t * tail;
};
list_t * makelist (void * x, list_t * xs) {
    list_t * ans = (list_t *)malloc(sizeof(list_t));
    ans->head = x;
    ans->tail = xs;
    return ans;
}
list_t * map(void* (*f)(void*,void*), void* env, list_t * xs) {
    if(xs==NULL)
    return NULL;
    return makelist(f(env,xs->head), map(f,env,xs->tail));
}
list_t * filter(bool (*f)(void*,void*), void* env, list_t * xs) {
    if(xs==NULL)
        return NULL;
    if(f(env,xs->head))
        return makelist(xs->head, filter(f,env,xs->tail));
    return filter(f,env,xs->tail);
}
int length(list_t* xs) {
    int ans = 0;
    while(xs != NULL) {
        ++ans;
        xs = xs->tail;
    }
    return ans;
}
void* doubleInt(void* ignore, void* i) { // type casts to match what map expects
    return (void*)(((intptr_t)i)*2);
}
list_t * doubleAll(list_t * xs) { // assumes list holds intptr_t fields
    return map(doubleInt, NULL, xs);
}
bool isN(void* n, void* i) { // type casts to match what filter expects
    return ((intptr_t)n)==((intptr_t)i);
}
int countNs(list_t * xs, intptr_t n) { // assumes list hold intptr_t fields
    return length(filter(isN, (void*)n, xs));
}
void printList(list_t * xs){
    while(xs != NULL){
        printf("%lu\n",((intptr_t)xs->head));
        xs = xs->tail;
    }
}
int main() {
    list_t *x1 = makelist((void *)1,NULL);
    list_t *x2 = makelist((void *)2,x1);
    printList(x2);
    list_t * x3 = doubleAll(x2);
    printList(x3);
    return 0;
}