higher order function in pure C

bool.c

/*
WE WANT THAT:

true = λx.λy.x
false = λx.λy.y

not = λx.x false true
and = λx.λy.x y false
or = λx.λy.x true y
*/

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

// Define the boolean function type
typedef void* (*bool)(void* x, void* y);

// Define 'true' and 'false' functions
void* true_fn(void* x, void* y) {
    return x;
}

void* false_fn(void* x, void* y) {
    return y;
}

// Define 'not' function
bool not_fn(bool x) {
    return (bool)x(false_fn, true_fn);
}

// Define 'and' function
bool and_fn(bool x, bool y) {
    return (bool)x(y, false_fn);
}

// Define 'or' function
bool or_fn(bool x, bool y) {
    return (bool)x(true_fn, y);
}

// Functions to represent boolean values as strings for testing
const char* true_str() {
    return "true";
}

const char* false_str() {
    return "false";
}

// Function to get the string representation of a boolean
const char* to_string(bool b) {
    return (const char*)b((void*)true_str(), (void*)false_str());
}

int main() {
    // Define the boolean values
    bool t = true_fn;
    bool f = false_fn;

    // Array of booleans for iteration
    bool bools[] = { t, f };
    const char* bool_names[] = { "true", "false" };

    // Print truth table for NOT
    printf("Truth table for NOT:\n");
    for (int i = 0; i < 2; i++) {
        bool a = bools[i];
        printf("NOT %s: %s\n", to_string(a), to_string(not_fn(a)));
    }
    printf("\n");

    // Print truth table for AND
    printf("Truth table for AND:\n");
    for (int i = 0; i < 2; i++) {
        bool a = bools[i];
        for (int j = 0; j < 2; j++) {
            bool b = bools[j];
            printf("%s AND %s: %s\n", to_string(a), to_string(b), to_string(and_fn(a, b)));
        }
    }
    printf("\n");

    // Print truth table for OR
    printf("Truth table for OR:\n");
    for (int i = 0; i < 2; i++) {
        bool a = bools[i];
        for (int j = 0; j < 2; j++) {
            bool b = bools[j];
            printf("%s OR %s: %s\n", to_string(a), to_string(b), to_string(or_fn(a, b)));
        }
    }

    return 0;
}

hof.c

#include <stdio.h>

typedef int (*FoldFunc)(int acc, int elem);

int foldl(int *arr, int acc, FoldFunc f, size_t len) {
  for (int i = 0; i < len; i++) {
    acc = f(acc, arr[i]);
  }

  return acc;
}

int add(int a, int b) {
  return a + b;
}

int main() {
  puts("i'm about to do something blasmefemous...\n");

  int arr[] = {1, 2, 3, 4, 5};
  puts("we have a simple array, why not to use HOF?\n");
  size_t len = sizeof(arr) / sizeof(arr[0]);

  int sum = foldl(arr, 0, add, len);
  printf("Sum: %d\n", sum);

  return 0;
}