lazy_eval.c

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

typedef void *(ThunkFunc)(void *);

typedef struct thunk Thunk;
struct thunk {
	void *result;
	ThunkFunc *func;
	void *param;
};

void *force_thunk(Thunk *thunk)
{
	if (thunk->result == NULL) {
		thunk->result = thunk->func(thunk->param);
	}
	return thunk->result;
}

void mkthunk_from_ref(Thunk *thunk, ThunkFunc *func, void *param)
{
	thunk->result = NULL;
	thunk->func = func;
	thunk->param = param;
}

void *thunk_func(void *param)
{
	int *p;

	(void) param;
	puts("foo");
	p = malloc(sizeof(int));
	*p = 10;
	return p;
}

int main(void)
{
	Thunk thunk;

	mkthunk_from_ref(&thunk, thunk_func, NULL);
	puts("bar");
	printf("%d\n", *(int *) force_thunk(&thunk));
	printf("%d\n", *(int *) force_thunk(&thunk));
	return 0;
}