allow unary operations.

c/mul/main.c

#include "mpc.h"
long eval_op(long x, char* op, long y) {
long eval_binary_op(char* op, long x, long y) {
	if (strcmp(op, "+") == 0) { return x + y; }
	if (strcmp(op, "-") == 0) { return x - y; }
	if (strcmp(op, "*") == 0) { return x * y; }
	return 0;
}
long eval_unary_op(char* op, long x) {
	if (strcmp(op, "-") == 0) { return -x; }
	return x;
}
long eval(mpc_ast_t* t) {
	if (strstr(t->tag, "number")) {
		return atoi(t->contents);
	char* op = t->children[1]->contents;
	long x = eval(t->children[2]);
	int i = 3;
	while (strstr(t->children[i]->tag, "expr")) {
		x = eval_op(x, op, eval(t->children[i]));
		i++;
	}
	if (t->children_num == 4) {
		return eval_unary_op(op, x);
	} else {
		int i = 3;
		while (strstr(t->children[i]->tag, "expr")) {
			x = eval_binary_op(op, x, eval(t->children[i]));
			i++;
		}
	return x;
		return x;
	}
}
int main(int argc, char** argv) {