Discriminated Tagged Union Expressions with Pattern Matching Dispatch

#include <stdio.h>
 
typedef enum { NODE_INT, NODE_ADD } NodeType;
 
struct Node;
 
typedef struct {
    struct Node* left;
    struct Node* right;
} BinaryOp;
 
typedef struct Node {
    NodeType type;
    union {
        int value;
        BinaryOp op;
    } as;
} Node;
 
int eval_node(const Node* node) {
    switch (node->type) {
        case NODE_INT:
            return node->as.value;
        case NODE_ADD:
            return eval_node(node->as.op.left) + eval_node(node->as.op.right);
    }
    return 0;
}