write-you-a-haskell/chapter10/eval.c

typedef struct T {
    enum { ADD, MUL, DIV, SUB, NUM } tag;
    union {
        struct {
            struct T *left, *right;
        } node;
        int value;
    };
} Expr;

int eval(Expr t)
{
    switch (t.tag) {
        case ADD:
            return eval(*t.node.left) + eval(*t.node.right);
            break;
        case MUL:
            return eval(*t.node.left) * eval(*t.node.right);
            break;
        case DIV:
            return eval(*t.node.left) / eval(*t.node.right);
            break;
        case SUB:
            return eval(*t.node.left) - eval(*t.node.right);
            break;
        case NUM:
            return t.value;
            break;
    }
}