Function needs_parentheses

prqlc/prqlc/src/sql/gen_expr.rs:1012–1030

For an operation represented as `a child b` with a surrounding parent operation (e.g., `(a child b) parent c` or `a parent (b child c)`): 1. When the child operator has higher precedence than the parent, parentheses *are not* required. 2. When the child operator has lower precedence than the parent, parentheses *are* required. 3. When the child and parent operators have the same precedence, the

(
    expr: &ExprOrSource,
    is_left: bool,
    parent_strength: i32,
    parent_associativity: Associativity,
)

// please do not make the code terser without being confident that it's easier
// to understand.
fn needs_parentheses(
    expr: &ExprOrSource,
    is_left: bool,
    parent_strength: i32,
    parent_associativity: Associativity,
) -> bool {
    let rule_3a = matches!(parent_associativity, Associativity::Both);
    let rule_3b_left = is_left && parent_associativity.left_associative();
    let rule_3b_right = !is_left && parent_associativity.right_associative();

    match expr.binding_strength().cmp(&parent_strength) {
        // Rule 1
        Ordering::Greater => false,
        // Rule 2
        Ordering::Less => true,
        // Rule 3
        Ordering::Equal => !(rule_3a || rule_3b_left || rule_3b_right),
    }
}

/// Associativity of an expression's operator.
/// Note that there's no exponent symbol in SQL, so we don't seem to require a `Right` variant.