/**
 * @factory/shared — the formula AST discriminated union (TASK-0084 R2).
 *
 * The AST produced by `packages/database-engine`'s parser is JSON-serializable
 * (stored in `formulas.ast`); THIS type is the single source the editor's
 * autocomplete, the Phase-12 "generate formula" AI tool, and the engine's
 * evaluator all consume. Keeping it here (not in the engine) is a recorded
 * decision: the editor and the AI gateway must not depend on the database
 * engine just for the AST shape.
 *
 * Node kinds:
 *  - `Literal` — number / string / date (ISO string) / bool.
 *  - `NullLiteral` — `null` (added to the spec's union; array aggregates need
 *    null elements and the null-matrix goldens — recorded).
 *  - `ArrayLiteral` — `[a, b, c]` (same reason — the function set takes
 *    list arguments; recorded).
 *  - `Prop` — `prop("Name")` (also reachable via bare `prop` calls).
 *  - `Call` — fixed function table, closed set.
 *  - `BinaryOp` — arithmetic / comparison / logical (left-assoc).
 *  - `UnaryOp` — `-x`, `not x`.
 *  - `If` — `if(c, a, b)`; the ternary `c ? a : b` parses to the same node.
 */

export type FormulaLiteralType = "number" | "string" | "date" | "bool";

export type FormulaAstNode =
  | { type: "Literal"; value: number | string | boolean; literalType: FormulaLiteralType }
  | { type: "NullLiteral" }
  | { type: "ArrayLiteral"; elements: FormulaAstNode[] }
  | { type: "Prop"; name: string }
  | { type: "Call"; name: string; args: FormulaAstNode[] }
  | { type: "BinaryOp"; op: string; left: FormulaAstNode; right: FormulaAstNode }
  | { type: "UnaryOp"; op: "-" | "not"; operand: FormulaAstNode }
  | { type: "If"; cond: FormulaAstNode; then: FormulaAstNode; else: FormulaAstNode };