let ( let* ) = Result.bind

open Expr
open Error
open Value

let value_of_literal (literal : literal) : Value.lox_value =
  match literal with String s -> String s | Number x -> Number x | Bool b -> Bool b | Nil -> Nil

let rec interpret_expr (expr : expr_node) : (lox_value, interpreter_error) result =
  let pos = expr.pos in
  match expr.expr with
  | Literal literal -> Ok (value_of_literal literal)
  | BinaryExpr { op; left; right } -> (
      let* left = interpret_expr left in
      let* right = interpret_expr right in
      match (left, op, right) with
      | String a, Plus, String b -> Ok (String (a ^ b))
      | Number x, Plus, Number y -> Ok (Number (x +. y))
      | Number x, Minus, Number y -> Ok (Number (x -. y))
      | Number x, Mul, Number y -> Ok (Number (x *. y))
      | Number x, Div, Number y -> Ok (Number (x /. y))
      | Number x, Equal, Number y -> Ok (Bool (x = y))
      | Number x, Greater, Number y -> Ok (Bool (x > y))
      | Number x, GreaterEqual, Number y -> Ok (Bool (x >= y))
      | Number x, Less, Number y -> Ok (Bool (x < y))
      | Number x, LessEqual, Number y -> Ok (Bool (x <= y))
      | Bool b, And, Bool c -> Ok (Bool (b && c))
      | Bool b, Or, Bool c -> Ok (Bool (b || c))
      | _, Equal, _ -> Ok (Bool (left = right))
      | _, _, _ ->
          let msg =
            Printf.sprintf "Invalid operands of type %s and %s to operator %s"
              (type_string_of_lox_value left) (type_string_of_lox_value right) (show_binary_op op)
          in
          Error (InterpreterError.make pos msg))
  | UnaryExpr { op; expr } -> (
      let* expr = interpret_expr expr in
      match (op, expr) with
      | Neg, Number x -> Ok (Number (-.x))
      | Not, Bool b -> Ok (Bool (not b))
      | _, _ ->
          let msg =
            Printf.sprintf "Invalid operant of type %s to operator %s"
              (type_string_of_lox_value expr) (show_unary_op op)
          in
          Error (InterpreterError.make pos msg))