type PrimValue =
    | BoolVal of bool
    | IntVal of int

let inline operator val1 val2 op = op val1 val2

let rec eval (expr : expr) =
    match expr with
    | CstI i                   -> IntVal i
    | Prim ("+", lOper, rOper) -> evalNumericExpr (+) lOper rOper
    | Prim ("-", lOper, rOper) -> evalNumericExpr (-) lOper rOper
    | Prim ("*", lOper, rOper) -> evalNumericExpr (*) lOper rOper
    | Prim ("/", lOper, rOper) -> evalNumericExpr (/) lOper rOper
    | Prim ("%", lOper, rOper) -> evalNumericExpr (%) lOper rOper
    | CstB b                   -> BoolVal b
    | Prim ("=", lOper, rOper)-> evalBoolExpr (=) lOper rOper
    | Prim ("<>",lOper, rOper)-> evalBoolExpr (<>) lOper rOper
    | Prim (">", lOper, rOper)-> evalBoolExpr (>) lOper rOper
    | Prim ("<", lOper, rOper)-> evalBoolExpr (<) lOper rOper
    | Prim (">=", lOper, rOper)-> evalBoolExpr (>=) lOper rOper
    | Prim ("<=", lOper, rOper)-> evalBoolExpr (<=) lOper rOper
    | _                 -> failwith "Syntax error"

and evalNumericExpr op lOper rOper =
    let left = eval lOper
    let right = eval rOper
    match (left, right) with
    | (IntVal a, IntVal b) ->  IntVal (op a b)
    | _                  -> failwith "Syntax error"

and evalBoolExpr op lOper rOper =
    let left = eval lOper
    let right = eval rOper
    match (left, right) with
    | (BoolVal a, BoolVal b) ->  BoolVal (op a b)
    | (IntVal a, IntVal b)  -> BoolVal (op a b)
    | _                  -> failwith "Syntax error"