open System
open Parser

(* AST *)

type Term =
| AAA
| BBB
| Parens of Expr
and Expr =
| Terms of Term list

(* grammar *)

// forward ref
let expr, exprImpl = recparser()

let parens = pbetween (pchar '{') (pchar '}') expr |>> (fun e -> Parens e) <!> "parens"
let aaa = pstr "aaa" |>> (fun _ -> AAA) <!> "aaa"
let bbb = pstr "bbb" |>> (fun _ -> BBB) <!> "bbb"
let term = aaa <|> bbb <|> parens <!> "term"
exprImpl := pmany1 term |>> (fun ts -> Terms ts) <!> "expr"
let grammar = pleft expr peof <!> "top"

(* parser function:
   Returns Some AST on success, otherwise None.
*)
let parse input : Expr option =
    match grammar (prepare input) with
    | Success(res,_) -> Some res
    | Failure        -> None

(*
   A simple read-eval-print loop.
   Actually, it's so simple that there's no eval.
   Note that, in the homework, I want you
   to handle input differently; the REPL is just for fun.
*)
let rec repl() : unit =
    printf "Enter an expression: "
    let input = System.Console.ReadLine()
    if input = "quit" then
        printfn "Goodbye!"
        exit 0
    else
        let asto = parse input
        match asto with
        | Some ast -> printfn "%A" ast
        | None     -> printfn "Invalid expression."
        repl()

[<EntryPoint>]
let main argv =
    repl()
    0 // return an integer exit code