module CS334

open Combinator

type Expr =
| Terms of Term list

and Term =
| AAA
| BBB
| Brace of Expr

let expr,exprImpl = recparser()
let aaa = pstr "aaa" |>> (fun _ -> AAA)
let bbb = pstr "bbb" |>> (fun _ -> BBB)
let brace = pbetween (pchar '{') expr (pchar '}') |>> Brace
let term = aaa <|> bbb <|> brace

exprImpl := pmany1 term |>> Terms
let grammar = pleft expr peof

let parse (input: string) : Expr option =
    let i = prepare input
    match grammar i with
    | Success (ast,_) -> Some ast
    | Failure _ -> None