FemtoML parser in F# using FParsec

FemtoMLParserUsingFParsec.fs

    > let pInt : Parser<_, unit> = puint32 |>> int .>> spaces;;

    > let str s = pstring s >>. spaces;;

    > let keywords =
        "if then else let rec in fun".Split ' '
        |> set;;

    > let pIdent : Parser<_, unit> =

        let isAlphaNum c = isLetter c || isDigit c

        let idStr = many1Satisfy2L isLetter isAlphaNum "identifier" .>> spaces

        let p state =
          let reply = idStr state
          if reply.Status = Ok && keywords.Contains reply.Result then
            Reply(Error, expected "identifier")
          else
            reply

        attempt p .>> spaces;;

    > let opp = OperatorPrecedenceParser<Expr,unit,unit>();;

    > let pExpr = opp.ExpressionParser .>> spaces;;

    > let pAtom =
        choice
          [ pInt |>> Int
            pIdent |>> Ident
            between (str "(") (str ")") pExpr ];;

    > opp.TermParser <-
        choice
          [ many1 pAtom |>> fun fs -> List.reduce (fun f g -> Apply(f, g)) fs
            pipe3 (str "if" >>. pExpr) (str "then" >>. pExpr) (str "else" >>. pExpr)
              (fun p t f -> If(p, t, f))
            pipe3 (str "let" >>. (str "rec" >>. pIdent .>> str "=")) pExpr (str "in" >>. pExpr)
              (fun var body rest -> LetRec(var, body, rest))
            pipe2 (str "fun" >>. pIdent .>> str "->") pExpr
              (fun par body -> Lambda(par, body)) ];;

    > opp.AddOperator(InfixOperator("<", spaces, 1, Associativity.Left, fun f g -> BinOp(f, Lt, g)));;

    > opp.AddOperator(InfixOperator("+", spaces, 2, Associativity.Left, fun f g -> BinOp(f, Add, g)));;

    > opp.AddOperator(InfixOperator("-", spaces, 2, Associativity.Left, fun f g -> BinOp(f, Sub, g)));;

    > let parse str = run (spaces >>. pExpr .>> eof) str;;

Shorter version here: https://gist.github.com/mrakgr/b16ced3ec9cb92c8adbc79d85914ce3f