FemtoML parser in F# using FParsec (rewrite of Jon Harrop's example)

FemtoMLParserUsingFParsec.fs

let pInt s = (puint32 |>> int .>> spaces) s
let str s = pstring s .>> spaces
let keywords = "if then else let rec in fun".Split ' ' |> set
let pIdent s = 
    let isAlphaNum c = isLetter c || isDigit c
    (many1Satisfy2L isLetter isAlphaNum "identifier" .>> spaces
    >>=? fun x _ -> if keywords.Contains x then Reply(Error, expected "identifier") else Reply(x)) s

let rec pExpr s =
    let pAtom s =
        choice [ 
            pInt |>> Int
            pIdent |>> Ident
            between (str "(") (str ")") pExpr ] s
    let pExprInner s =
        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)) ] s
    let pOp op expr = pipe2 expr (many (op .>>. expr)) (List.fold (fun a (op,b) -> Op(a,op,b)))
    List.foldBack pOp [str "<"; choice [str "+"; str "-"]] pExprInner s