open FParsec 

type Thing =
    | Value of string
    | Pair of Thing * Thing

 
let validVariableCharacter x = ['a' .. 'z'] |> Seq.contains x

let thingParser, thingParserImpl = createParserForwardedToRef()
let valueParser = many1Satisfy validVariableCharacter |>> Value
let pairParser = thingParser .>> spaces1 .>>. thingParser |>> Pair

do thingParserImpl := attempt(pairParser) <|> valueParser // this results in a stackoverflowexception

printfn "%A" <| run thingParser "a" // expect Value "a"
printfn "%A" <| run thingParser "a b c" // expect Pair(a, Pair(b, c))