JSon parser as per http://www.quanttec.com/fparsec/tutorial.html#fs-value-restriction

json.fs

type P<'t> = Parser<'t, unit>

type Json =
    | JString of string
    | JNumber of float
    | JBool of bool
    | JNull
    | JList of Json list
    | JObject of Map<string, Json>

let delim c = pchar c >>. spaces

let rec key : P<_> = pchar '"' >>. (manyChars (noneOf "\"")) .>> delim '"'
and jstring : P<_> = key |>> JString
and jnumber = pfloat .>> spaces |>> JNumber

and inline listBetweenChar sOpen sClose parser f : P<_> =
    between (delim sOpen) (delim sClose) (sepBy parser comma) |>> f

and jarray input = listBetweenChar '[' ']' json JList <| input

and jobject input =
    listBetweenChar '{' '}' (key .>>. (delim ':' >>. json)) (Map.ofList >> JObject) <| input

and jtrue = stringReturn "true" (JBool true)
and jfalse = stringReturn "false" (JBool false)
and comma = delim ','
and json = choice [jarray; jnumber; jstring; jobject; jtrue; jfalse; stringReturn "null" JNull]