Parsing in to a recursive data structure

I wish to parse a string in to a recursive data structure using F#. In this question I'm going to present a simplified example that cuts to the core of what I want to do.

I want to parse a string of nested square brackets in to the record type:

type Bracket = | Bracket of Bracket option

So:

• "[]" -> Bracket None
• "[[]]" -> Bracket ( Some ( Bracket None) )
• "[[[]]]" -> Bracket ( Some ( Bracket ( Some ( Bracket None) ) ) )

I would like to do this using the parser combinators in the FParsec library. Here is what I have so far:

let tryP parser = 
    parser |>> Some
    <|>
    preturn None

/// Parses up to nesting level of 3
let parseBrakets  : Parser<_> = 

let mostInnerLevelBracket = 
    pchar '[' 
    .>> pchar ']'
    |>> fun _ -> Bracket None

let secondLevelBracket = 
    pchar '['
    >>. tryP mostInnerLevelBracket
    .>> pchar ']'
    |>> Bracket

let firstLevelBracket = 
    pchar '['
    >>. tryP secondLevelBracket
    .>> pchar ']'
    |>> Bracket

firstLevelBracket

I even have some Expecto tests:

open Expecto

[<Tests>]
let parserTests = 
[ "[]", Bracket None 
    "[[]]", Bracket (Some (Bracket None))
    "[[[]]]", Bracket ( Some  (Bracket (Some (Bracket None))))  ]
|> List.map(fun (str, expected) -> 
    str
    |> sprintf "Trying to parse %s"
    |> testCase
    <| fun _ -> 
    match run parseBrakets str with
    | Success (x, _,_) -> Expect.equal x expected "These should have been equal"
    | Failure (m, _,_) -> failwithf "Expected a match: %s" m
)
|> testList "Bracket tests"

let tests = 
[ parserTests ]
|> testList "Tests"

runTests defaultConfig tests

The problem is of course how to handle and arbitrary level of nesting - the code above only works for up to 3 levels. The code I would like to write is:

let rec pNestedBracket = 
    pchar '['
    >>. tryP pNestedBracket
    .>> pchar ']'
    |>> Bracket

But F# doesn't allow this.

Am I barking up the wrong tree completely with how to solve this (I understand that there are easier ways to solve this particular problem)?