F#, FParsec, and Updating UserState - f#

Question

Okay, since my last question elicited no responses, I'm forging ahead in a different direction. Lol!
I can't find any examples beyond the official documentation on managing user state, or accessing the results of a prior parser.
N.b. This code does not compile.
namespace MultipartMIMEParser
open FParsec
open System.IO
type Header = { name : string
; value : string
; addl : (string * string) list option }
type Content = Content of string
| Post of Post list
and Post = { headers : Header list
; content : Content }
type private UserState = { Boundary : string }
with static member Default = { Boundary="" }
module internal P =
let ($) f x = f x
let undefined = failwith "Undefined."
let ascii = System.Text.Encoding.ASCII
let str cs = System.String.Concat (cs:char list)
let makeHeader ((n,v),nvps) = { name=n; value=v; addl=nvps}
let runP p s = match runParserOnStream p UserState.Default "" s ascii with
| Success (r,_,_) -> r
| Failure (e,_,_) -> failwith (sprintf "%A" e)
let blankField = parray 2 newline
let delimited d e =
let pEnd = preturn () .>> e
let part = spaces >>. (manyTill $ noneOf d $ (attempt (preturn () .>> pstring d) <|> pEnd)) |>> str
in part .>>. part
let delimited3 firstDelimiter secondDelimiter thirdDelimiter endMarker =
delimited firstDelimiter endMarker
.>>. opt (many (delimited secondDelimiter endMarker
>>. delimited thirdDelimiter endMarker))
// TODO: This is the parser I'm asking about.
let pHeader =
let includesBoundary s = undefined
let setBoundary b = { Boundary=b }
in delimited3 ":" ";" "=" blankField
|>> makeHeader
>>. fun stream -> if includesBoundary // How do I access the output from makeHeader here?
then stream.UserState <- setBoundary b // I need b to be read from the output of makeHeader.
Reply ()
else Reply ()
let pHeaders = manyTill pHeader $ attempt (preturn () .>> blankField)
// N.b. This is the mess I'm currently wrestling with. It does not compile, and is
// not sound yet.
let rec pContent boundary =
match boundary with
| "" -> // Content is text.
let line = restOfLine false
in pipe2 pHeaders (manyTill line $ attempt (preturn () .>> blankField))
$ fun h c -> { headers=h
; content=Content $ System.String.Join (System.Environment.NewLine,c) }
| _ -> // Content contains boundaries.
let b = "--"+boundary
let p = pipe2 pHeaders (pContent b) $ fun h c -> { headers=h; content=c }
in skipString b >>. manyTill p (attempt (preturn () .>> blankField))
let pStream = runP (pipe2 pHeaders pContent $ fun h c -> { headers=h; content=c })
type MParser (s:Stream) =
let r = P.pStream s
let findHeader name =
match r.headers |> List.tryFind (fun h -> h.name.ToLower() = name) with
| Some h -> h.value
| None -> ""
member p.Boundary =
let isBoundary ((s:string),_) = s.ToLower() = "boundary"
let header = r.headers
|> List.tryFind (fun h -> if h.addl.IsSome
then h.addl.Value |> List.exists isBoundary
else false)
in match header with
| Some h -> h.addl.Value |> List.find isBoundary |> snd
| None -> ""
member p.ContentID = findHeader "content-id"
member p.ContentLocation = findHeader "content-location"
member p.ContentSubtype = findHeader "type"
member p.ContentTransferEncoding = findHeader "content-transfer-encoding"
member p.ContentType = findHeader "content-type"
member p.Content = r.content
member p.Headers = r.headers
member p.MessageID = findHeader "message-id"
member p.MimeVersion = findHeader "mime-version"
A truncated example of the POST I am trying to parse follows:
content-type: Multipart/related; boundary="RN-Http-Body-Boundary"; type="multipart/related"
--RN-Http-Body-Boundary
Message-ID: <25845033.1160080657073.JavaMail.webmethods#exshaw>
Mime-Version: 1.0
Content-Type: multipart/related; type="application/xml";
boundary="----=_Part_235_11184805.1160080657052"
------=_Part_235_11184805.1160080657052
Content-Type: Application/XML
Content-Transfer-Encoding: binary
Content-Location: RN-Preamble
Content-ID: <1430586.1160080657050.JavaMail.webmethods#exshaw>
XML document begins here...

Answer 1

So basically, what you want to do in pHeader is to use the parser as a monad, rather than an applicative. Based on your code style you come from Haskell so I'll assume you know these words. Something like this then:
let pHeader =
let includesBoundary s = undefined
let setBoundary b = { Boundary=b }
in delimited3 ":" ";" "=" blankField
|>> makeHeader
>>= fun header stream ->
if includesBoundary header
then let b = undefined // some expression including header, if I understood correctly
stream.UserState <- setBoundary b
Reply ()
else Reply ()
Or you can write it in a computation expression (which would correspond to do-notation in Haskell):
let pHeader =
let includesBoundary s = undefined
let setBoundary b = { Boundary=b }
parse {
let! header =
delimited3 ":" ";" "=" blankField
|>> makeHeader
return! fun stream ->
if includesBoundary header
then let b = undefined // some expression including header, if I understood correctly
stream.UserState <- setBoundary b
Reply ()
else Reply ()
}