# Module Netmime_string

    module Netmime_string: sig .. end

Low-level functions to parse and print mail and MIME messages

`Netmime_string` contains a lot of functions to scan and print strings formatted as MIME messages. For a higher-level view on this topic, see the `Netmime` module.

Contents

- Splitting a string into lines
- Parsing and Printing Mail Headers
- Parsing Structured Values
- Parsing Certain Forms of Structured Values
- Printing Structured Values
- Scanning MIME Messages
- Helpers for MIME Messages

## Splitting a string into lines

These functions are all CR/LF-aware, i.e. lines can be terminated by either LF or CR/LF.

    val find_line_end : string -> int -> int -> int

`find_line_end s pos len`: Searches the next line end (CR/LF or only LF), and returns the position. The search starts at position `pos`, and covers the next `len` bytes. Raises `Not_found` if there is no line end.

    val find_line_end_poly : 's Netstring_tstring.tstring_ops -> 's -> int -> int -> int

polymorphic version

    val find_line_start : string -> int -> int -> int

`find_line_start s pos len`: Searches the next start, and returns its position. The line start is the position after the next line end (CR/LF or only LF). The search starts at position `pos`, and covers the next `len` bytes. Raises `Not_found` if there is no line end.

    val find_line_start_poly : 's Netstring_tstring.tstring_ops -> 's -> int -> int -> int

polymorphic version

    val find_double_line_start : string -> int -> int -> int

`find_double_line_start s pos len`: Searches two adjacent line ends (each may be a CR/LF combination or a single LF), and returns the position after the second line end. The search starts at position `pos`, and covers the next `len` bytes. Raises `Not_found` if the mentioned pattern is not found.

    val find_double_line_start_poly : 's Netstring_tstring.tstring_ops -> 's -> int -> int -> int

polymorphic version

    val skip_line_ends : string -> int -> int -> int

`skip_line_ends s pos len`: Skips over adjacent line ends (terminated by CR/LF or plain LF), and returns the position after the last line end. The search starts at position `pos`, and covers the next `len` bytes. Note that this function cannot raise `Not_found`.

    val skip_line_ends_poly : 's Netstring_tstring.tstring_ops -> 's -> int -> int -> int

polymorphic version

    val fold_lines_p : ('a -> int -> int -> int -> bool -> 'a) -> 'a -> string -> int -> int -> 'a

`fold_lines_p f acc0 s pos len`: Splits the substring of `s` from `pos` to `pos+len` into lines, and folds over these lines like `List.fold_left`. The function `f` is called as `f acc p0 p1 p2 is_last` where `acc` is the current accumulator (initialized with `acc0`), and

- `p0` is the start position of the line in `s`
- `p1` is the position of the line terminator in `s`
- `p2` is the position after the line terminator in `s`
- `is_last` is true if this is the last line in the iteration

The lines can be terminated with CR/LF or LF. For the last line the terminator is optional (`p1=p2` is possible).

The function is tail-recursive.

    val fold_lines_p_poly : 's Netstring_tstring.tstring_ops ->
           ('a -> int -> int -> int -> bool -> 'a) -> 'a -> 's -> int -> int -> 'a

even more polymorphic

    val fold_lines : ('a -> string -> 'a) -> 'a -> string -> int -> int -> 'a

`fold_lines f acc0 s pos len`: Splits the substring of `s` from `pos` to `pos+len` into lines, and folds over these lines like `List.fold_left`. The function `f` is called as `f acc line` where `acc` is the current accumulator (initialized with `acc0`), and `line` is the current line w/o terminator.

The lines can be terminated with CR/LF or LF.

The function is tail-recursive.

Example: Get the lines as list:

             List.rev(fold_lines (fun l acc -> acc::l) [] s pos len)

    val fold_lines_poly : 's Netstring_tstring.tstring_ops ->
           ('a -> 's -> 'a) -> 'a -> 's -> int -> int -> 'a

even more polymorphic

    val iter_lines : (string -> unit) -> string -> int -> int -> unit

`iter_lines f s pos len`: Splits the substring of `s` from `pos` to `pos+len` into lines, and calls `f line` for each line.

The lines can be terminated with CR/LF or LF.

    val iter_lines_poly : 's Netstring_tstring.tstring_ops -> ('s -> unit) -> 's -> int -> int -> unit

even more polymorphic

    val skip_whitespace_left : string -> int -> int -> int

`skip_whitespace_left s pos len`: Returns the smallest `p` with `p >= pos && p < pos+len` so that `s.[p]` is not a whitesapce character (space, TAB, CR, LF), and `s.[q]` is a whitespace character for all `q<p`. If this is not possible `Not_found` will be raised.

    val skip_whitespace_right : string -> int -> int -> int

`skip_whitespace_right s pos len`: Returns the biggest `p` with `p >= pos && p < pos+len` so that `s.[p]` is not a whitesapce character (space, TAB, CR, LF), and `s.[q]` is a whitespace character for all `q>p`. If this is not possible `Not_found` will be raised.

## Parsing and Printing Mail Headers

The Format of Mail Messages

Messages consist of a header and a body; the first empty line separates both parts. The header contains lines "param-name`:` param-value" where the param-name must begin on column 0 of the line, and the "`:`" separates the name and the value. So the format is roughly:

       param1-name: param1-value
       ...
       paramN-name: paramN-value
       _
       body

(Where "_" denotes an empty line.)

Details

Note that parameter values are restricted; you cannot represent arbitrary strings. The following problems can arise:

- Values cannot begin with whitespace characters, because there may be an arbitrary number of whitespaces between the "`:`" and the value.
- Values (and names of parameters, too) must only be formed of 7 bit ASCII characters. (If this is not enough, the MIME standard knows the extension RFC 2047 that allows that header values may be composed of arbitrary characters of arbitrary character sets. See below how to decode such characters in values returned by this function.)
- Header values may be broken into several lines. Continuation lines must begin with whitespace characters. This means that values must not contain line breaks as semantic part of the value. And it may mean that one whitespace character is not distinguishable from several whitespace characters.
- Header lines must not be longer than 78 characters (soft limit) or 998 characters (hard limit). Values that would result into longer lines must be broken into several lines. This means that you cannot represent strings that contain too few whitespace characters. (Note: The soft limit is to avoid that user agents have problems with long lines. The hard limit means that transfer agents sometimes do not transfer longer lines correctly.)
- Some old gateways pad the lines with spaces at the end of the lines.

This implementation of a mail scanner tolerates a number of deviations from the standard: long lines are not rejected; 8 bit values are generally accepted; lines may be ended only with LF instead of CRLF.

Compatibility

These functions can parse all mail headers that conform to RFC 822 or RFC 2822.

But there may be still problems, as RFC 822 allows some crazy representations that are actually not used in practice. In particular, RFC 822 allows it to use backslashes to "indicate" that a CRLF sequence is semantically meant as line break. As this function normally deletes CRLFs, it is not possible to recognize such indicators in the result of the function.

    val fold_header : ?downcase:bool ->
           ?unfold:bool ->
           ?strip:bool ->
           ('a -> string -> string -> 'a) -> 'a -> string -> int -> int -> 'a

`fold_header f acc0 s pos len`: Parses a MIME header in the string `s` from `pos` to exactly `pos+len`. The MIME header must be terminated by an empty line.

A folding operation is done over the header values while the lines are extracted from the string, very much like `List.fold_left`. For each header `(n,v)` where `n` is the name and `v` is the value, the function `f` is called as `f acc n v`.

If the header cannot be parsed, a `Failure` is raised.

Certain transformations may be applied (default: no transformations):

- If `downcase` is set, the header names are converted to lowercase characters
- If `unfold` is set, the line terminators are not included in the resulting values. This covers both the end of line characters at the very end of a header and the end of line characters introduced by continuation lines.
- If `strip` is set, preceding and trailing white space is removed from the value (including line terminators at the very end of the value)

    val list_header : ?downcase:bool ->
           ?unfold:bool -> ?strip:bool -> string -> int -> int -> (string * string) list

`list_header s pos len`: Returns the headers as list of pairs `(name,value)`.

For the meaning of the arguments see `fold_header` above.

    val scan_header : ?downcase:bool ->
           ?unfold:bool ->
           ?strip:bool ->
           string -> start_pos:int -> end_pos:int -> (string * string) list * int

`let params, header_end_pos = scan_header s start_pos end_pos`:

Deprecated.

Scans the mail header that begins at position `start_pos` in the string `s` and that must end somewhere before position `end_pos`. It is intended that in `end_pos` the character position following the end of the body of the MIME message is passed.

Returns the parameters of the header as `(name,value)` pairs (in `params`), and in `header_end_pos` the position of the character following directly after the header (i.e. after the blank line separating the header from the body).

- If `downcase`, header names are converted to lowercase characters
- The arguments `unfold` and `strip` have a slightly different meaning as for the new function `fold_header` above. In particular, whitespace is already stripped off the returned values if any of `unfold` or `strip` are enabled. (This is for backward compatibility.)

Also, this function is different because `downcase` and `unfold` are enabled by default, and only `strip` is not enabled.

    val scan_header_tstring : ?downcase:bool ->
           ?unfold:bool ->
           ?strip:bool ->
           Netsys_types.tstring ->
           start_pos:int -> end_pos:int -> (string * string) list * int

The same for tagged strings

    val scan_header_poly : ?downcase:bool ->
           ?unfold:bool ->
           ?strip:bool ->
           's Netstring_tstring.tstring_ops ->
           's -> start_pos:int -> end_pos:int -> (string * string) list * int

Polymorphic version

    val read_header : ?downcase:bool ->
           ?unfold:bool ->
           ?strip:bool -> Netstream.in_obj_stream -> (string * string) list

This function expects that the current position of the passed `in_obj_stream` is the first byte of the header. The function scans the header and returns it. After that, the stream position is after the header and the terminating empty line (i.e. at the beginning of the message body).

The options `downcase`, `unfold`, and `strip` have the same meaning as in `scan_header`.

Example

To read the mail message "`file.txt`":

     let ch = new Netchannels.input_channel (open_in "file.txt") in
     let stream = new Netstream.input_stream ch in
     let header = read_header stream in
     stream#close_in()  (* no need to close ch *)

    val write_header : ?soft_eol:string ->
           ?eol:string -> Netchannels.out_obj_channel -> (string * string) list -> unit

This function writes the header to the passed `out_obj_channel`. The empty line following the header is also written.

Exact output format:

- The header is not folded, i.e. no additional CRLF sequences are inserted into the header to avoid long header lines. In order to produce correct headers, the necessary CRLF bytes must already exist in the field values. (You can use the function `write_value` below for this.)
- However, this function helps getting some details right. First, whitespace at the beginning of field values is suppressed. Example: `write_header ch ["x","Field value"; "y"," Other value"]` outputs:

       x: Field value\r\n
       y: Other value\r\n
       \r\n

- The end-of-line sequences LF, and CRLF, followed by whitespace are replaced by the passed `soft_eol` string. If the necessary space or tab character following the eol is missing, an additional space character will be inserted. Example: `write_header ch ["x","Field\nvalue"; "y","Other\r\n\tvalue"]` outputs:

       x: Field\r\n
        value
       y: Other\r\n
       \tvalue

- Empty lines (and lines only consisting of whitespace) are suppressed if they occur inside the header. Example: `write_header ch ["x","Field\n\nvalue"]` outputs:

       x: Field\r\n
        value

- Whitespace at the end of a header field is suppressed. One field is separated from the next field by printing `eol` once.

These rules ensure that the printed header will be well-formed with two exceptions:

- Long lines (> 72 characters) are neither folded nor rejected
- True 8 bit characters are neither properly encoded nor rejected

These two problems cannot be addressed without taking the syntax of the header fields into account. See below how to create proper header fields from `s_token` lists.

## Parsing Structured Values

The following types and functions allow it to build scanners for structured mail and MIME values in a highly configurable way.

Structured Values

RFC 822 (together with some other RFCs) defines lexical rules how formal mail header values should be divided up into tokens. Formal mail headers are those headers that are formed according to some grammar, e.g. mail addresses or MIME types.

Some of the characters separate phrases of the value; these are the "special" characters. For example, '@' is normally a special character for mail addresses, because it separates the user name from the domain name (as in `user@domain`). RFC 822 defines a fixed set of special characters, but other RFCs use different sets. Because of this, the following functions allow it to configure the set of special characters.

Every sequence of characters may be embraced by double quotes, which means that the sequence is meant as literal data item; special characters are not recognized inside a quoted string. You may use the backslash to insert any character (including double quotes) verbatim into the quoted string (e.g. "He said: \"Give it to me!\""). The sequence of a backslash character and another character is called a quoted pair.

Structured values may contain comments. The beginning of a comment is indicated by '(', and the end by ')'. Comments may be nested. Comments may contain quoted pairs. A comment counts as if a space character were written instead of it.

Control characters are the ASCII characters 0 to 31, and 127. RFC 822 demands that mail headers are 7 bit ASCII strings. Because of this, this module also counts the characters 128 to 255 as control characters.

Domain literals are strings embraced by '[' and ']'; such literals may contain quoted pairs. Today, domain literals are used to specify IP addresses (rare), e.g. `user@[192.168.0.44]`.

Every character sequence not falling in one of the above categories is an atom (a sequence of non-special and non-control characters). When recognized, atoms may be encoded in a character set different than US-ASCII; such atoms are called encoded words (see RFC 2047).

Scanning Using the Extended Interface

In order to scan a string containing a structured value, you must first create a `mime_scanner` using the function `create_mime_scanner`. The scanner contains the reference to the scanned string, and a specification how the string is to be scanned. The specification consists of the lists `specials` and `scan_options`.

The character list `specials` specifies the set of special characters. These are the characters that are not regarded as part of atoms, because they work as delimiters that separate atoms (like `@` in the above example). In addition to this, when '"', '(', and '[' are seen as regular characters not delimiting quoted string, comments, and domain literals, respectively, these characters must also be added to `specials`. In detail, these rules apply:

- Spaces:
  - If `' '` in `specials`: A space character is returned as `Special ' '`. Note that there may also be an effect on how comments are returned (see below).
  - If `' '` not in `specials`: Spaces are not returned, although they still delimit atoms.
- Tabs, CRs, LFs:
  - If `'\t'` in `specials`: A tab character is returned as `Special '\t'`.
  - If `'\t'` not in `specials`: Tabs are not returned, although they still delimit atoms.
  - If `'\r'` in `specials`: A CR character is returned as `Special '\r'`.
  - If `'\r'` not in `specials`: CRs are not returned, although they still delimit atoms.
  - If `'\n'` in `specials`: A LF character is returned as `Special '\n'`.
  - If `'\n'` not in `specials`: LFs are not returned, although they still delimit atoms.
- Comments:
  - If `'('` in `specials`: Comments are not recognized. The character '(' is returned as `Special '('`.
  - If `'('` not in `specials`: Comments are recognized. How comments are returned, depends on the following:
    1. If `Return_comments` in `scan_options`: Outer comments are returned as `Comment` (note that inner comments are recognized but are not returned as tokens)
    2. If otherwise `' '` in `specials`: Outer comments are returned as `Special ' '`
    3. Otherwise: Comments are recognized but not returned at all.
- Quoted strings:
  - If `'"'` in `specials`: Quoted strings are not recognized, and double quotes are returned as `Special '"'`.
  - If `'"'` not in `specials`: Quoted strings are returned as `QString` tokens.
- Domain literals:
  - If '[' in `specials`: Domain literals are not recognized, and left brackets are returned as `Special` '['.
  - If '[' not in `specials`: Domain literals are returned as `DomainLiteral` tokens.

If recognized, quoted strings are returned as `QString s`, where `s` is the string without the embracing quotes, and with already decoded quoted pairs.

Control characters `c` are returned as `Control c`.

If recognized, comments may either be returned as spaces (in the case you are not interested in the contents of comments), or as `Comment` tokens. The contents of comments are not further scanned; you must start a subscanner to analyze comments as structured values.

If recognized, domain literals are returned as `DomainLiteral s`, where `s` is the literal without brackets, and with decoded quoted pairs.

Atoms are returned as `Atom s` where `s` is a longest sequence of atomic characters (all characters which are neither special nor control characters nor delimiters for substructures). If the option `Recognize_encoded_words` is on, atoms which look like encoded words are returned as `EncodedWord` tokens. (Important note: Neither '?' nor '=' must be special in order to enable this functionality.)

After the `mime_scanner` has been created, you can scan the tokens by invoking `scan_token` which returns one token at a time, or by invoking `scan_token_list` which returns all following tokens.

There are two token types: `s_token` is the base type and is intended to be used for pattern matching. `s_extended_token` is a wrapper that additionally contains information where the token occurs.

Scanning Using the Simple Interface

Instead of creating a `mime_scanner` and calling the scan functions, you may also invoke `scan_structured_value`. This function returns the list of tokens directly; however, it is restricted to `s_token`.

Examples

- Simple address:

       scan_structured_value "user@domain.com" [ '@'; '.' ] []
         = [ Atom "user"; Special '@'; Atom "domain"; Special '.'; Atom "com" ]

- Spaces are not returned:

       scan_structured_value "user @ domain . com" [ '@'; '.' ] []
         = [ Atom "user"; Special '@'; Atom "domain"; Special '.'; Atom "com" ]

- Comments are not returned:

       scan_structured_value "user(Do you know him?)@domain.com" [ '@'; '.' ] []
         = [ Atom "user"; Special '@'; Atom "domain"; Special '.'; Atom "com" ]

- Comments are indicated if requested:

       scan_structured_value "user(Do you know him?)@domain.com" [ '@'; '.' ]
           [ Return_comments ]
         = [ Atom "user"; Comment; Special '@'; Atom "domain"; Special '.';
             Atom "com" ]

- Spaces are returned if special:

       scan_structured_value "user (Do you know him?) @ domain . com"
           [ '@'; '.'; ' ' ] []
         = [ Atom "user"; Special ' '; Special ' '; Special ' '; Special '@';
             Special ' '; Atom "domain";
             Special ' '; Special '.'; Special ' '; Atom "com" ]

- Both spaces and comments are requested:

       scan_structured_value "user (Do you know him?) @ domain . com"
           [ '@'; '.'; ' ' ] [ Return_comments ]
         = [ Atom "user"; Special ' '; Comment; Special ' '; Special '@';
             Special ' '; Atom "domain";
             Special ' '; Special '.'; Special ' '; Atom "com" ]

- Another case:

       scan_structured_value "user @ domain . com" [ '@'; '.'; ' ' ] []
         = [ Atom "user"; Special ' '; Special '@'; Special ' '; Atom "domain";
             Special ' '; Special '.'; Special ' '; Atom "com" ]

- '(' is special:

       scan_structured_value "user(Do you know him?)@domain.com" ['@'; '.'; '(']
           []
         = [ Atom "user"; Special '('; Atom "Do"; Atom "you"; Atom "know";
             Atom "him?)"; Special '@'; Atom "domain"; Special '.'; Atom "com" ]

- Quoted strings:

       scan_structured_value "\"My.name\"@domain.com" [ '@'; '.' ] []
         = [ QString "My.name"; Special '@'; Atom "domain"; Special '.';
             Atom "com" ]

- Encoded words are not returned:

       scan_structured_value "=?ISO-8859-1?Q?Keld_J=F8rn_Simonsen?="
           [ ] [ ]
         = [ Atom "=?ISO-8859-1?Q?Keld_J=F8rn_Simonsen?=" ]

- Encoded words are returned if requested:

       scan_structured_value "=?ISO-8859-1?Q?Keld_J=F8rn_Simonsen?="
           [ ] [ Recognize_encoded_words ]
         = [ EncodedWord(("ISO-8859-1",""), "Q", "Keld_J=F8rn_Simonsen") ]

    type s_token =

`|` | `Atom of `string``
`|` | `EncodedWord of `((string * string) * string * string)`` | `(*` | Args: `((charset,lang),encoding,encoded_word)` | `*)`
`|` | `QString of `string``
`|` | `Control of `char``
`|` | `Special of `char``
`|` | `DomainLiteral of `string``
`|` | `Comment`
`|` | `End`

A token may be one of:

- `QString s`: The quoted string `s`, i.e a string between double quotes. Quoted pairs are already decoded in `s`.
- `Control c`: The control character `c` (0-31, 127, 128-255)
- `Special c`: The special character `c`, i.e. a character from the `specials` list
- `DomainLiteral s`: The bracketed string `s`, i.e. a string between brackets. Quoted pairs are already decoded in `s`.
- `Comment`: A string between parentheses. This kind of token is only generated when the option `Return_comments` is in effect.
- `EncodedWord((charset,lang),encoding,encoded_word)`: An RFC-2047 style encoded word: `charset` is the name of the character set; `lang` is the language specifier (from RFC 2231) or ""; `encoding` is either "Q" or "B"; and `encoded_word` is the word encoded in `charset` and `encoding`. This kind of token is only generated when the option `Recognize_encoded_words` is in effect (if not, `Atom` is generated instead).
- `Atom s`: A string which is neither quoted not bracketed nor written in RFC 2047 notation, and which is not a control or special character, i.e. the "rest"
- `End`: The end of the string

    type s_option =

`|` | `No_backslash_escaping` | `(*` | Do not handle backslashes in quoted string and comments as escape characters; backslashes are handled as normal characters. For example: The wrong qstring `"C:\dir\file"` will be returned as `QString "C:\dir\file"` when this option is in effect, and not as `QString "C:dirfile"` as by default. -- This is a common error in many MIME implementations. | `*)`
`|` | `Return_comments` | `(*` | Comments are returned as token `Comment` (unless '(' is included in the list of special characters, in which case comments are not recognized at all). You may get the exact location of the comment by applying `get_pos` and `get_length` to the extended token. | `*)`
`|` | `Recognize_encoded_words` | `(*` | Enables that encoded words are recognized and returned as `EncodedWord` instead of `Atom`. | `*)`

    type s_extended_token

An opaque type containing the information of `s_token` plus:

- where the token occurs
- RFC-2047 access functions

    val get_token : s_extended_token -> s_token

Return the `s_token` within the `s_extended_token`

    val get_decoded_word : s_extended_token -> string

    val get_charset : s_extended_token -> string

Return the decoded word (the contents of the word after decoding the "Q" or "B" representation), and the character set of the decoded word (uppercase).

These functions not only work for `EncodedWord`. The function `get_decoded_word` returns for the other kinds of token:

- `Atom`: Returns the atom without decoding it
- `QString`: Returns the characters inside the double quotes, and ensures that any quoted pairs are decoded
- `Control`: Returns the one-character string
- `Special`: Returns the one-character string
- `DomainLiteral`: Returns the characters inside the brackets, and ensures that any quoted pairs are decoded
- `Comment`: Returns `""`

The function `get_charset` returns `"US-ASCII"` for them.

    val get_language : s_extended_token -> string

Returns the language if the token is an `EncodedWord`, and `""` for all other tokens.

    val get_pos : s_extended_token -> int

Return the byte position where the token starts in the string (the first byte has position 0)

    val get_line : s_extended_token -> int

Return the line number where the token starts (numbering begins usually with 1)

    val get_column : s_extended_token -> int

Return the column of the line where the token starts (first column is number 0)

    val get_length : s_extended_token -> int

Return the length of the token in bytes

    val separates_adjacent_encoded_words : s_extended_token -> bool

True iff the current token is white space (i.e. `Special ' '`, `Special '\t'`, `Special '\r'` or `Special '\n'`) and the last non-white space token was `EncodedWord` and the next non-white space token will be `EncodedWord`.

The background of this function is that white space between encoded words does not have a meaning, and must be ignored by any application interpreting encoded words.

    type mime_scanner

The opaque type of a scanner for structured values

    val create_mime_scanner : specials:char list ->
           scan_options:s_option list ->
           ?pos:int -> ?line:int -> ?column:int -> string -> mime_scanner

Creates a new `mime_scanner` scanning the passed string.

`specials` : The list of characters recognized as special characters.
`scan_options` : The list of global options modifying the behaviour of the scanner
`pos` : The position of the byte where the scanner starts in the passed string. Defaults to 0.
`line` : The line number of this first byte. Defaults to 1.
`column` : The column number of this first byte. Default to 0.

Note for `create_mime_scanner`:

The optional parameters `pos`, `line`, `column` are intentionally placed after `scan_options` and before the string argument, so you can specify scanners by partially applying arguments to `create_mime_scanner` which are not yet connected with a particular string:

     let my_scanner_spec = create_mime_scanner my_specials my_options in
     ...
     let my_scanner = my_scanner_spec my_string in
     ...

    val get_pos_of_scanner : mime_scanner -> int

    val get_line_of_scanner : mime_scanner -> int

    val get_column_of_scanner : mime_scanner -> int

Return the current position, line, and column of a `mime_scanner`. The primary purpose of these functions is to simplify switching from one `mime_scanner` to another within a string:

     let scanner1 = create_mime_scanner ... s in
     ... now scanning some tokens from s using scanner1 ...
     let scanner2 = create_mime_scanner ...
                      ?pos:(get_pos_of_scanner scanner1)
                      ?line:(get_line_of_scanner scanner1)
                      ?column:(get_column_of_scanner scanner1)
                      s in
     ... scanning more tokens from s using scanner2 ...

Restriction: These functions are not available if the option `Recognize_encoded_words` is on. The reason is that this option enables look-ahead scanning; please use the location of the last scanned token instead.

Note: To improve the performance of switching, it is recommended to create scanner specs in advance (see the example `my_scanner_spec` above).

    val scan_token : mime_scanner ->
           s_extended_token * s_token

Returns the next token, or `End` if there is no more token. The token is returned both as extended and as normal token.

    val scan_token_list : mime_scanner ->
           (s_extended_token * s_token) list

Returns all following tokens as a list (excluding `End`)

    val scan_structured_value : string ->
           char list -> s_option list -> s_token list

This function is included for backwards compatibility, and for all cases not requiring extended tokens.

It scans the passed string according to the list of special characters and the list of options, and returns the list of all tokens.

    val specials_rfc822 : char list

    val specials_rfc2045 : char list

The sets of special characters defined by the RFCs 822 and 2045.

## Parsing Certain Forms of Structured Values

    val scan_encoded_text_value : string -> s_extended_token list

Scans a "text" value. The returned token list contains only `Special`, `Atom` and `EncodedWord` tokens. Spaces, TABs, CRs, LFs are returned (as `Special`) unless they occur between adjacent encoded words in which case they are suppressed. The characters '(', '[', and '"' are also returned as `Special` tokens, and are not interpreted as delimiters.

For instance, this function can be used to scan the "Subject" field of mail messages.

    val scan_value_with_parameters : string -> s_option list -> string * (string * string) list

`let name, params = scan_value_with_parameters s options`: Scans values with annotations like `name ; p1=v1 ; p2=v2 ; ...` For example, MIME types like "text/plain;charset=ISO-8859-1" can be parsed.

The values may or may not be quoted. The characters ";", "=", and even "," are only accepted as part of values when they are quoted. On sytax errors, the function fails.

RFC 2231: This function supports some features of this RFC: Continued parameter values are concatenated. For example:

     Content-Type: message/external-body; access-type=URL;
        URL*0="ftp://";
        URL*1="cs.utk.edu/pub/moore/bulk-mailer/bulk-mailer.tar"

This is returned as:

    "message/external-body",
       [ ("access-type", "URL");
         ("URL", "ftp://cs.utk.edu/pub/moore/bulk-mailer/bulk-mailer.tar") ]
          )

However, encoded parameter values are not handled specially. The parameter `title*=us-ascii'en-us'This%20is%20%2A%2A%2Afun%2A%2A%2A` would be returned as `("title*", "us-ascii'en-us'This%20is%20%2A%2A%2Afun%2A%2A%2A")`. Use `scan_values_with_parameters_ep` instead (see below).

Raises `Failure` on syntax errors.

    type s_param

The type of encoded parameters (RFC 2231)

    val param_value : s_param -> string

    val param_charset : s_param -> string

    val param_language : s_param -> string

Return the decoded value of the parameter, the charset (uppercase), and the language. If the charset is not available, `""` will be returned. If the language is not available, `""` will be returned.

    val mk_param : ?charset:string -> ?language:string -> string -> s_param

Creates a parameter from a value (in decoded form). The parameter may have a charset and a language.

    val print_s_param : Stdlib.Format.formatter -> s_param -> unit

Prints a parameter to the formatter (as toploop printer)

    val scan_value_with_parameters_ep : string ->
           s_option list ->
           string * (string * s_param) list

`let name, params = scan_value_with_parameters_ep s options`: This version of the scanner copes with encoded parameters according to RFC 2231. Note: "ep" means "encoded parameters".

Example: `doc.html;title*=us-ascii'en-us'This%20is%20%2A%2A%2Afun%2A%2A%2A`

The parameter `title` would be returned as:

- name is `"title"`
- value is `"This is ***fun***"`
- charset is `"US-ASCII"`
- language is `"en-us"`

Raises `Failure` on syntax errors.

    val scan_mime_type : string -> s_option list -> string * (string * string) list

`let name, params = scan_mime_type s options`: Scans MIME types like `text/plain; charset=iso-8859-1` The name of the type and the names of the parameters are converted to lower case.

Raises `Failure` on syntax errors.

    val scan_mime_type_ep : string ->
           s_option list ->
           string * (string * s_param) list

`let name, params = scan_mime_type_ep s options`: This version copes with RFC-2231-encoded parameters.

Raises `Failure` on syntax errors.

    val split_mime_type : string -> string * string

`let (main_type, sub_type) = split_mime_type content_type`: Splits the MIME type into main and sub type, for example `split_mime_type "text/plain" = ("text", "plain")` . The returned strings are always lowercase.

Raises `Failure` on syntax errors.

## Printing Structured Values

    exception Line_too_long

Raised when the hard limit of the line length is exceeded

    val write_value : ?maxlen1:int ->
           ?maxlen:int ->
           ?hardmaxlen1:int ->
           ?hardmaxlen:int ->
           ?fold_qstring:bool ->
           ?fold_literal:bool ->
           ?unused:int Stdlib.ref ->
           ?hardunused:int Stdlib.ref ->
           Netchannels.out_obj_channel -> s_token list -> unit

Writes the list of `s_token` to the `out_obj_channel`. The value is optionally folded into several lines while writing, but this is off by default. To enable folding, pass both `maxlen1` and `maxlen`: The `maxlen1` parameter specifies the length of the first line to write, the `maxlen` parameter specifies the length of the other lines.

If enabled, folding tries to ensure that the value is written in several lines that are not longer as specified by `maxlen1` and `maxlen`. The value is split into lines by inserting "folding space" at certain locations (which is usually a linefeed followed by a space character, see below). The following table specifies between which tokens folding may happen:

                   +=========================================================+
     1st   \   2nd | Atom | QString | DLiteral | EncWord | Special | Spec ' '|
     ==============+======+=========+==========+=========+=========+=========+
              Atom | FS   |  FS     |   FS     |   FS    |    -    |    F    |
           QString | FS   |  FS     |   FS     |   FS    |    -    |    F    |
     DomainLiteral | FS   |  FS     |   FS     |   FS    |    -    |    F    |
       EncodedWord | FS   |  FS     |   FS     |   FS    |    -    |    F    |
           Special | -    |  -      |   -      |   -     |    -    |    F    |
       Special ' ' | -    |  -      |   -      |   -     |    -    |    -    |
     ==============+======+=========+==========+=========+=========+=========+

The table shows between which two types of tokens a space or a folding space is inserted:

- `FS`: folding space
- `F`: linefeed without extra space
- `-`: nothing can be inserted here

Folding space is `"\n "`, i.e. only LF, not CRLF is used as end-of-line character. The function `write_header` will convert these LF to CRLF if needed.

`Special '\t'` is handled like `Special ' '`. Control characters are just printed, without folding. Comments, however, are substituted by either space or folding space. The token `End` is ignored.

Furthermore, folding may also happen within tokens:

- `Atom`, `Control`, and `Special` are never split up into parts. They are simply printed.
- `EncodedWord`s, however, are reformatted. This especially means: adjacent encoded words are first concatenated if possible (same character set, same encoding, same language), and then split up into several pieces with optimally chosen lengths. Note: Because this function gets `s_token` as input and not `s_extended_token`, it is not known whether `Special ' '` tokens (or other whitespace) between adjacent EncodedWords must be ignored. Because of this, `write_value` only reformats adjacent encoded words when there is not any whitespace between them.
- `QString` may be split up in a special way unless `fold_qstring` is set to `false`. For example, `"One Two Three"` may be split up into three lines `"One\n Two\n \ Three"`. Because some header fields explicitly forbid folding of quoted strings, it is possible to set `~fold_qstring:false` (it is `true` by default). Note: Software should not rely on that the different types of whitespace (especially space and TAB) remain intact at the beginning of a line. Furthermore, it may also happen that additional whitespace is added at the end of a line by the transport layer.
- `DomainLiteral`: These are handled like `QString`. The parameter `~fold_literal:false` turns folding off if it must be prevented, it is `true` by default.
- `Comment`: Comments are effectively omitted! Instead of `Comment`, a space or folding space is printed. However, you can output comments by passing sequences like `Special "("; ...; Special ")"` .

It is possible to get the actual number of characters back that can still be printed into the last line without making the line too long. Pass an `int ref` as `unused` to get this value (it may be negative!). Pass an `int ref` as `hardunused` to get the number of characters that may be printed until the hard limit is exceeded.

The function normally does not fail when a line becomes too long, i.e. it exceeds `maxlen1` or `maxlen`. However, it is possible to specify a hard maximum length (`hardmaxlen1` and `hardmaxlen`). If these are exceeded, the function will raise `Line_too_long`.

For electronic mail, a `maxlen` of 78 and a `hardmaxlen` of 998 is recommended.

Known Problems:

- The reformatter for EncodedWords takes into account that multi-byte characters must not be split up. However, this works only when the multi-byte character set is known to `Netconversion`. You can assume that UTF-8 and UTF-16 always work. If the character set is not known the reformatter may split the string at wrong positions.
- The reformatter for EncodedWords may parse the token, and if this fails, you will get the exception `Malformed_code`. This is only done in some special cases, however.
- The function prints spaces between adjacent atoms. Although this is allowed in principal, other MIME implementations might fail when there are spaces at unexpected locations. Workaround: If no spaces are desired, concatenate adjacent atoms before passing them to this function.

Further Tips:

- Pass ~maxlen1:0 and ~maxlen:0 to get shortest lines
- Use the reformatter for encoded words! It works well. For example, to output a long sentence, just wrap it into one `EncodedWord`. The reformatter takes care to fold the word into several lines.

    val param_tokens : ?maxlen:int ->
           (string * s_param) list -> s_token list

Formats a parameter list. For example, `[ "a", "b"; "c", "d" ]` is transformed to the token sequence corresponding to `; a=b; c=d`. If `maxlen` is specified, it is ensured that the individual parameter (e.g. `"a=b;"`) is not longer than `maxlen-1`, such that it will fit into a line with maximum length `maxlen`. By default, no maximum length is guaranteed. If `maxlen` is passed, or if a parameter specifies a character set or language, the encoding of RFC 2231 will be applied. If these conditions are not met, the parameters will be encoded traditionally.

    val split_uri : string -> s_token list

Splits a long URI according to the algorithm of RFC 2017. The input string must only contain 7 bit characters, and must be, if necessary, already be URL-encoded.

## Scanning MIME Messages

    val scan_multipart_body : string ->
           start_pos:int ->
           end_pos:int -> boundary:string -> ((string * string) list * string) list

`let [params1, value1; params2, value2; ...] = scan_multipart_body s start_pos end_pos boundary`:

Scans the string `s` that is the body of a multipart message. The multipart message begins at position `start_pos` in `s`, and `end_pos` is the position of the character following the message. In `boundary` the boundary string must be passed (this is the "boundary" parameter of the multipart MIME type, e.g. `multipart/mixed;boundary="some string"` ).

The return value is the list of the parts, where each part is returned as pair `(params, value)`. The left component `params` is the list of name/value pairs of the header of the part. The right component is the raw content of the part, i.e. if the part is encoded ("content-transfer-encoding"), the content is returned in the encoded representation. The caller is responsible for decoding the content.

The material before the first boundary and after the last boundary is not returned.

Multipart Messages

The MIME standard defines a way to group several message parts to a larger message (for E-Mails this technique is known as "attaching" files to messages); these are the so-called multipart messages. Such messages are recognized by the major type string "multipart", e.g. `multipart/mixed` or `multipart/form-data`. Multipart types MUST have a `boundary` parameter because boundaries are essential for the representation.

Multipart messages have a format like (where "_" denotes empty lines):

     ...Header...
     Content-type: multipart/xyz; boundary="abc"
     ...Header...
     _
     Body begins here ("prologue")
     --abc
     ...Header part 1...
     _
     ...Body part 1...
     --abc
     ...Header part 2...
     _
     ...Body part 2
     --abc
     ...
     --abc--
     Epilogue

The parts are separated by boundary lines which begin with "--" and the string passed as boundary parameter. (Note that there may follow arbitrary text on boundary lines after "--abc".) The boundary is chosen such that it does not occur as prefix of any line of the inner parts of the message.

The parts are again MIME messages, with header and body. Note that it is explicitely allowed that the parts are even multipart messages.

The texts before the first boundary and after the last boundary are ignored.

Note that multipart messages as a whole MUST NOT be encoded. Only the PARTS of the messages may be encoded (if they are not multipart messages themselves).

Please read RFC 2046 if want to know the gory details of this brain-dead format.

    val scan_multipart_body_and_decode : string ->
           start_pos:int ->
           end_pos:int -> boundary:string -> ((string * string) list * string) list

Same as `scan_multipart_body`, but decodes the bodies of the parts if they are encoded using the methods "base64" or "quoted printable". Fails, if an unknown encoding is used.

    val scan_multipart_body_from_netstream : Netstream.in_obj_stream ->
           boundary:string ->
           create:((string * string) list -> 'a) ->
           add:('a -> Netstream.in_obj_stream -> int -> int -> unit) ->
           stop:('a -> unit) -> unit

`scan_multipart_body_from_netstream s boundary create add stop`:

Reads the MIME message from the netstream `s` block by block. The parts are delimited by the `boundary`.

Once a new part is detected and begins, the function `create` is called with the MIME header as argument. The result `p` of this function may be of any type.

For every chunk of the part that is being read, the function `add` is invoked: `add p s k n`.

Here, `p` is the value returned by the `create` invocation for the current part. `s` is the netstream. The current window of `s` contains the read chunk completely; the chunk begins at position `k` of the window (relative to the beginning of the window) and has a length of `n` bytes.

When the part has been fully read, the function `stop` is called with `p` as argument.

That means, for every part the following is executed:

- `let p = create h`
- `add p s k1 n1`
- `add p s k2 n2`
- ...
- `add p s kN nN`
- `stop p`

Important Precondition:

- The block size of the netstream `s` must be at least `String.length boundary + 4`

Exceptions:

- Exceptions can happen because of ill-formed input, and within the callbacks of the functions `create`, `add`, `stop`.
- If the exception happens while part `p` is being read, and the `create` function has already been called (successfully), the `stop` function is also called (you have the chance to close files). The exception is re-raised after `stop` returns.

    val read_multipart_body : (Netstream.in_obj_stream -> 'a) ->
           string -> Netstream.in_obj_stream -> 'a list

This is the "next generation" multipart message parser. It is called as follows:

`let parts = read_multipart_body f boundary s`

As precondition, the current position of the stream `s` must be at the beginning of the message body. The string `boundary` must be the message boundary (without "--"). The function `f` is called for every message part, and the resulting list `parts` is the concatentation of the values returned by `f`.

The stream passed to `f` is a substream of `s` that begins at the first byte of the header of the message part. The function `f` can read data from the substream as necessary. The substream terminates at the end of the message part. This means that `f` can simply read the data of the substream from the beginning to the end. It is not necessary that `f` reads the substream until EOF, however.

After all parts have been read, the trailing material of stream `s` is skipped until EOF of `s` is reached.

## Helpers for MIME Messages

    val create_boundary : ?random:string list -> ?nr:int -> unit -> string

Creates a boundary string that can be used to separate multipart messages. The string is 63 characters long and has the following "features":

- Most of the string consists of the minus character yielding a clear optical effect
- The string contains "=__". This sequence cannot be obtained by the quoted-printable encoding, so you need not to care whether strings encoded as quoted-printable contain the boundary.
- The string contains "<&>;" which is illegal in HTML, XML, and SGML.
- The string does not contain double quotes or backslashes, so you can safely put double quotes around it in the MIME header.
- The string contains `nr`, so you can safely distinguish between several boundaries occurring in the same MIME body if you assign different `nr`.
- The string contains a hash value composed of the first 256 bytes of all strings passed as `random`, and influenced by the current GC state.