# Module Netmime

    module Netmime: sig .. end

Netmime contains high-level classes and functions to process mail and MIME messages.

Contents

- Types
- Classes
- Parsing MIME messages
- Printing MIME Messages

The tutorial has been moved to `Netmime_tut`.

# Types

    type store = [ `File of string | `Memory ]

Specifies where to store the body of a mail message. ``Memory` means in-memory, ``File name` means in the file `name`. The body is stored in decoded form (i.e. without transfer encoding).

    exception Immutable of string

Raised if it is tried to modify a read-only value. The string denotes the function or method where the incident happened.
MIME headers and bodies are defined in two steps. First the subtype describing read access is defined (`mime_header_ro`, and `mime_body_ro`), and after that the full class type including write access is defined (`mime_header`, and `mime_body`).

The idea is that you can write functions that take an ro value as input to indicate that they do not modify the value. For example:

     let number_of_fields (h:#mime_header_ro) =
       List.length (h#fields)

This function accepts both `mime_header`, and `mime_header_ro` values as input, but the typing ensures that the function cannot mutate anything.

There is another way to ensure that a header or body is not modified. The read-only flag can be set when creating the object, and this flag causes that all trials to modify the value will raise the exception `Immutable`. Of course, such trials of mutation are only detected at run-time.

The advantage of the read-only flag is that it even works if mutation depends on a condition, but it can be ensured that this condition is never true. Furthermore, typing is much simpler (getting subtyping correct can be annoying).

    class type mime_header_ro = object .. end

This is the read-only version of a MIME header.

    class type mime_header = object .. end

A MIME header with both read and write method.

    class type mime_body_ro = object .. end

This is the read-only version of a MIME body.

    class type mime_body = object .. end

A MIME body with both read and write method.
One can consider the pair `(mime_header, mime_body)` as simple MIME message with one header and one body. Of course, this simple representation does not support multi-part messages (attachments). For that reason, the `complex_mime_message` was invented: The body can be further structured as a sequence of parts that are complex messages themselves.

For example, a mail message with an attachment is usually represented as

     (mail_header, `Parts [ (main_header, `Body main_body);
                            (att_header, `Body att_body) ] )

Here, `mail_header` is the real header of the mail message. `main_header` is the header of the main message, usually only containing the content type of `main_body`, the body of the main message. The attachment has also its own `att_header`, again usually only containing the content type, and the data of the attachment can be found in `att_body`.

Nowadays, mails have often even a more complicated structure with ``Parts` containing nested ``Parts`. As `complex_mime_message` is recursive, any kind of nesting can be easily represented.

    type complex_mime_message = mime_header * complex_mime_body

    type complex_mime_body = [ `Body of mime_body | `Parts of complex_mime_message list ]

    type complex_mime_message_ro = mime_header_ro * complex_mime_body_ro

    type complex_mime_body_ro = [ `Body of mime_body_ro
           | `Parts of complex_mime_message_ro list ]

The read-only view of a complex_mime_message
Note: ``Parts []`, i.e. ``Parts` together with an empty list, is considered as illegal. Such a value cannot be transformed into printable text.

    type mime_message = mime_header * [ `Body of mime_body ]

Simple MIME message, in a form that is compatible with complex ones.

    type mime_message_ro = mime_header_ro * [ `Body of mime_body_ro ]

Read-only variant of simple messages

# Classes

    class basic_mime_header : ?ro:bool -> (string * string) list -> mime_header

An implementation of `mime_header`.

    class memory_mime_body : ?ro:bool -> string -> mime_body

An implementation of `mime_body` where the value is stored in-memory.

    class file_mime_body : ?ro:bool -> ?fin:bool -> string -> mime_body

An implementation of `mime_body` where the value is stored in an external file.

# Parsing MIME messages

    val read_mime_header : ?unfold:bool ->
           ?strip:bool -> ?ro:bool -> Netstream.in_obj_stream -> mime_header

Decodes the MIME header that begins at the current position of the netstream, and returns the header as class `basic_mime_header`. After returning, the stream is advanced to the byte following the empty line terminating the header.

Example: To read the header at the beginning of the file "f", use:

     let ch = new Netchannels.input_channel (open_in "f") in
     let stream = new Netstream.input_stream ch in
     let h = read_mime_header stream in
     ...
     stream#close_in();    (* no need to close ch *)

Note that although the `stream` position after parsing is exactly known, the position of `ch` cannot be predicted.

`unfold` : whether linefeeds are replaced by spaces in the values of the header fields (Note: defaults to `false` here in contrast to `Mimestring.scan_header`!)
`strip` : whether whitespace at the beginning and at the end of the header fields is stripped
`ro` : whether the returned header is read-only (default: false)
Hint: To write the header `h` into the channel `ch`, use

     Mimestring.write_header ch h#fields

Link: `Mimestring.write_header`

    type multipart_style = [ `Deep | `Flat | `None ]

How to parse multipart messages:

- ``None`: Do not handle multipart messages specially. Multipart bodies are not further decoded, and returned as ``Body b` where `b` is the transfer-encoded text representation.
- ``Flat`: If the top-level message is a multipart message, the parts are separated and returned as list. If the parts are again multipart messages, these inner multipart messages are not furher decoded and returned as ``Body b`.
- ``Deep`: Multipart messages are recursively decoded and returned as tree structure.

This value determines how far the `complex_mime_message` structure is created for a parsed MIME message. ``None` means that no parts are decoded, and messages have always only a simple ``Body b`, even if `b` is in reality a multi-part body. With ``Flat`, the top-level multi-part bodies are decoded (if found), and messages can have a structured ``Parts [_, `Body b1; _, `Body b1; ...]` body. Finally, ``Deep` allows that inner multi-part bodies are recursively decoded, and messages can have an arbitrarily complex form.

    val decode_mime_body : #mime_header_ro ->
           Netchannels.out_obj_channel -> Netchannels.out_obj_channel

`let ch' = decode_mime_body hdr ch`: According to the value of the Content-transfer-encoding header field in `hdr` the encoded MIME body written to `ch'` is decoded and transferred to `ch`.

Handles 7bit, 8bit, binary, quoted-printable, base64.

Example: The file "f" contains base64-encoded data, and is to be decoded and to be stored in "g":

     let ch_f = new Netchannels.input_channel (open_in "f") in
     let ch_g = new Netchannels.output_channel (open_out "g") in
     let hdr = new basic_mime_header ["content-transfer-encoding", "base64" ] in
     let ch = decode_mime_body hdr ch_g in
     ch # output_channel ch_f;
     ch # close_out();
     ch_g # close_out();
     ch_f # close_in();

Note: This function is internally used by `read_mime_message` to decode bodies. There is usually no need to call it directly.

    val storage : ?ro:bool ->
           ?fin:bool -> store -> mime_body * Netchannels.out_obj_channel

Creates a new storage facility for a mime body according to `store`. This function can be used to build the `storage_style` argument of the class `read_mime_message` (below). For example, this is useful to store large attachments in external files, as in:

     let storage_style hdr =
       let filename = hdr ... (* extract from hdr *) in
       storage (`File filename)

`ro` : whether the returned mime_bodies are read-only or not. Note that it is always possible to write into the body using the returned out_obj_channel regardless of the value of ~ro. Default: false
`fin` : whether to finalize bodies stored in files. Default: false

    val read_mime_message : ?unfold:bool ->
           ?strip:bool ->
           ?ro:bool ->
           ?multipart_style:multipart_style ->
           ?storage_style:(mime_header ->
                           mime_body * Netchannels.out_obj_channel) ->
           Netstream.in_obj_stream -> complex_mime_message

Decodes the MIME message that begins at the current position of the passed netstream. It is expected that the message continues until EOF of the netstream.

Multipart messages are decoded as specified by `multipart_style` (see above).

Message bodies with content-transfer-encodings of 7bit, 8bit, binary, base64, and quoted-printable can be processed. The bodies are stored without content-transfer-encoding (i.e. in decoded form), but the content-transfer-encoding header field is not removed from the header.

The `storage_style` function determines where every message body is stored. The corresponding header of the body is passed to the function as argument; the result of the function is a pair of a new `mime_body` and an `out_obj_channel` writing into this body. You can create such a pair by calling `storage` (above).

By default, the `storage_style` is `storage ?ro `Memory` for every header. Here, the designator ``Memory` means that the body will be stored in an O'Caml string. The designator ``File fn` would mean that the body will be stored in the file `fn`. The file would be created if it did not yet exist, and it would be overwritten if it did already exist.

Note that the `storage_style` function is called for every non-multipart body part.

Large message bodies (> maximum string length) are supported if the bodies are stored in files. The memory consumption is optimized for this case, and usually only a small constant amount of memory is needed.

Example:

Parse the MIME message stored in the file f:

     let m = read_mime_message
               (new input_stream (new input_channel (open_in f)))

`unfold` : whether linefeeds are replaced by spaces in the values of the header fields (Note: defaults to `false` here in contrast to `Mimestring.scan_header`!)
`strip` : whether whitespace at the beginning and at the end of the header fields is stripped
`ro` : Whether the created MIME headers are read-only or not. Furthermore, the default `storage_style` uses this parameter for the MIME bodies, too. However, the MIME bodies may have a different read-only flag in general.

# Printing MIME Messages

    val encode_mime_body : ?crlf:bool ->
           #mime_header_ro ->
           Netchannels.out_obj_channel -> Netchannels.out_obj_channel

`let ch' = encode_mime_body hdr ch`: According to the value of the Content-transfer-encoding header field in `hdr` the unencoded MIME body written to ch' is encoded and transferred to ch.

Handles 7bit, 8bit, binary, quoted-printable, base64.

For an example, see `decode_mime_body` which works in a similar way but performs decoding instead of encoding.

`crlf` : if set (this is by default the case) CR/LF will be used for end-of-line (eol) termination, if not set LF will be used. For 7bit, 8bit and binary encoding the existing eol delimiters are not rewritten, so this option has only an effect for quoted-printable and base64.

    val write_mime_message : ?wr_header:bool ->
           ?wr_body:bool ->
           ?nr:int ->
           ?ret_boundary:string Pervasives.ref ->
           ?crlf:bool ->
           Netchannels.out_obj_channel -> complex_mime_message -> unit

Writes the MIME message to the output channel. The content-transfer- encoding of the leaves is respected, and their bodies are encoded accordingly. The content-transfer-encoding of multipart messages is always "fixed", i.e. set to "7bit", "8bit", or "binary" depending on the contents.

The function fails if multipart messages do not have a multipart content type field (i.e. the content type does not begin with "multipart"). If only the boundary parameter is missing, a good boundary parameter is added to the content type. "Good" means here that it is impossible that the boundary string occurs in the message body if the content-transfer-encoding is quoted-printable or base64, and that such an occurrence is very unlikely if the body is not encoded. If the whole content type field is missing, a "multipart/mixed" type with a boundary parameter is added to the printed header.

Note that already existing boundaries are used, no matter whether they are of good quality or not.

No other header fields are added, deleted or modified. The mentioned modifications are _not_ written back to the passed MIME message but only added to the generated message text.

It is possible in some cases that the boundary does not work (both the existing boundary, and the added boundary). This causes that a wrong and unparseable MIME message is written. In order to ensure a correct MIME message, it is recommended to parse the written text, and to compare the structure of the message trees. It is, however, very unlikely that a problem arises.

Note that if the passed message is a simple message like (_,`Body _), and if no content-transfer-encoding is set, the written message might not end with a linefeed character.

`wr_header` : If true, the outermost header is written. Inner headers of the message parts are written unless ~wr_body=false.
`wr_body` : If true, the body of the whole message is written; if false, no body is written at all.
`nr` : This argument sets the counter that is included in generated boundaries to a certain minimum value.
`ret_boundary` : if passed, the boundary of the outermost multipart message is written to this reference. (Internally used.)
`crlf` : if set (this is by default the case) CR/LF will be used for end-of-line (eol) termination, if not set LF will be used. The eol separator is used for the header, the multipart framing, and for bodies encoded as quoted-printable or base64. Other eol separators are left untouched.