[web-devel] WAI enhancement: request body size

[Michael Snoyman]

I'd like to make an addition to the WAI protocol: a field in the Request
data type to track the request body size. I would like this to be:

    requestBodyLength :: Maybe Word64

I think the choice of Word64 makes sense without further explanation. The
perhaps surprising part is the use of `Maybe`. The reason for this is that
in the case of a chunked request body, there's no way to know the size of
the request body from the headers alone. So the logic for populating this
field would go something like:

   - If content-length is supplied by the user, parse it and populate the
   field with Just content-length.
   - If the request is chunked, populate with Nothing.
   - Otherwise, there is a zero-sized request body, and populate with Just
   0.

One other idea would be to create a special datatype such as `data
RequestBodyLength = ChunkedBody | KnownLength Word64`, but I don't believe
it really adds much above the `Maybe Word64`.

Any thoughts on this proposal?

Note: This is related to Yesod issue:
https://github.com/yesodweb/yesod/issues/468
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