No matter what we will need the bytestring dependency to define the CIByteString datatype. But is there an advantage to this polymorphism?
Yes, there is no need to define CIByteString at all - that can come in the WAI package, and since it has a IsString instance it will work with the http-types library automatically.
I can't think of a use case for a status message or a header to be anything but a ByteString.
Perhaps, but by making them IsString you keep full flexibility, remove all debates over things like whether hdrContentType should be CIByteString, what functions should be on CIByteString etc. Thanks, Neil PS. I'm not actually too interested in the results, as long as they slot into WAI I don't mind what the types are - the IsString was just an idea that might make things simpler and more flexible (or might not).