An HTTP POST doesn’t necessarily have to create a persisted resource. In RFC 2616, section 9.5 we find:
The action performed by the POST method might not result in a resource
that can be identified by a URI. In this case, either 200 (OK) or 204
(No Content) is the appropriate response status, depending on whether
or not the response includes an entity that describes the result.
This means we can think about “creating a calculation” without having to persist the result of that calculation at it’s own URL. Your API could look like this:
Request:
POST /calculations
Content-Type: text/plain
3 + 3 / 12 ^ ( 3 * PI)
Response:
200 OK
Content-Type: text/plain
3.005454
This has the benefit that you’re not passing “content” through the URL, which will break when you try to submit a long calculation through a client or proxy with limited length for URLs. You could also make support different representations for requests and responses.