This introduces a `Delayed` type in `RoutingApplication.hs` that
contains a handler together with delayed checks. There are several
blocks of delayed checks, so that we can ultimately execute them in the
order we desire.
The process is documented in more detail in `RoutingApplication.hs`.
The main `Server.Internal` module was getting a bit large for my taste.
It now contains just the instances. All the administrative utilities
are in their own dedicated modules.
Instead of directly interpreting a server as a `RoutingApplication`,
this change introduces the concept of a `Router`, which is a datatype
with several constructors.
In particular, the type of the `route` function changes from
route :: Proxy layout -> Server layout -> RoutingApplication
to
route :: Proxy layout -> IO (RouteResult (Server layout)) -> Router
Most important in practice is the case of the `StaticRouter` constructor
in `Router`. For choices between statically known paths, we can now use
a lookup table to dispatch requests rather than trying each request
individually.
This brings down routing complexity of a common case from
O(n) to O(log n).
Another important change is that the handler that is passed down by
`route` is no longer of type `Server layout`, but of type
`IO (RouteResult (Server layout))`. This means that API constructs
can "delay" checks and failure. For example, `ReqBody` does not have
to fetch the request body and feed it to the handler immediately; it
can instead record these actions in the handler that is passed down.
The code will only be executed at a leaf / endpoint of the API.
This is desired behaviour: We prefer to save work by doing all matching
on static path components first. Furthermore, we get better error codes
by doing so.
