Middleware that returns a handler which transforms any
:value slots in
messages sent via the request's
Transport to strings via the provided
:printer function, delegating all actual message handling to the provided
If no custom printer is set, this falls back to using
print-method. The function will be called with the value and any resolved
:print-options from the message.
Requires that results of eval operations are sent in messages in a
:value is already a string, and a sent message's
contains any truthy value, then
:value will not be re-printed. This allows
evaluation contexts to produce printed results in
:value if they so choose,
and opt out of the printing here.