Anatomy an endpoint

An endpoint is represented as a value of type Endpoint[I, E, O, S], where:

  • I is the type of the input parameters
  • E is the type of the error-output parameters
  • O is the type of the output parameters
  • S is the type of streams that are used by the endpoint’s inputs/outputs

Input/output parameters (I, E and O) can be:

  • of type Unit, when there’s no input/ouput of the given type
  • a single type
  • a tuple of types

Hence, an empty, initial endpoint (tapir.endpoint), with no inputs and no outputs, from which all other endpoints are derived has the type:

val endpoint: Endpoint[Unit, Unit, Unit, Nothing] = ...

An endpoint which accepts two parameters of types UUID and Int, upon error returns a String, and on normal completion returns a User, would have the type:

Endpoint[(UUID, Int), String, User, Nothing]

You can think of an endpoint as a function, which takes input parameters of type I and returns a result of type Either[E, O], where inputs or outputs can contain streaming bodies of type S.

Infallible endpoints

Note that the empty endpoint description maps no values to either error and success outputs, however errors are still represented and allowed to occur. If you would prefer to use an endpoint description, where errors can not happen, use infallibleEndpoint: Endpoint[Unit, Nothing, Unit, Nothing]. This might be useful when interpreting endpoints as a client.

Defining an endpoint

The description of an endpoint is an immutable case class, which includes a number of methods:

  • the name, description, etc. methods allow modifying the endpoint information, which will then be included in the endpoint documentation
  • the get, post etc. methods specify the HTTP method which the endpoint should support
  • the in, errorOut and out methods allow adding a new input/output parameter
  • mapIn, mapInTo, … methods allow mapping the current input/output parameters to another value or to a case class

An important note on mapping: in tapir, all mappings are bi-directional. That’s because each mapping can be used to generate a server or a client, as well as in many cases can be used both for input and for output.