Servant is a Haskell library that implements a domain-specific language for web APIs at the type level. By specifying the API as types, you get a clean separation of HTTP logic and the value-level handlers. Also, since the type checker has knowledge about the various endpoints and where they live, it will be able to verify that all handlers return the correct types and headers, and that internal URIs are correct.

Implementing a server (i.e. a set of handlers for the HTTP endpoints) is just one of several uses of an API type, however. By traversing the type, Servant is also able to derive a Swagger/OpenAPI specification for the API, and even client code in JavaScript and other languages.

We will look at how Servant achieves a clean separation of HTTP and business logic – true to the core philosophy of HTTP and REST. By not just implementing a server for an API, but also deriving a JavaScript client and a Swagger/OpenAPI specification, we will end up with an API with strong correctness guarantees and also documentation and client code that is guaranteed to be correct and up to date.