IsMono is a type-constraint that is used by the middleware api for allowing some operators only when the following condition is met.

Condition: Since a middleware takes in an Http and returns a new Http, IsMono, makes sure that the type parameters of the incoming Http and the ones for the outgoing Http is the same.

For Eg: IsMono will be defined for a middleware that looks as follows val mid: Middleware[Any, Nothing, Request, Response, Request, Response]

This is because both the middleware is defined from (Request, Response) => (Request, Response). Consider another example:

val mid: Middleware[Any, Nothing, Request, Response, UserRequest, UserResponse]

In this case, the incoming and outgoing types are different viz. (Request, Response) => (UserRequest, UserResponse), hence there is no IsMono defined for such middlewares.