Testing
When using the colog loggers in our code, we have the posibility of testing that the expected log statements are actually produced by using a pure logger type in our testing code.
The pure logger can be instantiated using Loggers.pure and we need to provide it with a type that is able to buffer the log statements in memory for us. In the core module we have the MemLog (or it's transformer sibling MemLogT) type which we could use out of the box for that purpose:
type BufferedLog[A] = MemLog[String, A]
val bufferL = Loggers.pure[BufferedLog, String]
// bufferL: Logger[BufferedLog, String] = Logger(
// colog.Loggers$$$Lambda$16019/411295653@7191245b
// )
So now we can use that newly defined logger to emit our log statements:
val logged = bufferL.log("Hello")
// logged: BufferedLog[Unit] = WriterT((Vector("Hello"), ()))
The log statements have been accumulated in memory by a Writer monad, so we need to run it to obtain them:
val (logs, _) = logged.run
// logs: Vector[String] = Vector("Hello")
Loggers.pure can work with any functor that supports the tell operation (known as FunctorTell in cats-mtl), a writer monad is one of them (and that's what the MemLog type is) although it is not required to use it as long as a resolvable instance of a FunctorTell[F[_], Vector[A]] is available.