I will try in this channel. I cannot understand what's going on in the declaration of Filter here: https://github.com/http4k/http4k/blob/master/http4k-core/src/main/kotlin/org/http4k/core/Http4k.kt To make it worse, when I isolated that declaration and tried to compile it, the compiler insists that "'invoke' overrides nothing". Here's my simple test: http://rextester.com/live/FQUC99965 So now I am stuck - I noticed that inheritance hierarchy in IntelliJ mentions some Function1 interface that seems not to be inherited there (but just spitballing here.)

You could try in #http4k, but this seems like more of a language question 🙂 The invoke method in question is inherited from

(HttpHandler) -> HttpHandler

, which is extended by the Filter interface. Which versions of Idea/Kotlin are you using?