in ktor-server-html-builder-jvm, there’s an intere...

# compiler

[August Lilleaas]

in ktor-server-html-builder-jvm, there’s an interesting Kotlin detail that I’m wondering if anyone here knows more about: respondHtml(status) { with(template) { apply() } } . The use of with here seems to be used to make sure that apply() refers to the method that’s defined as a method on HTML<T> that template implements. If you write template.apply() it resolves to the built in scope function apply . What is it about the use of with that makes Kotlin invoke the apply method on HTML<T> rather than the scope function?

[Big Chungus]

With overrides default receiver context to template, which allows you to access extenstion methods defined within that type

[August Lilleaas]

so I guess that the “rule” is that the default receiver context gets presedence somehow?

[Big Chungus]

Yes, since with places template as dispatch receiver thus allowing access to extension functions defined inside it

[August Lilleaas]

interestingly, this.apply inside the with block makes apply refer to the extension function

[August Lilleaas]

so it seems to be a presedence rule that specifically only applies to an implicit receiver

[Big Chungus]

Note that extension functions defined inside the type are not the same as extension functions defined on the type

[August Lilleaas]

right, I’m talking about the built-in apply that’s defined on a generic type actually, so it’s available for everything

[Big Chungus]

Doesn't really matter in this case, same rules apply

[August Lilleaas]

pun intended? 🦜 😄

[August Lilleaas]

I wonder if this rule we’re discussing here is encoded somewhere in the docs 🤔 https://kotlinlang.org/docs/reference/grammar.html

[August Lilleaas]

Calling such a function is special because the receiver parameter is not supplied as an argument of the call, but as the receiver of the call, be it implicit or explicit. This parameter is available inside the scope of the function as the implicit receiver or this-expression, while nested scopes may introduce additional receivers that take precedence over this one.

[August Lilleaas]

from https://kotlinlang.org/spec/kotlin-spec.html

[August Lilleaas]

The implicit receiver having the highest priority is also called the default implicit receiver. The default implicit receiver is available in a scope as this.

[Big Chungus]

Probably easier to understand with example

class Type {
  fun String.doStuff() = println("Using dispatch receiver")
}

fun String.doStuff() = println("Using extension receiver")


fun main() {
  val type = Type()
  // Calls top-level extension function
  "HI".doStuff()
  // Calls a method from Type class
  with(type) {
    "HI".doStuff()
  }
}

[August Lilleaas]

that makes sense - in Template<T> the apply function is defined as an extension function on T