Function in default parameter value closing over variables defined later in the enclosing function

Defaults of variables are evaluated in the evaluation frame of the
call. So the inside() function is created in the evaluation frame,
and it's environment will be that frame.

When it is called it will create a new evaluation frame (empty in
your example), with a parent being its environment, i.e. the
evaluation frame from when it was created, so it will be able to see
your secret variable.

in my opinion it would be fine to write it as

  outside <- function(inside = defaultInsideFn) {
     defaultInsideFn <- function() print(secret)
     secret <- 'secret'
     inside()
  }