gistfile1.txt

# The latch function remembers what it was called with
# and returns what it was previously called with.
#
# This is using global namespace - in your code use a module namespace
#
Puppet::Functions.create_function(:latch) do
  dispatch :example do
    repeated_param 'Any', :arg
  end

  def example(*args)
    adapter = adapterclass.adapt(closure_scope().compiler)
    result = adapter.thing
    adapter.thing = args
    result
  end

  def adapterclass
    @adapterclass ||= Class.new(Puppet::Pops::Adaptable::Adapter) do
      attr_accessor :thing
      def self.name()
        # This is using global namespace - in your code use a module namespace
        "LatchFunctionAdapter"
      end
    end
    @adapterclass
  end
end

The above makes the adapter have the lifecycle of the compiler. If it should instead have the lifecycle of the environment change line 12 to be:

    adapter = adapterclass.adapt(closure_scope().environment)

You can try it with something like notice([latch(first), latch(second), latch(third)]) to get it to output the array [undef, "first", "second"] (this works with both lifecycles).

Have not tested the env lifecycle - to do that - have an agent request a catalog from master with a notify { "testlatch": message => "Time was: ${latch(Timestamp())}"} - then do it again a little while later.
The first should have the message "Time was:", and the second compile should have "Time was: ".

The env that is used must not have 0 timeout.

To be safe in general use of the pattern showed in the gist, the name of the class returned by name should actually include the id of the class as well - this because it could exist in a different version in memory in another environment - it will however not be a problem as long as it is being used to adapt to the environment or compiler (as shown in this gist) since any other version of the adapter would be adapted to other instances of the compiler/environment...