A fun example using Deferred to write puppet code in a functional style

functional_puppet.pp

# Resolves a given value - if given a Deferred it will be called and all other values are returned as is.
#
function resolve(Any $x) {
  if $x =~ Deferred { $x.call } else { $x }
}

# Resolves and evaluates a condition and either resolves and evaluates the given when_true,
# or the given when_false depending on the outcome of the evaluation of the condition.
# The result is undef if the selected when_true/when_false is undefined.
# 
function if_true(
  Any $test, 
  Optional[Any] $when_true = undef,
  Optional[Any] $when_false = undef
  ) {
  if resolve($test) { resolve($when_true) }
  else              { resolve($when_false) }
}

# Example function
function do_this() {
  notice "Doing this"
}
# Example function
function do_that() {
  notice "Doing that"
}
# Example - using the if_true() function (notices "Doing that")
false.if_true(Deferred('do_this'), Deferred('do_that'))