append.pp

[~]$ cat tmp/append.pp
class foo::bar($foo = []) {
    inline_template('<%= @foo.push("bar") %>')
    # do things
    inline_template('<%= @foo.push("foo") %>')
    # more things
    inline_template('<%= @foo.push("hello") %>')
    # even more things
    inline_template('<%= @foo.push("test") %>')
    notify{$foo.join(','):}
}

$foo = []

class { 'foo::bar':
  foo => $foo,
}

notify{'another':
  message => $foo.join(','),
}

[~]$ puppet apply tmp/append.pp
Notice: Compiled catalog for nyx.local in environment production in 0.03 seconds
Notice: bar,foo,hello,test
Notice: /Stage[main]/Foo::Bar/Notify[bar,foo,hello,test]/message: defined 'message' as 'bar,foo,hello,test'
Notice:
Notice: /Stage[main]/Main/Notify[another]/message: defined 'message' as ''
Notice: Applied catalog in 0.02 seconds