How to map an array within a resource

I've got stuck trying to use Puppet 3.8 to tackle what seems like it should be a simple problem. My nodes have a number of independent installations of a single system. I have defined a top-level type which creates instances of many resources. My nodes then declare a number of instances, and all the resources are instantiated, just as I'd like. Now, I'd like to augment each installation with its own set of externally node-specified directories. So when my node declares...

installation { "user1":
    subdirs => ["subdir1", "subdir2"]
 }

I'd like to end up creating (say)

          /home/user1/subdir1
          /home/user1/subdir2

How do I do that? In particular, how (within the class) do I transform the list of subdirectory names

["subdir1", "subdir2"] 

into names that are unique to the instance of the class, such as

 ["installation1/$subdir1", "installation1/$subdir2" ]