Completion of two items separated by a dash

["Scott Frazer (scfrazer)" <frazer.scott@xxxxxxxxx>]

I am trying to write a completion function for a command that takes two 
items separated by a dash, and the second item depends on the first. 
For example, suppose possible full completions were:

foo-bar
foo-baz
abc-def
abc-xyz

Ideally, the completion function would first present options "foo" and 
"abc", then after the user chooses one it inserts the "-', and when you 
hit tab again it presents either "bar" and "baz" or "def" and "xyz".

For this simple example I could just show all four, but in real life 
there are many first items and many second items and the list would be huge.

I have written some simple completions before, but this has me stumped. 
Is there a way to do this?

Thanks,
Scott