[Haskell-beginners] List combination function?

[Ian Duncan]

Hello all,

I'm trying to build a function that takes a string such as "123" and 
gives me permutations including permutations with lesser list lengths. 
I'm not sure how to phrase it, but here is what the output could look like:

foo "123" => ["123","213","321","231","312","132", "12", "13", "21", 
"23", "31", "32", "1", "2", "3", ""]

The ordering doesn't matter, and that null list at the end doesn't 
particularly matter, but I don't really know the mathematical phrasing 
of what I'm asking for. I'm trying to build a scrabble helper that can 
find the optimal score given a set of letters to work with.

Thanks for your help,

Ian Duncan