Adding numbers as digits in a list

In a phone interview with an un-named company :)  I was asked to come up with solution for hypothetical adder where  input data are integers  but are given as list of single digits.     For instance 345 would be [3,4,5].   Problem is to write an adder for the numbers.    I kind of like the simplicity of the solution in Haskell ( as oppose to my java-like doodle in the interview).

You can find my code Here