Am I missing something?

[ghost]

On both OpenBSD amd64 and Snow Leopard I get the following.

[james@thinkpad potion]$ ./potion                                                                                                                                                                                  
>> add = (x, y): x + y.
=> function(x, y)
>> add(2, 4) string print
nil=> nil
>>

Is this expect behavior? I would think I would get 6 returned. All tests pass when I run make test yet calling a simple function doesn't?

[ghost]

Yes, this is expected behavior because the scope in which the interactive interpreter runs commands changes with each line. In your example, on the second line, add was nil. If you seperated the lines with a comma, it would work:
>> add = (x, y): x + y., add(2, 4) string print
6=> nil