Date: Sat, 19 Sep 2002 12:03:13 -0400 (EDT)
Subject: homework question

> What are Alt and Repeat in ML? Are they reserved words or special
> functions in ML?(see ex1). 

Alt and Repeat are names you give to construct your own datatype.
They are NOT reserved words or special functions in ML.
 

> How do we  know which case can be empty and which case is not?
> Could you give us more examples on what cases can be emty and what cases are not?

You do pattern matching. 
Examples:
Empty is empty
Repeat (Char #"a") can be empty
Alt (Empty, Char #"a") can be empty
Conc(Empty, Empty) is empty
...


Please let me know if you still have problem.

Li