Date: Thu, 19 Sep 2002 16:53:22 -0400
To: Li Liao <lliao@mail.eecis.udel.edu>
Subject: Re: homework question


Thank you for your reply. It's really helpfull to see some examples,as I
didn't take a compiler class and some thing are new to me.
I also had some additional question in the secons email I sent you.
And here is one more question for ex1:
it's fairly easy to write a funcion for ex1 if we consider only simle
strings like:
Repeat(Char #"a"); Char #"a"
The function will look like this
fun canBeEmpty Empty = true
|   canBeEmpty (Repeat a) = true
|   canBeEmpty (Alt (a, b))= false
|   canBeEmpty (Conc (a, b))=false
|   canBeEmpty (Char a) = false;

But we consider more complicated regular expressions like: Alt(Conc(Char
#"a", Repeat(Char #"b")), Conc(Repeat(Char #"a"),Char #"b"))
we'll have to parse the string and consider every case, if any Repeat or
Empty is found, we'll return true. What is the best way to parse this string
in ML and do comparisons?