[Notes by Jeff Vaughan]

AST/Interpreters

Using SML datatypes we can implement a very basic interpreter to operate on 
sets.

type set =  int list
datatype opr = Union | Intersection
datatype exp = Set_exp of set | Opr_exp of exp * opr *exp

fun eval(e: exp): set =
   case e of
     Set_exp(s) => s
   | Opr_exp(e1, Union, e2) => (eval(e1))@(eval(e2))
   | Opr_exp(e1, Intersection, e2) =>
      List.filter ( fn(x:int) => List.exists (fn(y)=>y=x) (eval e2)) (eval e1)

I think it's all straightforward.  Please notice that multiple copies of a 
single element can wind up in a set after a union.  This is a bug - it's 
not true to the definition of a set, and waste's space.  One way to fix it 
is to call ListMergeSort.uniqueSort after appending the lists.


I'm going to cop out and post a link to the notes from the 2002sp
lecture on ASTs and substitution
http://www.cs.cornell.edu/courses/cs312/2002sp/lectures/lec14.asp

There are few basics examples of non-recursive substitution, and an
overview of an Abstract syntax for a more interesting language than my
set language.