eval.sig

(* Authors: David Walker, Yitzhak Mandelbaum, Andrew Appel, etc.
*)
signature EVAL = sig

    datatype value =
	     IntV of int            (* integer value *)
	   | FunV of Absyn.func     (* function value *)
           | ExternV of Absyn.id    (* external function value *)
	   | TupleV of value list   (* tuple value *)
           | LocV of int            (* new - location value*)
			       
    (* Evaluate a program and return a value.
       In some circumstances, it is impossible to continue evaluation.
       In these cases, raise the exception Eval
       For example, it is impossible to continue evaluation when:
       1) a program tries to apply a primitive operation to an object of the 
       wrong type.  For instance, PrintInt can only print integers,
       not tuples or functions.  multiply can only multiply two
       integers.  
       2) a program tries to apply a primitive operation to the wrong number
       of arguments.  Multiply, subtract and add all work on exactly 2
       arguments.
       3) a program tries to project (Proj) from something that is not
       a tuple (eg: it tries to project from an integer or a function).
       4) a program tries to do a call operation on something that is not a
       function
       5) a program tries to do an If test on something that is not an integer
     *)
    exception Eval
    val eval_prog : Absyn.prog -> value
end