(*
 * This file is an attempt to represent an rsl runtime value in rsl.  We note
 * that we seem not to need an explicit type tag, since rsl provides this for
 * us automatically.  This seems kinda sweet, if it pans out.
 *)

object Value =
    i: integer or
    r: real or
    b: boolean or
    s: string or
    l: ListValue or
    t: TupleValue or
    u: UnionValue or
    o: OpValue;

object ListValue = Value*;
object TupleValue = TupleFieldValue*;
object UnionValue = t:Tag and v:TupleValue;
object OpValue = op(Value)->(Value);

object TupleFieldValue = n:FieldName and v:Value;

object FieldName = string;
object Tag = string;

operation Dot(v:Value,fn:FieldName)->(Value) =
    if (v?.t) then
	FieldSelect(v.t, fn)
    else if (v?.u) then
	FieldSelect(v.u.v, fn)
    else
	nil

    description: (*
	This is an overloading of '.' for Value's.  It allows almost
	syntactically nice record refs of the form

	    x."y"

	for some TupleValue x with field named "y".

	WARNING: Overloading '.' may not be allowable, since it built-in def
	seems to be a fexper.  THIS REQUIES SOME MORE THOUGHT.

    *);
end Dot;

function FieldSelect(tv:TupleValue, fn:FieldName) -> (Value) =
    if tv = nil
	then nil
    else if tv[1].n = fn then
	tv[1].v
    else
	FieldSelect(tv[2:#tv], fn);