/*
	Class representing elements of form (a . b) in D in Jones, Chapter 2
	Written by:  Kim Bruce, 9/16/2002
*/

public class Composite implements D{
	private D head;		// composite represents (head . tail)
	private D tail;
	
	// Create new composite with "head" as new head and "tail" as new tail
	// I.e., new element is (head . tail)
	public Composite(D head, D tail) {
		this.head = head;
		this.tail = tail;
	}
	
	// create new list with other as head and this as tail
	public D cons(D other) {
		return new Composite(other,this);
	}
	
	// return head of this
	public D hd(){
		return head;
	}
	
	// return tail of this    
	public D tl(){
		return tail;
	}

	// returns whether argument is nil	
	private boolean isNil(D elt) {
		return elt == D.nil;
	}
	
	// return true iff this equals other
	public boolean booleq(D other) {
	    if (other instanceof Composite) {
			Composite oc = (Composite)other;
			D otherhead = oc.head;
			D othertail = oc.tail;
			return (head.booleq(otherhead) && tail.booleq(othertail));
		} else {
			return false;
		}
	}

	// return dTrue if this equals other and dFalse otherwise
	public D eq(D other){
	    if (booleq(other))
	    	return D.dTrue;
	    else
	    	return D.dFalse;
	}
	
	// return string representation of composite	
	public String toString() {
		return "( " + head +" . "+tail+")";
	}
	
	public boolean isList() {
		return (head instanceof Atom) && tail.isList();
	}
	
	public String asList() {
		if (isList()){
			String val = "( "+head;
			D curTail = tail;
			while (!isNil(curTail)) {
				val = val + ", "+curTail.hd();
				curTail = curTail.tl();
			}
			return val+" )";
		} else {
			return "error: not a list";
		}
	}
}