/*
	Class representing atoms in D in Jones, Chapter 2
	Written by:  Kim Bruce, 9/16/2002
*/

public class Atom implements D {
   
	private String value;	// value of atom
   
   	// Create atom with value val
	public Atom(String val) {
		value = val;
	}
	
	// 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 D.nil;
	}
	
	// return tail of this    
	public D tl(){
		return D.nil;
	}

	// return true iff this equals other
	public boolean booleq(D other){
		return (other instanceof Atom && ((Atom)other).value.equals(value));
	}

	// 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 atom	
	public String toString() {
		return value;
	}
	
	public boolean isList() {
		return value.equals("nil");
	}
	
	public String asList() {
		if (isList())
			return "()";
		else
			return "error: not a list";
	}
}