To your point, here's my Java implementation of the few key Scheme functions we
need in actions. It's kind of amusing to see "car" and "cons" in Java :-).
Yes, I know Java doesn't guarantee tail-recursion, but for its expected use and
environment that shouldn't matter. If we need a few more, we can just add
them; this means that we can use Scheme procedures in the ANTLR prototype
actions.
--- David A. Wheeler
=============================================
package scheme;
// (C) 2013 David A. Wheeler, released under MIT license.
// Provide a few traditional Lisp-like functions in Java,
// just barely enough functionality for sweet.g, but
// accurate for what it does.
public class Pair extends Object {
private Object car_value, cdr_value;
public Pair(Object x, Object y) {
this.car_value = x; this.cdr_value = y;
}
public static Pair cons(Object x, Object y) {
return new Pair(x, y);
}
public static Object car(Pair x) {
return x.car_value;
}
public static Object cdr(Pair x) {
return x.cdr_value;
}
public static Boolean nullp(Object x) { // Scheme "null?"
return x == null;
}
public static Boolean pairp(Object x) { // Scheme "pair?"
return x instanceof Pair;
}
// This does not check for cycles
private static Boolean ends_in_null(Pair x) {
if (nullp(cdr(x))) {
return true;
} else if (! pairp(cdr(x))) {
return false;
} else {
return ends_in_null( (Pair) cdr(x));
}
}
public static Boolean listp(Object x) { // Scheme "list?"
if (nullp(x)) return false;
else if (! pairp(x)) return false;
else return ends_in_null( (Pair) x);
}
public static Pair list(Object x) {
return cons(x, null);
}
public static Pair list(Object x, Object y) {
return cons(x, list(y));
}
// "y" really needs to be a pair, but that makes it harder to
// to work with ANTLR
public static Object append(Object x, Object y) {
if (x == null) {
return y;
} else if (x instanceof Pair) {
Pair p = (Pair) x;
return cons(car(p), append(cdr(p), y));
} else {
throw new Error(); // for now.
}
}
private static String string_datum_tail(Object x) {
if (x == null) {
return ")";
} else if (x instanceof Pair) {
Pair p = (Pair) x;
return " " + string_datum(car(p)) + string_datum_tail(cdr(p));
} else
return " . " + string_datum(x) + ")";
}
public static String string_datum(Object x) {
if (x == null) {
return "()";
} else if (x instanceof Pair) {
Pair p = (Pair) x;
return "(" + string_datum(car(p)) + string_datum_tail(cdr(p));
} else {
return x.toString();
}
}
// (define (monify x)
// (cond
// ((not (pair? x)) x)
// ((null? (cdr x)) (car x))
// (#t x)))
public static Object monify(Object x) {
if (! pairp(x)) {
return x;
} else if (nullp(cdr( (Pair) x))) {
return car( (Pair) x);
} else {return x;}
}
}
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