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Functional Java is an open source library that supports closures for the Java programming language
package fj.data;
import fj.Equal;
import fj.F;
import fj.F2;
import fj.F2Functions;
import fj.F3;
import fj.Function;
import fj.Ord;
import fj.P;
import fj.P1;
import fj.P2;
import fj.P3;
import fj.Show;
import fj.function.Integers;
import java.util.Iterator;
import static fj.Function.compose;
import static fj.Function.curry;
import static fj.Function.flip;
import static fj.Function.join;
import static fj.Function.uncurryF2;
import static fj.data.Option.none;
import static fj.data.Option.some;
import static fj.data.Stream.nil;
import static fj.data.Stream.repeat;
/**
* Provides a pointed stream, which is a non-empty zipper-like stream structure that tracks an index (focus)
* position in a stream. Focus can be moved forward and backwards through the stream, elements can be inserted
* before or after the focused position, and the focused item can be deleted.
*
* Based on the pointedlist library by Jeff Wheeler.
*/
public final class Zipper implements Iterable> {
private final Stream left;
private final A focus;
private final Stream right;
private Zipper(final Stream left, final A focus, final Stream right) {
this.left = left;
this.focus = focus;
this.right = right;
}
/**
* Creates a new Zipper with the given streams before and after the focus, and the given focused item.
*
* @param left The stream of elements before the focus.
* @param focus The element under focus.
* @param right The stream of elements after the focus.
* @return a new Zipper with the given streams before and after the focus, and the given focused item.
*/
public static Zipper zipper(final Stream left, final A focus, final Stream right) {
return new Zipper(left, focus, right);
}
/**
* Creates a new Zipper from the given triple.
*
* @param p A triple of the elements before the focus, the focus element, and the elements after the focus,
* respectively.
* @return a new Zipper created from the given triple.
*/
public static Zipper zipper(final P3, A, Stream> p) {
return new Zipper(p._1(), p._2(), p._3());
}
/**
* First-class constructor of zippers.
*
* @return A function that yields a new zipper given streams on the left and right and a focus element.
*/
public static F3, A, Stream, Zipper> zipper() {
return (l, a, r) -> zipper(l, a, r);
}
/**
* Returns the product-3 representation of this Zipper.
*
* @return the product-3 representation of this Zipper.
*/
public P3, A, Stream> p() {
return P.p(left, focus, right);
}
/**
* A first-class function that yields the product-3 representation of a given Zipper.
*
* @return A first-class function that yields the product-3 representation of a given Zipper.
*/
public static F, P3, A, Stream>> p_() {
return a -> a.p();
}
/**
* An Ord instance for Zippers.
*
* @param o An Ord instance for the element type.
* @return An Ord instance for Zippers.
*/
public static Ord> ord(final Ord o) {
final Ord> so = Ord.streamOrd(o);
return Ord.p3Ord(so, o, so).contramap(Zipper.p_());
}
/**
* An Equal instance for Zippers.
*
* @param e An Equal instance for the element type.
* @return An Equal instance for Zippers.
*/
public static Equal> eq(final Equal e) {
final Equal> se = Equal.streamEqual(e);
return Equal.p3Equal(se, e, se).contramap(Zipper.p_());
}
/**
* A Show instance for Zippers.
*
* @param s A Show instance for the element type.
* @return A Show instance for Zippers.
*/
public static Show> show(final Show s) {
final Show> ss = Show.streamShow(s);
return Show.p3Show(ss, s, ss).contramap(Zipper.p_());
}
/**
* Maps the given function across the elements of this zipper (covariant functor pattern).
*
* @param f A function to map across this zipper.
* @return A new zipper with the given function applied to all elements.
*/
public Zipper map(final F f) {
return zipper(left.map(f), f.f(focus), right.map(f));
}
/**
* Performs a right-fold reduction across this zipper.
*
* @param f The function to apply on each element of this zipper.
* @param z The beginning value to start the application from.
* @return the final result after the right-fold reduction.
*/
public B foldRight(final F> f, final B z) {
return left.foldLeft(flip(f),
right.cons(focus).foldRight(compose(
Function., B, B>andThen().f(P1.__1()), f), z));
}
/**
* Creates a new zipper with a single element.
*
* @param a The focus element of the new zipper.
* @return a new zipper with a single element which is in focus.
*/
public static Zipper single(final A a) {
return zipper(Stream.nil(), a, Stream.nil());
}
/**
* Possibly create a zipper if the provided stream has at least one element, otherwise None.
* The provided stream's head will be the focus of the zipper, and the rest of the stream will follow
* on the right side.
*
* @param a The stream from which to create a zipper.
* @return a new zipper if the provided stream has at least one element, otherwise None.
*/
@SuppressWarnings({"IfMayBeConditional"})
public static Option> fromStream(final Stream a) {
if (a.isEmpty())
return none();
else
return some(zipper(Stream.nil(), a.head(), a.tail()._1()));
}
/**
* Possibly create a zipper if the provided stream has at least one element, otherwise None.
* The provided stream's last element will be the focus of the zipper, following the rest of the stream in order,
* to the left.
*
* @param a The stream from which to create a zipper.
* @return a new zipper if the provided stream has at least one element, otherwise None.
*/
public static Option> fromStreamEnd(final Stream a) {
if (a.isEmpty())
return none();
else {
final Stream xs = a.reverse();
return some(zipper(xs.tail()._1(), xs.head(), Stream.nil()));
}
}
/**
* Returns the focus element of this zipper.
*
* @return the focus element of this zipper.
*/
public A focus() {
return focus;
}
/**
* Possibly moves the focus to the next element in the list.
*
* @return An optional zipper with the focus moved one element to the right, if there are elements to the right of
* focus, otherwise None.
*/
public Option> next() {
return right.isEmpty() ? Option.>none() : some(tryNext());
}
/**
* Attempts to move the focus to the next element, or throws an error if there are no more elements.
*
* @return A zipper with the focus moved one element to the right, if there are elements to the right of
* focus, otherwise throws an error.
*/
public Zipper tryNext() {
if (right.isEmpty())
throw new Error("Tried next at the end of a zipper.");
else
return zipper(left.cons(focus), right.head(), right.tail()._1());
}
/**
* Possibly moves the focus to the previous element in the list.
*
* @return An optional zipper with the focus moved one element to the left, if there are elements to the left of
* focus, otherwise None.
*/
public Option> previous() {
return left.isEmpty() ? Option.>none() : some(tryPrevious());
}
/**
* Attempts to move the focus to the previous element, or throws an error if there are no more elements.
*
* @return A zipper with the focus moved one element to the left, if there are elements to the left of
* focus, otherwise throws an error.
*/
public Zipper tryPrevious() {
if (left.isEmpty())
throw new Error("Tried previous at the beginning of a zipper.");
else
return zipper(left.tail()._1(), left.head(), right.cons(focus));
}
/**
* First-class version of the next() function.
*
* @return A function that moves the given zipper's focus to the next element.
*/
public static F, Option>> next_() {
return as -> as.next();
}
/**
* First-class version of the previous() function.
*
* @return A function that moves the given zipper's focus to the previous element.
*/
public static F, Option>> previous_() {
return as -> as.previous();
}
/**
* Inserts an element to the left of the focus, then moves the focus to the new element.
*
* @param a A new element to insert into this zipper.
* @return A new zipper with the given element in focus, and the current focus element on its right.
*/
public Zipper insertLeft(final A a) {
return zipper(left, a, right.cons(focus));
}
/**
* Inserts an element to the right of the focus, then moves the focus to the new element.
*
* @param a A new element to insert into this zipper.
* @return A new zipper with the given element in focus, and the current focus element on its left.
*/
public Zipper insertRight(final A a) {
return zipper(left.cons(focus), a, right);
}
/**
* Possibly deletes the element at the focus, then moves the element on the left into focus.
* If no element is on the left, focus on the element to the right.
* Returns None if the focus element is the only element in this zipper.
*
* @return A new zipper with this zipper's focus element removed, or None if deleting the focus element
* would cause the zipper to be empty.
*/
public Option> deleteLeft() {
return left.isEmpty() && right.isEmpty()
? Option.>none()
: some(zipper(left.isEmpty() ? left : left.tail()._1(),
left.isEmpty() ? right.head() : left.head(),
left.isEmpty() ? right.tail()._1() : right));
}
/**
* Possibly deletes the element at the focus, then moves the element on the right into focus.
* If no element is on the right, focus on the element to the left.
* Returns None if the focus element is the only element in this zipper.
*
* @return A new zipper with this zipper's focus element removed, or None if deleting the focus element
* would cause the zipper to be empty.
*/
public Option> deleteRight() {
return left.isEmpty() && right.isEmpty()
? Option.>none()
: some(zipper(right.isEmpty() ? left.tail()._1() : left,
right.isEmpty() ? left.head() : right.head(),
right.isEmpty() ? right : right.tail()._1()));
}
/**
* Deletes all elements in the zipper except the focus.
*
* @return A new zipper with the focus element as the only element.
*/
public Zipper deleteOthers() {
final Stream nil = nil();
return zipper(nil, focus, nil);
}
/**
* Returns the length of this zipper.
*
* @return the length of this zipper.
*/
public int length() {
return foldRight(Function.>constant(Integers.add.f(1)), 0);
}
/**
* Returns whether the focus is on the first element.
*
* @return true if the focus is on the first element, otherwise false.
*/
public boolean atStart() {
return left.isEmpty();
}
/**
* Returns whether the focus is on the last element.
*
* @return true if the focus is on the last element, otherwise false.
*/
public boolean atEnd() {
return right.isEmpty();
}
/**
* Creates a zipper of variations of this zipper, in which each element is focused,
* with this zipper as the focus of the zipper of zippers (comonad pattern).
*
* @return a zipper of variations of the provided zipper, in which each element is focused,
* with this zipper as the focus of the zipper of zippers.
*/
public Zipper> positions() {
final Stream> left = Stream.unfold(
p -> p.previous().map(join(P., Zipper>p2())), this);
final Stream> right = Stream.unfold(
p -> p.next().map(join(P., Zipper>p2())), this);
return zipper(left, this, right);
}
/**
* Maps over variations of this zipper, such that the given function is applied to each variation (comonad pattern).
*
* @param f The comonadic function to apply for each variation of this zipper.
* @return A new zipper, with the given function applied for each variation of this zipper.
*/
public Zipper cobind(final F, B> f) {
return positions().map(f);
}
/**
* Zips the elements of this zipper with a boolean that indicates whether that element has focus.
* All of the booleans will be false, except the focused element.
*
* @return A new zipper of pairs, with each element of this zipper paired with a boolean that is true if that
* element has focus, and false otherwise.
*/
public Zipper> zipWithFocus() {
return zipper(left.zip(repeat(false)), P.p(focus, true), right.zip(repeat(false)));
}
/**
* Move the focus to the specified index.
*
* @param n The index to which to move the focus.
* @return A new zipper with the focus moved to the specified index, or none if there is no such index.
*/
public Option> move(final int n) {
final int ll = left.length();
final int rl = right.length();
Option> p = some(this);
if (n < 0 || n >= length())
return none();
else if (ll >= n)
for (int i = ll - n; i > 0; i--)
p = p.bind(Zipper.previous_());
else if (rl >= n)
for (int i = rl - n; i > 0; i--)
p = p.bind(Zipper.next_());
return p;
}
/**
* A first-class version of the move function.
*
* @return A function that moves the focus of the given zipper to the given index.
*/
public static F, Option>>> move() {
return curry((i, a) -> a.move(i));
}
/**
* Moves the focus to the element matching the given predicate, if present.
*
* @param p A predicate to match.
* @return A new zipper with the nearest matching element focused if it is present in this zipper.
*/
public Option> find(final F p) {
if (p.f(focus()))
return some(this);
else {
final Zipper> ps = positions();
return ps.lefts().interleave(ps.rights()).find(zipper -> p.f(zipper.focus()));
}
}
/**
* Returns the index of the focus.
*
* @return the index of the focus.
*/
public int index() {
return left.length();
}
/**
* Move the focus to the next element. If the last element is focused, loop to the first element.
*
* @return A new zipper with the next element focused, unless the last element is currently focused, in which case
* the first element becomes focused.
*/
public Zipper cycleNext() {
if (left.isEmpty() && right.isEmpty())
return this;
else if (right.isEmpty()) {
final Stream xs = left.reverse();
return zipper(Stream.nil(), xs.head(), xs.tail()._1().snoc(P.p(focus)));
} else
return tryNext();
}
/**
* Move the focus to the previous element. If the first element is focused, loop to the last element.
*
* @return A new zipper with the previous element focused, unless the first element is currently focused,
* in which case the last element becomes focused.
*/
public Zipper cyclePrevious() {
if (left.isEmpty() && right.isEmpty())
return this;
else if (left.isEmpty()) {
final Stream xs = right.reverse();
return zipper(xs.tail()._1().snoc(P.p(focus)), xs.head(), Stream.nil());
} else
return tryPrevious();
}
/**
* Possibly deletes the element at the focus, then move the element on the left into focus. If no element is on the
* left, focus on the last element. If the deletion will cause the list to be empty, return None.
*
* @return A new zipper with the focused element removed, and focus on the previous element to the left, or the last
* element if there is no element to the left.
*/
public Option> deleteLeftCycle() {
if (left.isEmpty() && right.isEmpty())
return none();
else if (left.isNotEmpty())
return some(zipper(left.tail()._1(), left.head(), right));
else {
final Stream xs = right.reverse();
return some(zipper(xs.tail()._1(), xs.head(), Stream.nil()));
}
}
/**
* Possibly deletes the element at the focus, then move the element on the right into focus. If no element is on the
* right, focus on the first element. If the deletion will cause the list to be empty, return None.
*
* @return A new zipper with the focused element removed, and focus on the next element to the right, or the first
* element if there is no element to the right.
*/
public Option> deleteRightCycle() {
if (left.isEmpty() && right.isEmpty())
return none();
else if (right.isNotEmpty())
return some(zipper(left, right.head(), right.tail()._1()));
else {
final Stream xs = left.reverse();
return some(zipper(Stream.nil(), xs.head(), xs.tail()._1()));
}
}
/**
* Replaces the element in focus with the given element.
*
* @param a An element to replace the focused element with.
* @return A new zipper with the given element in focus.
*/
public Zipper replace(final A a) {
return zipper(left, a, right);
}
/**
* Returns the Stream representation of this zipper.
*
* @return A stream that contains all the elements of this zipper.
*/
public Stream toStream() {
return left.reverse().snoc(P.p(focus)).append(right);
}
/**
* Returns a Stream of the elements to the left of focus.
*
* @return a Stream of the elements to the left of focus.
*/
public Stream lefts() {
return left;
}
/**
* Returns a Stream of the elements to the right of focus.
*
* @return a Stream of the elements to the right of focus.
*/
public Stream rights() {
return right;
}
/**
* Zips this Zipper with another, applying the given function lock-step over both zippers in both directions.
* The structure of the resulting Zipper is the structural intersection of the two Zippers.
*
* @param bs A Zipper to zip this one with.
* @param f A function with which to zip together the two Zippers.
* @return The result of applying the given function over this Zipper and the given Zipper, location-wise.
*/
public Zipper zipWith(final Zipper bs, final F2 f) {
return F2Functions.zipZipperM(f).f(this, bs);
}
/**
* Zips this Zipper with another, applying the given function lock-step over both zippers in both directions.
* The structure of the resulting Zipper is the structural intersection of the two Zippers.
*
* @param bs A Zipper to zip this one with.
* @param f A function with which to zip together the two Zippers.
* @return The result of applying the given function over this Zipper and the given Zipper, location-wise.
*/
public Zipper zipWith(final Zipper bs, final F> f) {
return zipWith(bs, uncurryF2(f));
}
/**
* Returns an iterator of all the positions of this Zipper, starting from the leftmost position.
*
* @return An iterator of all the positions of this Zipper, starting from the leftmost position.
*/
public Iterator> iterator() { return positions().toStream().iterator(); }
}