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Optics (adapted from Scala Monocle library)
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‎core/src/main/java/fj/data/optic/Fold.java‎

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package fj.data.optic;
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import fj.F;
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import fj.Function;
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import fj.Monoid;
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import fj.data.Either;
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import fj.data.List;
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import fj.data.Option;
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/**
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* A {@link Fold} can be seen as a {@link Getter} with many targets or a weaker {@link PTraversal} which cannot modify its
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* target.
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*
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* {@link Fold} is on the top of the Optic hierarchy which means that {@link Getter}, {@link PTraversal}, {@link POptional},
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* {@link PLens}, {@link PPrism} and {@link PIso} are valid {@link Fold}
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*
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* @param <S> the source of a {@link Fold}
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* @param <A> the target of a {@link Fold}
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*/
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public abstract class Fold<S, A> {
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/**
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* map each target to a {@link Monoid} and combine the results underlying representation of {@link Fold}, all {@link Fold}
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* methods are defined in terms of foldMap
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*/
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public abstract <M> F<S, M> foldMap(Monoid<M> m, F<A, M> f);
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/** combine all targets using a target's {@link Monoid} */
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public final F<S, A> fold(final Monoid<A> m) {
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return foldMap(m, Function.identity());
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}
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/**
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* get all the targets of a {@link Fold} TODO: Shall it return a Stream as there might be an infinite number of targets?
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*/
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public final List<A> getAll(final S s) {
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return foldMap(Monoid.listMonoid(), List::single).f(s);
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}
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/** find the first target of a {@link Fold} matching the predicate */
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public final F<S, Option<A>> find(final F<A, Boolean> p) {
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return foldMap(Monoid.optionMonoid(), a -> p.f(a) ? Option.some(a) : Option.none());
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}
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/** get the first target of a {@link Fold} */
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public final Option<A> headOption(final S s) {
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return find(__ -> true).f(s);
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}
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/** check if at least one target satisfies the predicate */
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public final F<S, Boolean> exist(final F<A, Boolean> p) {
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return foldMap(Monoid.disjunctionMonoid, p);
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}
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/** check if all targets satisfy the predicate */
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public final F<S, Boolean> all(final F<A, Boolean> p) {
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return foldMap(Monoid.conjunctionMonoid, p);
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}
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/** join two {@link Fold} with the same target */
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public final <S1> Fold<Either<S, S1>, A> sum(final Fold<S1, A> other) {
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return new Fold<Either<S, S1>, A>() {
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@Override
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public <B> F<Either<S, S1>, B> foldMap(final Monoid<B> m, final F<A, B> f) {
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return s -> s.either(Fold.this.foldMap(m, f), other.foldMap(m, f));
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}
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};
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}
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/**********************************************************/
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/** Compose methods between a {@link Fold} and another Optics */
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/**********************************************************/
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/** compose a {@link Fold} with a {@link Fold} */
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public final <B> Fold<S, B> composeFold(final Fold<A, B> other) {
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return new Fold<S, B>() {
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@Override
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public <C> F<S, C> foldMap(final Monoid<C> m, final F<B, C> f) {
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return Fold.this.<C> foldMap(m, other.<C> foldMap(m, f));
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}
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};
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}
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/** compose a {@link Fold} with a {@link Getter} */
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public final <C> Fold<S, C> composeGetter(final Getter<A, C> other) {
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return composeFold(other.asFold());
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}
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/** compose a {@link Fold} with a {@link POptional} */
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public final <B, C, D> Fold<S, C> composeOptional(final POptional<A, B, C, D> other) {
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return composeFold(other.asFold());
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}
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/** compose a {@link Fold} with a {@link PPrism} */
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public final <B, C, D> Fold<S, C> composePrism(final PPrism<A, B, C, D> other) {
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return composeFold(other.asFold());
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}
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/** compose a {@link Fold} with a {@link PLens} */
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public final <B, C, D> Fold<S, C> composeLens(final PLens<A, B, C, D> other) {
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return composeFold(other.asFold());
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}
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/** compose a {@link Fold} with a {@link PIso} */
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public final <B, C, D> Fold<S, C> composeIso(final PIso<A, B, C, D> other) {
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return composeFold(other.asFold());
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}
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public static final <A> Fold<Either<A, A>, A> codiagonal() {
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return new Fold<Either<A, A>, A>() {
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@Override
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public <B> F<Either<A, A>, B> foldMap(final Monoid<B> m, final F<A, B> f) {
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return e -> e.either(f, f);
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}
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};
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}
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}

‎core/src/main/java/fj/data/optic/Getter.java‎

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package fj.data.optic;
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import fj.F;
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import fj.Function;
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import fj.Monoid;
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import fj.P;
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import fj.P2;
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import fj.data.Either;
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/**
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* A {@link Getter} can be seen as a glorified get method between a type S and a type A.
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*
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* A {@link Getter} is also a valid {@link Fold}
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*
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* @param <S> the source of a {@link Getter}
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* @param <A> the target of a {@link Getter}
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*/
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public abstract class Getter<S, A> {
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Getter() {
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super();
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}
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/** get the target of a {@link Getter} */
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public abstract A get(S s);
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/** join two {@link Getter} with the same target */
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public final <S1> Getter<Either<S, S1>, A> sum(final Getter<S1, A> other) {
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return getter(e -> e.either(this::get, other::get));
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}
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/** pair two disjoint {@link Getter} */
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public final <S1, A1> Getter<P2<S, S1>, P2<A, A1>> product(final Getter<S1, A1> other) {
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return getter(p2 -> P.p(this.get(p2._1()), other.get(p2._2())));
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}
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public final <B> Getter<P2<S, B>, P2<A, B>> first() {
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return getter(p -> P.p(this.get(p._1()), p._2()));
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}
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public final <B> Getter<P2<B, S>, P2<B, A>> second() {
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return getter(p -> P.p(p._1(), this.get(p._2())));
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}
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/*************************************************************/
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/** Compose methods between a {@link Getter} and another Optics */
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/*************************************************************/
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/** compose a {@link Getter} with a {@link Fold} */
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public final <B> Fold<S, B> composeFold(final Fold<A, B> other) {
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return asFold().composeFold(other);
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}
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/** compose a {@link Getter} with a {@link Getter} */
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public final <B> Getter<S, B> composeGetter(final Getter<A, B> other) {
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return getter(s -> other.get(get(s)));
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}
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/** compose a {@link Getter} with a {@link POptional} */
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public final <B, C, D> Fold<S, C> composeOptional(final POptional<A, B, C, D> other) {
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return asFold().composeOptional(other);
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}
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/** compose a {@link Getter} with a {@link PPrism} */
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public final <B, C, D> Fold<S, C> composePrism(final PPrism<A, B, C, D> other) {
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return asFold().composePrism(other);
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}
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/** compose a {@link Getter} with a {@link PLens} */
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public final <B, C, D> Getter<S, C> composeLens(final PLens<A, B, C, D> other) {
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return composeGetter(other.asGetter());
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}
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/** compose a {@link Getter} with a {@link PIso} */
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public final <B, C, D> Getter<S, C> composeIso(final PIso<A, B, C, D> other) {
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return composeGetter(other.asGetter());
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}
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/******************************************************************/
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/** Transformation methods to view a {@link Getter} as another Optics */
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/******************************************************************/
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/** view a {@link Getter} with a {@link Fold} */
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public final Fold<S, A> asFold() {
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return new Fold<S, A>() {
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@Override
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public <B> F<S, B> foldMap(final Monoid<B> m, final F<A, B> f) {
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return s -> f.f(get(s));
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}
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};
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}
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public static final <A> Getter<Either<A, A>, A> codiagonal() {
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return getter(e -> e.either(Function.identity(), Function.identity()));
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}
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public static final <S, A> Getter<S, A> getter(final F<S, A> get) {
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return new Getter<S, A>() {
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@Override
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public A get(final S s) {
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return get.f(s);
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}
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};
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}
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}

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