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Immutable Clojure collections and a Transformation abstraction for Java 8+, immutably, type-safely, and with good performance. Name will change to "Paguro" in November 2016.
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// Copyright (c) 2014-03-09 PlanBase Inc. & Glen Peterson
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package org.organicdesign.fp;

import org.organicdesign.fp.function.Function0;
import org.organicdesign.fp.function.Function1;

import java.util.Objects;

/**
 Indicates presence or absence of a value (null is a valid, present value) or end-of-stream.
 This is NOT a type-safe null.
 @param 
 */
public interface Option { // extends UnmodSortedIterable {

    /** Return the value wrapped in this Option.  Only safe to call this on Some. */
    T get();

    /** If this is Some, return the value wrapped in this Option.  Otherwise, return the given value. */
    T getOrElse(T t);

    /** Is this Some? */
    boolean isSome();

    /** Pass in a function to execute if its Some and another to execute if its None. */
     U patMat(Function1 has, Function0 hasNot);

    // ==================================================== Static ====================================================
    /** None is a singleton and this is its only instance. */
    Option NONE = new None();

    /** Calling this instead of referring to NONE directly can make the type infrencer happy. */
    @SuppressWarnings("unchecked")
    static  Option none() { return NONE; }

    /** Public static factory method for contructing Options. */
    static  Option of(T t) {
        if (NONE.equals(t)) {
            return none();
        }
        return new Some<>(t);
    }

    /** Construct an option, but if t is null, make it None instead of Some. */
    static  Option someOrNullNoneOf(T t) {
        if ( (t == null) || NONE.equals(t) ) {
            return none();
        }
        return new Some<>(t);
    }

    /** Represents the absence of a value */
    final class None implements Option {
        /** Private constructor for singleton. */
        private None() {}

        @Override public T get() { throw new IllegalStateException("Called get on None"); }

        @Override public T getOrElse(T t) { return t; }

        @Override public boolean isSome() { return false; }
//
//        @Override public UnmodSortedIterator iterator() {
//            return UnmodSortedIterator.empty();
//        }

        @Override public  U patMat(Function1 has, Function0 hasNot) { return hasNot.get(); }

        /** Valid, but deprecated because it's usually an error to call this in client code. */
        @Deprecated // Has no effect.  Darn!
        @Override public int hashCode() { return 0; }

        /** Valid, but deprecated because it's usually an error to call this in client code. */
        @Deprecated // Has no effect.  Darn!
        @Override public boolean equals(Object other) { return (this == other) || (other instanceof None); }

        // Defend our singleton property in the face of deserialization.  Not sure this is necessary, but probably
        // won't hurt.
        private Object readResolve() { return NONE; }
    }

    /** Represents the presence of a value, even if that value is null. */
    class Some implements Option {
        private final T item;
        private Some(T t) { item = t; }

        //public static Some of(T t) { return new Option(t); }

        @Override public T get() { return item; }

        @Override public T getOrElse(T t) { return item; }

        @Override public boolean isSome() { return true; }
//
//        @Override public UnmodSortedIterator iterator() {
//            return new UnmodSortedIterator() {
//                private boolean hasNext = true;
//                @Override public boolean hasNext() { return hasNext; }
//                @Override public T next() {
//                    if (hasNext) {
//                        hasNext = false;
//                        return item;
//                    }
//                    throw new NoSuchElementException();
//                }
//            };
//        }

        @Override public  U patMat(Function1 has, Function0 hasNot) {
            return has.apply(item);
        }

        /** Valid, but deprecated because it's usually an error to call this in client code. */
        @Deprecated // Has no effect.  Darn!
        @Override public int hashCode() {
            // We return Integer.MIN_VALUE for null to make it different from None which always returns zero.
            return item == null ? Integer.MIN_VALUE : item.hashCode();
        }

        /** Valid, but deprecated because it's usually an error to call this in client code. */
        @Deprecated // Has no effect.  Darn!
        @Override public boolean equals(Object other) {
            if (this == other) { return true; }
            if ( !(other instanceof Option) ) { return false; }

            final Option that = (Option) other;
            return that.isSome() && Objects.equals(this.item, that.get());
        }
    }
}