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P1.java
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package fj;
import fj.data.List;
import fj.data.Stream;
import fj.data.Array;
import java.lang.ref.SoftReference;
/**
* A product-1. Also, the identity monad.
*
* @version %build.number%
*/
public interface P1<A> {
/**
* Access the first element of the product.
*
* @return The first element of the product.
*/
public abstract A _1();
/**
* Map the element of the product.
*
* @param f The function to map with.
* @return A product with the given function applied.
*/
public default <X> P1<X> map(final F<A, X> f) {
return () -> f.f(_1());
}
/**
* Returns a function that returns the first element of a product.
*
* @return A function that returns the first element of a product.
*/
public static <A> F<P1<A>, A> __1() {
return P1::_1;
}
/**
* Promote any function to a transformation between P1s.
*
* @param f A function to promote to a transformation between P1s.
* @return A function promoted to operate on P1s.
*/
public static <A, B> F<P1<A>, P1<B>> fmap(final F<A, B> f) {
return a -> a.map(f);
}
/**
* Binds the given function to the value in a product-1 with a final join.
*
* @param a A value in a product-1 to which to apply a function.
* @param f A function to apply to the value in a product-1.
* @return The result of applying the given function to the value of given product-1.
*/
public static <A, B> P1<B> bind(final P1<A> a, final F<A, P1<B>> f) {
return () -> f.f(a._1())._1();
}
/**
* Promotes the given function so that it returns its value in a P1.
*
* @param f A function to have its result wrapped in a P1.
* @return A function whose result is wrapped in a P1.
*/
public static <A, B> F<A, P1<B>> curry(final F<A, B> f) {
return a -> () -> f.f(a);
}
/**
* Performs function application within a P1 (applicative functor pattern).
*
* @param ca The P1 to which to apply a function.
* @param cf The P1 function to apply.
* @return A new P1 after applying the given P1 function to the first argument.
*/
public static <A, B> P1<B> apply(final P1<A> ca, final P1<F<A, B>> cf) {
return bind(cf, f -> fmap(f).f(ca));
}
/**
* Binds the given function to the values in the given P1s with a final join.
*
* @param ca A given P1 to bind the given function with.
* @param cb A given P1 to bind the given function with.
* @param f The function to apply to the values in the given P1s.
* @return A new P1 after performing the map, then final join.
*/
public static <A, B, C> P1<C> bind(final P1<A> ca, final P1<B> cb, final F<A, F<B, C>> f) {
return apply(cb, fmap(f).f(ca));
}
/**
* Joins a P1 of a P1 with a bind operation.
*
* @param a The P1 of a P1 to join.
* @return A new P1 that is the join of the given P1.
*/
public static <A> P1<A> join(final P1<P1<A>> a) {
return bind(a, Function.identity());
}
/**
* Promotes a function of arity-2 to a function on P1s.
*
* @param f The function to promote.
* @return A function of arity-2 promoted to map over P1s.
*/
public static <A, B, C> F<P1<A>, F<P1<B>, P1<C>>> liftM2(final F<A, F<B, C>> f) {
return pa -> pb -> bind(pa, pb, f);
}
/**
* Turns a List of P1s into a single P1 of a List.
*
* @param as The list of P1s to transform.
* @return A single P1 for the given List.
*/
public static <A> P1<List<A>> sequence(final List<P1<A>> as) {
return as.foldRight(liftM2(List.cons()), () -> List.nil());
}
/**
* A first-class version of the sequence method for lists of P1s.
*
* @return A function from a List of P1s to a single P1 of a List.
*/
public static <A> F<List<P1<A>>, P1<List<A>>> sequenceList() {
return as -> sequence(as);
}
/**
* Turns a stream of P1s into a single P1 of a stream.
*
* @param as The stream of P1s to transform.
* @return A single P1 for the given stream.
*/
public static <A> P1<Stream<A>> sequence(final Stream<P1<A>> as) {
return as.foldRight(liftM2(Stream.cons()), () -> Stream.nil());
}
/**
* Turns an array of P1s into a single P1 of an array.
*
* @param as The array of P1s to transform.
* @return A single P1 for the given array.
*/
public static <A> P1<Array<A>> sequence(final Array<P1<A>> as) {
return () -> as.map(P1::_1);
}
/**
* Provides a memoising P1 that remembers its value.
*
* @return A P1 that calls this P1 once and remembers the value for subsequent calls.
*/
public default P1<A> memo() {
final P1<A> self = this;
return new P1<A>() {
private final Object latch = new Object();
@SuppressWarnings({"InstanceVariableMayNotBeInitialized"})
private volatile SoftReference<A> v;
public A _1() {
A a = v != null ? v.get() : null;
if (a == null)
synchronized (latch) {
if (v == null || v.get() == null)
a = self._1();
v = new SoftReference<A>(a);
}
return a;
}
};
}
/**
* Returns a constant function that always uses this value.
*
* @return A constant function that always uses this value.
*/
public default <B> F<B, A> constant() {
return b -> _1();
}
}