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Functional patterns for Java

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package com.jnape.palatable.lambda.functions.builtin.fn2;

import com.jnape.palatable.lambda.functions.Fn2;
import com.jnape.palatable.lambda.functions.specialized.Kleisli;
import com.jnape.palatable.lambda.functor.builtin.Lazy;

import static com.jnape.palatable.lambda.functions.specialized.Kleisli.kleisli;
import static com.jnape.palatable.lambda.functor.builtin.Lazy.lazy;

/**
 * Given a {@link Fn2} that receives a recursive function and an input and yields a {@link Lazy lazy} result, and an
 * input, produce a {@link Lazy lazy} result that, when forced, will recursively invoke the function until it terminates
 * in a stack-safe way.
 * 
 * Example:
 * 
 * {@code
 * Lazy lazyFactorial = lazyRec((fact, x) -> x.equals(ONE)
 *                                                       ? lazy(x)
 *                                                       : fact.apply(x.subtract(ONE)).fmap(y -> y.multiply(x)),
 *                                                  BigInteger.valueOf(50_000));
 * BigInteger value = lazyFactorial.value(); // 3.34732050959714483691547609407148647791277322381045 x 10^213236
 * }
 * 
 *
 * @param  the input type
 * @param  the output type
 */
public final class LazyRec implements
        Fn2, Lazy>, A, Lazy>, A, Lazy> {

    private static final LazyRec INSTANCE = new LazyRec<>();

    private LazyRec() {
    }

    @Override
    public Lazy checkedApply(Fn2, Lazy>, A, Lazy> fn, A a) {
        return lazy(a).flatMap(fn.apply(lazyRec(fn)));
    }

    @SuppressWarnings("unchecked")
    public static  LazyRec lazyRec() {
        return (LazyRec) INSTANCE;
    }

    public static  Kleisli, Lazy> lazyRec(
            Fn2, Lazy>, A, Lazy> fn) {
        return kleisli(LazyRec.lazyRec().apply(fn));
    }

    public static  Lazy lazyRec(Fn2, Lazy>, A, Lazy> fn, A a) {
        return lazyRec(fn).apply(a);
    }
}