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Functional patterns for Java

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package com.jnape.palatable.lambda.functions.recursion;

import com.jnape.palatable.lambda.adt.coproduct.CoProduct2;
import com.jnape.palatable.lambda.functions.Fn1;
import com.jnape.palatable.lambda.functions.Fn2;
import com.jnape.palatable.lambda.functions.builtin.fn2.Unfoldr;
import com.jnape.palatable.lambda.functions.recursion.RecursiveResult.Recurse;
import com.jnape.palatable.lambda.functions.recursion.RecursiveResult.Terminate;

/**
 * Given an {@link Fn1}<A, {@link CoProduct2}<A, B, ?>> (analogous to "recurse" and "return"
 * tail position instructions, respectively), produce a {@link Fn1}<A, B> that unrolls the original
 * function by iteratively passing each result that matches the input (A) back to the original function,
 * and then terminating on and returning the first output (B).
 * 
 * This is isomorphic to - though presumably faster than - taking the last element of an {@link Unfoldr} call.
 *
 * @param  the trampolined function's input type
 * @param  the trampolined function's output type
 */
public final class Trampoline implements Fn2>, A, B> {

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

    @Override
    public B checkedApply(Fn1> fn, A a) {
        RecursiveResult next = fn.apply(a);
        while (next instanceof Recurse)
            next = fn.apply(((Recurse) next).a);
        return ((Terminate) next).b;
    }

    @SuppressWarnings("unchecked")
    public static  Trampoline trampoline() {
        return (Trampoline) INSTANCE;
    }

    public static  Fn1 trampoline(Fn1> fn) {
        return Trampoline.trampoline().apply(fn);
    }

    public static  B trampoline(Fn1> fn, A a) {
        return trampoline(fn).apply(a);
    }
}