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Functional patterns for Java

There is a newer version: 5.5.0

package spike;

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 spike.RecursiveResult.Recurse;
import spike.RecursiveResult.Terminate;

import java.util.function.Function;

/**
 * Given a {@link Function}<A, {@link CoProduct2}<A, B, ?>> (analogous to "recurse" and
 * "return" tail position instructions, respectively), produce a {@link Function}<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 apply(Function> 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 INSTANCE;
    }

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

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