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Functional patterns for Java

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package com.jnape.palatable.lambda.functor;

import com.jnape.palatable.lambda.functions.Fn1;

import static com.jnape.palatable.lambda.functions.builtin.fn1.Downcast.downcast;

/**
 * An interface for the generic covariant functorial operation map over some parameter A.
 * Functors are foundational to many of the classes provided by this library; generally, anything that can be thought of
 * as "mappable" is an instance of at least this interface.
 * 
 * For more information, read about Functors.
 *
 * @param  The type of the parameter
 * @param  The unification parameter
 * @see Bifunctor
 * @see Profunctor
 * @see Fn1
 * @see com.jnape.palatable.lambda.adt.hlist.Tuple2
 * @see com.jnape.palatable.lambda.adt.Either
 */
@FunctionalInterface
public interface Functor> {

    /**
     * Covariantly transmute this functor's parameter using the given mapping function. Generally this method is
     * specialized to return an instance of the class implementing Functor.
     *
     * @param  the new parameter type
     * @param fn  the mapping function
     * @return a functor over B (the new parameter type)
     */
     Functor fmap(Fn1 fn);

    /**
     * Convenience method for coercing this functor instance into another concrete type. Unsafe.
     *
     * @param  the concrete functor instance to coerce this functor to
     * @return the coerced functor
     */
    default > Concrete coerce() {
        return downcast(this);
    }
}