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July 26, 2019 04:42
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Passing common variables in Java
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| import java.util.function.Function; | |
| /** | |
| * AKA function monad | |
| * @param <R> values to be passed | |
| * @param <A> resulting value upon application of instance of R | |
| */ | |
| public final class Reader<R, A> { | |
| private final Function<R, A> g; | |
| /** | |
| * Wrap the function as a reader | |
| * @param g the function | |
| */ | |
| private Reader(Function<R, A> g) { | |
| this.g = g; | |
| } | |
| public static <A, B, E> Reader<E, B> lift2( | |
| Reader<E, A> reader, Function<A, Reader<E, B>> previousAwareBind){ | |
| return Reader.unit(r -> | |
| reader.bind(a -> previousAwareBind.apply(a.apply(r))).apply(r)); | |
| } | |
| /** | |
| * Lift the function as a reader | |
| * @param f the function | |
| * @param <E> the type of input which will be the type of env variables | |
| * @param <O> the output to be resolved once E is applied | |
| * @return the resulting reader | |
| */ | |
| public static <E, O> Reader<E, O> unit(Function<E, O> f){ | |
| return new Reader<>(f); | |
| } | |
| /** | |
| * Flat map operations of reader as a functor. In FP | |
| * Functions are first classed which is equivalent to objects here in Java; | |
| * however the f here is the function that relates the resulting output of this | |
| * reader to result of the new reader. The same env should be applied to the resulting | |
| * reader | |
| * @param f the function relating the this reader to the new reader | |
| * @param <O> the resulting value of the resulting reader upon application of env | |
| * @return the elevated function f | |
| */ | |
| public <I, O> Function<Reader<R, I>, Reader<R, O>> fMap(Function<Function<R, I>, Function<R, O>> f){ | |
| return r -> Reader.unit(f.apply(r.g)); | |
| } | |
| /** | |
| * Perform fMap operation on this instance | |
| * @param f the function relating the this reader to the new reader | |
| * @param <B> the resulting value of the resulting reader upon application of env | |
| * @return the resulting reader | |
| */ | |
| public <B> Reader<R, B> map(Function<Function<R, A>, Function<R, B>> f) { | |
| return fMap(f).apply(this); | |
| } | |
| /** | |
| * Bind operations of reader as a monad. Allows chain operations | |
| * of function that returns a reader that uses the same env variables. | |
| * Allows implicit passing of the env variable | |
| * | |
| * @param f function to be bind | |
| * @param <V> new resolution type | |
| * @return the resulting reader | |
| */ | |
| public <V> Reader<R, V> bind(Function<Function<R, A>, Reader<R, V>> f){ | |
| return new Reader<>(r -> f.apply(g).apply(r)); | |
| } | |
| /** | |
| * Applying the env variable results to the resolution of the elevated function g | |
| * @param r the env variable | |
| * @return the result of g applied with the env variable r | |
| */ | |
| public A apply(R r) { | |
| return g.apply(r); | |
| } | |
| } |
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