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RecursiveResult.java
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152 lines (124 loc) · 4.78 KB
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package spike;
import com.jnape.palatable.lambda.adt.coproduct.CoProduct2;
import com.jnape.palatable.lambda.functor.Applicative;
import com.jnape.palatable.lambda.functor.Bifunctor;
import com.jnape.palatable.lambda.monad.Monad;
import com.jnape.palatable.lambda.traversable.Traversable;
import java.util.Objects;
import java.util.function.Function;
/**
* Specialized {@link CoProduct2} representing the possible results of a primitive recursive function.
* Used by {@link Trampoline} to cheat around {@link CoProduct2#match} and quickly unpack values via
* <code>instanceof</code> checks to package private inner subtypes.
*
* @param <A> the recursive function's input type
* @param <B> the recursive function's output type
* @see Trampoline
*/
public abstract class RecursiveResult<A, B> implements CoProduct2<A, B, RecursiveResult<A, B>>, Bifunctor<A, B, RecursiveResult<?, ?>>, Monad<B, RecursiveResult<A, ?>>, Traversable<B, RecursiveResult<A, ?>> {
private RecursiveResult() {
}
@Override
@SuppressWarnings("unchecked")
public <C> RecursiveResult<C, B> biMapL(Function<? super A, ? extends C> fn) {
return (RecursiveResult<C, B>) Bifunctor.super.biMapL(fn);
}
@Override
@SuppressWarnings("unchecked")
public <C> RecursiveResult<A, C> biMapR(Function<? super B, ? extends C> fn) {
return (RecursiveResult<A, C>) Bifunctor.super.biMapR(fn);
}
@Override
public <C, D> RecursiveResult<C, D> biMap(Function<? super A, ? extends C> lFn,
Function<? super B, ? extends D> rFn) {
return match(a -> recurse(lFn.apply(a)), b -> terminate(rFn.apply(b)));
}
@Override
public <C> RecursiveResult<A, C> flatMap(
Function<? super B, ? extends Monad<C, RecursiveResult<A, ?>>> f) {
return match(RecursiveResult::recurse, b -> f.apply(b).coerce());
}
@Override
public <C> RecursiveResult<A, C> pure(C c) {
return terminate(c);
}
@Override
public <C> RecursiveResult<A, C> fmap(Function<? super B, ? extends C> fn) {
return Monad.super.<C>fmap(fn).coerce();
}
@Override
public <C> RecursiveResult<A, C> zip(
Applicative<Function<? super B, ? extends C>, RecursiveResult<A, ?>> appFn) {
return Monad.super.zip(appFn).coerce();
}
@Override
public <C> RecursiveResult<A, C> discardL(Applicative<C, RecursiveResult<A, ?>> appB) {
return Monad.super.discardL(appB).coerce();
}
@Override
public <C> RecursiveResult<A, B> discardR(Applicative<C, RecursiveResult<A, ?>> appB) {
return Monad.super.discardR(appB).coerce();
}
@Override
@SuppressWarnings("unchecked")
public <C, App extends Applicative> Applicative<RecursiveResult<A, C>, App> traverse(
Function<? super B, ? extends Applicative<C, App>> fn,
Function<? super Traversable<C, RecursiveResult<A, ?>>, ? extends Applicative<? extends Traversable<C, RecursiveResult<A, ?>>, App>> pure) {
return match(__ -> pure.apply(coerce()).fmap(x -> (RecursiveResult<A, C>) x),
b -> fn.apply(b).fmap(this::pure));
}
public static <A, B> RecursiveResult<A, B> recurse(A a) {
return new Recurse<>(a);
}
public static <A, B> RecursiveResult<A, B> terminate(B b) {
return new Terminate<>(b);
}
static final class Recurse<A, B> extends RecursiveResult<A, B> {
final A a;
private Recurse(A a) {
this.a = a;
}
@Override
public <R> R match(Function<? super A, ? extends R> aFn, Function<? super B, ? extends R> bFn) {
return aFn.apply(a);
}
@Override
public boolean equals(Object other) {
return other instanceof Recurse && Objects.equals(a, ((Recurse) other).a);
}
@Override
public int hashCode() {
return Objects.hash(a);
}
@Override
public String toString() {
return "Recurse{" +
"a=" + a +
'}';
}
}
static final class Terminate<A, B> extends RecursiveResult<A, B> {
final B b;
private Terminate(B b) {
this.b = b;
}
@Override
public <R> R match(Function<? super A, ? extends R> aFn, Function<? super B, ? extends R> bFn) {
return bFn.apply(b);
}
@Override
public boolean equals(Object other) {
return other instanceof Terminate && Objects.equals(b, ((Terminate) other).b);
}
@Override
public int hashCode() {
return Objects.hash(b);
}
@Override
public String toString() {
return "Terminate{" +
"b=" + b +
'}';
}
}
}