com.jnape.palatable.lambda.lens.lenses.EitherLens Maven / Gradle / Ivy
package com.jnape.palatable.lambda.lens.lenses;
import com.jnape.palatable.lambda.adt.Either;
import com.jnape.palatable.lambda.adt.Maybe;
import com.jnape.palatable.lambda.adt.coproduct.CoProduct2;
import com.jnape.palatable.lambda.lens.Lens;
import static com.jnape.palatable.lambda.lens.Lens.simpleLens;
/**
* Lenses that operate on {@link Either}s.
*/
public final class EitherLens {
private EitherLens() {
}
/**
* Convenience static factory method for creating a lens over right values, wrapping them in a {@link Maybe}. When
* setting, a {@link Maybe#nothing()} value means to leave the {@link Either} unaltered, where as a
* {@link Maybe#just} value replaces the either with a right over the {@link Maybe}.
*
* Note that this lens is NOT lawful, since "you get back what you put in" fails for {@link Maybe#nothing()}.
*
* @param the left parameter type
* @param the right parameter type
* @return a lens that focuses on right values
*/
public static Lens.Simple, Maybe> right() {
return simpleLens(CoProduct2::projectB, (lOrR, maybeR) -> maybeR.>fmap(Either::right).orElse(lOrR));
}
/**
* Convenience static factory method for creating a lens over left values, wrapping them in a {@link Maybe}. When
* setting, a {@link Maybe#nothing()} value means to leave the {@link Either} unaltered, where as a
* {@link Maybe#just} value replaces the either with a left over the {@link Maybe}.
*
* Note that this lens is NOT lawful, since "you get back what you put in" fails for {@link Maybe#nothing()}.
*
* @param the left parameter type
* @param the right parameter type
* @return a lens that focuses on left values
*/
public static Lens.Simple, Maybe> left() {
return simpleLens(CoProduct2::projectA, (lOrR, maybeL) -> maybeL.>fmap(Either::left).orElse(lOrR));
}
}