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Functional Java is an open source library that supports closures for the Java programming language
package fj.function;
import fj.Equal;
import fj.F;
import fj.F0;
import fj.F2;
import fj.Function;
import fj.Monoid;
import fj.P1;
import fj.P2;
import fj.data.List;
import fj.data.Option;
import static fj.Function.compose;
import static fj.Function.curry;
import static fj.data.List.lookup;
/**
* The essence of the visitor design pattern expressed polymorphically.
*
* @version %build.number%
*/
public final class Visitor {
private Visitor() {
throw new UnsupportedOperationException();
}
/**
* Returns the first value available in the given list of optional values. If none is found return the given default value.
*
* @param values The optional values to search.
* @param def The default value if no value is found in the list.
* @return The first value available in the given list of optional values. If none is found return the given default value.
*/
public static X findFirst(final List> values, final F0 def) {
return Monoid.firstOptionMonoid().sumLeft(values).orSome(def);
}
/**
* Returns the first non-null value in the given list of optional values. If none is found return the given default value.
*
* @param values The potentially null values to search.
* @param def The default value if no value is found in the list.
* @return The first non-null value in the given list of optional values. If none is found return the given default value.
*/
public static X nullablefindFirst(final List values, final F0 def) {
return findFirst(values.map(Option.fromNull()), def);
}
/**
* Returns the first value found in the list of visitors after application of the given value, otherwise returns the
* given default.
*
* @param visitors The list of visitors to apply.
* @param def The default if none of the visitors yield a value.
* @param value The value to apply to the visitors.
* @return The first value found in the list of visitors after application of the given value, otherwise returns the
* given default.
*/
public static B visitor(final List>> visitors, final F0 def, final A value) {
return findFirst(visitors.map(Function.>apply(value)), def);
}
/**
* Returns the first non-null value found in the list of visitors after application of the given value,
* otherwise returns the given default.
*
* @param visitors The list of visitors to apply looking for a non-null.
* @param def The default if none of the visitors yield a non-null value.
* @param value The value to apply to the visitors.
* @return The first value found in the list of visitors after application of the given value, otherwise returns the
* given default.
*/
public static B nullableVisitor(final List> visitors, final F0 def, final A value) {
return visitor(visitors.map(new F, F>>() {
public F> f(final F k) {
return compose(Option.fromNull(), k);
}
}), def, value);
}
/**
* Uses an association list to perform a lookup with equality and returns a function that can be applied to a default,
* followed by the associated key to return a value.
*
* @param x The association list.
* @param eq The equality for the association list keys.
* @return A function that can be applied to a default value (there is no association) and an associated key.
*/
public static F> association(final List> x, final Equal eq) {
return curry(new F2() {
public B f(final B def, final A a) {
return lookup(eq, x, a).orSome(def);
}
});
}
/**
* Uses an association list to perform a lookup with equality and returns a function that can be applied to a default,
* followed by the associated key to return a value.
*
* @param x The association list.
* @param eq The equality for the association list keys.
* @return A function that can be applied to a default value (there is no association) and an associated key.
*/
public static F, F> associationLazy(final List> x, final Equal eq) {
return curry(new F2, A, B>() {
public B f(final P1 def, final A a) {
return lookup(eq, x, a).orSome(def);
}
});
}
}