Combinator Functions

combinatorfunctions

package datastructures2;
...
public class ListModule {
public static interface List<T> {
...
public
List<T> filter(Function1<T,Boolean> f);
public <T2> List<T2> map(Function1<T,T2> f);
public <T2> T2 foldLeft (T2 seed, Function2<T2,T,T2> f);
public <T2> T2 foldRight (T2 seed, Function2<T,T2,T2> f);
public void foreach(Function1Void<T> f);
}
public static final class NonEmptyList<T> implements List<T> {
...
public List<T> filter (Function1<T,Boolean> f) {
if (f.apply(head())) {
return list(head(), tail().filter(f));
} else {
return tail().filter(f);
}
}
public <T2> List<T2> map (Function1<T,T2> f) {
return list(f.apply(head()), tail().map(f));
}
public <T2> T2 foldLeft (T2 seed, Function2<T2,T,T2> f) {
return tail().foldLeft(f.apply(seed, head()), f);
}
public <T2> T2 foldRight (T2 seed, Function2<T,T2,T2> f) {
return f.apply(head(), tail().foldRight(seed, f));
}
}
public void foreach (Function1Void<T> f) {
f.apply(head());
tail().foreach(f);
}
public static final List<? extends Object> EMPTY = new List<Object>() {
...
public
List<Object> filter (Function1<Object,Boolean> f) { return this; }
public <T2> List<T2> map (Function1<Object,T2> f) { return emptyList(); }
public <T2> T2 foldLeft (T2 seed, Function2<T2,Object,T2> f) { return seed; }
public <T2> T2 foldRight (T2 seed, Function2<Object,T2,T2> f) { return seed; }
public void foreach (Function1Void<Object> f) {}
};
}

foldleft

List<Integer> listI =list(1, list(2, list(3, list(4, emptyList()))));
listI.foldLeft(0, new Function2<Integer, Integer, Integer>() {
public Integer apply(Integer seed, Integer item) { return seed + item; }
});


//((((0 + 1) + 2) + 3) + 4) == 10

foldright

List<Integer> listI =list(1, list(2, list(3, list(4, emptyList()))));
listI.foldRight(0, new Function2<Integer, Integer, Integer>() {
public Integer apply(Integer item, Integer seed) { return item + seed; }
});


//(1 + (2 + (3 + (4 + 0)))) == 10