44 lines
1.0 KiB
Java
44 lines
1.0 KiB
Java
package cs2030s.fp;
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public class Memo<T> extends Lazy<T> {
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private Actually<T> value;
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protected Memo(Constant<? extends T> c) {
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super(c);
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// if actually is err, it is uninitialised
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this.value = Actually.<T>err(null);
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}
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protected Memo(T v) {
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super(() -> v);
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this.value = Actually.ok(v);
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}
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public static <T> Memo<T> from(T v) {
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return new Memo<T>(v);
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}
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public static <T> Memo<T> from(Constant<? extends T> v) {
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return new Memo<T>(v);
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}
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public T get() {
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T result = this.value.except(() -> super.get());
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this.value = Actually.ok(result);
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return result;
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}
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@Override
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public <R> Memo<R> transform(Immutator<? extends R, ? super T> f) {
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return Memo.<R>from(() -> f.invoke(this.get()));
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}
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public <R> Memo<R> next(Immutator<? extends Lazy<? extends R>, ? super T> f) {
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return Memo.<R>from(() -> f.invoke(this.get()).get());
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}
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@Override
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public String toString() {
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return this.value.next(t -> Actually.ok(String.valueOf(t))).unless("?");
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}
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} |