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diff --git a/src/main/java/bjc/data/internals/HalfBoundLazyPair.java b/src/main/java/bjc/data/internals/HalfBoundLazyPair.java
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+++ b/src/main/java/bjc/data/internals/HalfBoundLazyPair.java
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+package bjc.data.internals;
+
+import java.util.function.BiFunction;
+import java.util.function.Function;
+import java.util.function.Supplier;
+
+import bjc.data.IHolder;
+import bjc.data.IPair;
+import bjc.data.Identity;
+import bjc.data.LazyPair;
+
+/*
+ * @NOTE
+ * 	I am not convinced that this code works correctly. Tests should be
+ * 	written to make sure things only ever get instantiated once.
+ *
+ * 	Namely, my main concern is to whether the places that bind the pair
+ * 	without setting pairBound are doing the right thing.
+ */
+/**
+ * A lazy pair, with only one side bound.
+ *
+ * @author Ben Culkin
+ */
+@SuppressWarnings("javadoc")
+public class HalfBoundLazyPair<OldType, NewLeft, NewRight> implements IPair<NewLeft, NewRight> {
+	/* The supplier of the old value. */
+	private final Supplier<OldType> oldSupplier;
+
+	/* The function to transform the old value into a new pair. */
+	private final Function<OldType, IPair<NewLeft, NewRight>> binder;
+
+	/* The new bound pair. */
+	private IPair<NewLeft, NewRight> boundPair;
+	/* Has the pair been bound yet or not? */
+	private boolean pairBound;
+
+	/**
+	 * Create a new half-bound lazy pair.
+	 *
+	 * @param oldSupp
+	 *        The supplier of the old value.
+	 *
+	 * @param bindr
+	 *        The function to use to create the pair from the old value.
+	 */
+	public HalfBoundLazyPair(final Supplier<OldType> oldSupp,
+			final Function<OldType, IPair<NewLeft, NewRight>> bindr) {
+		oldSupplier = oldSupp;
+		binder = bindr;
+	}
+
+	@Override
+	public <BoundLeft, BoundRight> IPair<BoundLeft, BoundRight> bind(
+			final BiFunction<NewLeft, NewRight, IPair<BoundLeft, BoundRight>> bindr) {
+		final IHolder<IPair<NewLeft, NewRight>> newPair = new Identity<>(boundPair);
+		final IHolder<Boolean> newPairMade = new Identity<>(pairBound);
+
+		final Supplier<NewLeft> leftSupp = () -> {
+			if(!newPairMade.getValue()) {
+				/* Bind the pair if it hasn't been bound yet. */
+				newPair.replace(binder.apply(oldSupplier.get()));
+				newPairMade.replace(true);
+			}
+
+			return newPair.unwrap((pair) -> pair.getLeft());
+		};
+
+		final Supplier<NewRight> rightSupp = () -> {
+			if(!newPairMade.getValue()) {
+				/* Bind the pair if it hasn't been bound yet. */
+				newPair.replace(binder.apply(oldSupplier.get()));
+				newPairMade.replace(true);
+			}
+
+			return newPair.unwrap((pair) -> pair.getRight());
+		};
+
+		return new BoundLazyPair<>(leftSupp, rightSupp, bindr);
+	}
+
+	@Override
+	public <BoundLeft> IPair<BoundLeft, NewRight> bindLeft(
+			final Function<NewLeft, IPair<BoundLeft, NewRight>> leftBinder) {
+		final Supplier<NewLeft> leftSupp = () -> {
+			IPair<NewLeft, NewRight> newPair = boundPair;
+
+			if(!pairBound) {
+				newPair = binder.apply(oldSupplier.get());
+			}
+
+			return newPair.getLeft();
+		};
+
+		return new HalfBoundLazyPair<>(leftSupp, leftBinder);
+	}
+
+	@Override
+	public <BoundRight> IPair<NewLeft, BoundRight> bindRight(
+			final Function<NewRight, IPair<NewLeft, BoundRight>> rightBinder) {
+		final Supplier<NewRight> rightSupp = () -> {
+			IPair<NewLeft, NewRight> newPair = boundPair;
+
+			if(!pairBound) {
+				newPair = binder.apply(oldSupplier.get());
+			}
+
+			return newPair.getRight();
+		};
+
+		return new HalfBoundLazyPair<>(rightSupp, rightBinder);
+	}
+
+	@Override
+	public <OtherLeft, OtherRight, CombinedLeft, CombinedRight> IPair<CombinedLeft, CombinedRight> combine(
+			final IPair<OtherLeft, OtherRight> otherPair,
+			final BiFunction<NewLeft, OtherLeft, CombinedLeft> leftCombiner,
+			final BiFunction<NewRight, OtherRight, CombinedRight> rightCombiner) {
+		return otherPair.bind((otherLeft, otherRight) -> {
+			return bind((leftVal, rightVal) -> {
+				CombinedLeft cLeft = leftCombiner.apply(leftVal, otherLeft);
+				CombinedRight cRight = rightCombiner.apply(rightVal, otherRight);
+
+				return new LazyPair<>(cLeft, cRight);
+			});
+		});
+	}
+
+	@Override
+	public <NewLeftType> IPair<NewLeftType, NewRight> mapLeft(final Function<NewLeft, NewLeftType> mapper) {
+		final Supplier<NewLeftType> leftSupp = () -> {
+			if(pairBound) return mapper.apply(boundPair.getLeft());
+
+			final NewLeft leftVal = binder.apply(oldSupplier.get()).getLeft();
+
+			return mapper.apply(leftVal);
+		};
+
+		final Supplier<NewRight> rightSupp = () -> {
+			if(pairBound) return boundPair.getRight();
+
+			return binder.apply(oldSupplier.get()).getRight();
+		};
+
+		return new LazyPair<>(leftSupp, rightSupp);
+	}
+
+	@Override
+	public <NewRightType> IPair<NewLeft, NewRightType> mapRight(final Function<NewRight, NewRightType> mapper) {
+		final Supplier<NewLeft> leftSupp = () -> {
+			if(pairBound) return boundPair.getLeft();
+
+			return binder.apply(oldSupplier.get()).getLeft();
+		};
+
+		final Supplier<NewRightType> rightSupp = () -> {
+			if(pairBound) return mapper.apply(boundPair.getRight());
+
+			final NewRight rightVal = binder.apply(oldSupplier.get()).getRight();
+
+			return mapper.apply(rightVal);
+		};
+
+		return new LazyPair<>(leftSupp, rightSupp);
+	}
+
+	@Override
+	public <MergedType> MergedType merge(final BiFunction<NewLeft, NewRight, MergedType> merger) {
+		if(!pairBound) {
+			boundPair = binder.apply(oldSupplier.get());
+
+			pairBound = true;
+		}
+
+		return boundPair.merge(merger);
+	}
+}