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diff --git a/src/main/java/bjc/funcdata/theory/Functor.java b/src/main/java/bjc/funcdata/theory/Functor.java
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+++ b/src/main/java/bjc/funcdata/theory/Functor.java
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+package bjc.funcdata.theory;
+
+import java.util.function.Function;
+
+/**
+ * Represents a container or context some sort usually, but the precise
+ * definition is that it represents exactly what it is defined as.
+ *
+ * @author ben
+ *
+ * @param <ContainedType>
+ *        The value inside the functor.
+ */
+public interface Functor<ContainedType> {
+	/**
+	 * Converts a normal function to operate over values in a functor..
+	 *
+	 * N.B: Even though the type signature implies that you can apply the
+	 * resulting function to any type of functor, it is only safe to call it
+	 * on instances of the type of functor you called fmap on..
+	 *
+	 * @param <ArgType>
+	 *        The argument of the function.
+	 *
+	 * @param <ReturnType>
+	 *        The return type of the function.
+	 *
+	 * @param func
+	 *        The function to convert.
+	 *
+	 * @return The passed in function converted to work over a particular
+	 *         type of functors.
+	 */
+	public <ArgType, ReturnType> Function<Functor<ArgType>, Functor<ReturnType>> fmap(
+			Function<ArgType, ReturnType> func);
+
+	/**
+	 * Retrieve the thing inside this functor.
+	 *
+	 * @return The thing inside this functor.
+	 */
+	public ContainedType getValue();
+}