Define reader/writer interfaces with variant

partiql-spi/src/main/java/org/partiql/spi/value/DatumReader.java

@@ -0,0 +1,21 @@
+package org.partiql.spi.value;
+
+import org.jetbrains.annotations.Nullable;
+
+/**
+ * The {@link DatumReader} interface is a low-level reader interface for reading streams of PartiQL data.
+ * <br>
+ * {@see java.io.Reader}
+ * <br>
+ * TODO
+ *  - public void reset();
+ *  - public void skip(long n);
+ */
+public interface DatumReader extends AutoCloseable {
+
+    /**
+     * @return next Datum or null.
+     */
+    @Nullable
+    public Datum read();
+}

partiql-spi/src/main/java/org/partiql/spi/value/DatumWriter.java

@@ -0,0 +1,16 @@
+package org.partiql.spi.value;
+
+/**
+ * The {@link DatumWriter} interface is a low-level writer interface for writing streams of PartiQL data.
+ * <br>
+ * {@see java.io.Writer}
+ */
+public interface DatumWriter extends AutoCloseable {
+
+    /**
+     * Like java.io.Reader with combined `append` and `write` since this does not implement Appendable.
+     *
+     * @param datum to write.
+     */
+    public DatumWriter write(Datum datum);
+}

partiql-spi/src/main/java/org/partiql/spi/value/Variant.java

@@ -0,0 +1,43 @@
+package org.partiql.spi.value;
+
+import org.jetbrains.annotations.NotNull;
+
+import java.nio.charset.Charset;
+
+/**
+ * Variant is a {@link Datum} with the ability to pack and unpack a value.
+ *
+ * @param <T>
+ */
+public interface Variant<T> extends Datum {
+
+    /**
+     * Unpack the inner variant value.
+     *
+     * @return T
+     */
+    T unpack();
+
+    /**
+     * TODO move to writer?
+     * <br>
+     * Pack the variant into a byte array.
+     *
+     * @return byte[]
+     */
+    default byte[] pack() {
+        throw new UnsupportedOperationException("variant does not have a byte[] encoding.");
+    }
+
+    /**
+     * TODO move to writer?
+     * <br>
+     * Pack the variant into a byte array with the given charset.
+     *
+     * @param charset   charset
+     * @return          byte[]
+     */
+    default byte[] pack(@NotNull  Charset charset) {
+        throw new UnsupportedOperationException("variant does not have an encoding for charset: " + charset.name());
+    }
+}