ScriptEngine engine = new VTLScriptEngine(connector);

Bindings bindings = engine.getBindings(ScriptContext.ENGINE_SCOPE);
engine.eval("ds1 := get(\"foo\")" +
            "ds2 := get(\"bar\")" +
            "ds3 := [ds1, ds2] {" +
            "   filter ds1.id = \"string\"," +
            "   total := ds1.measure + ds2.measure" +
            "}");

System.out.println(bindings.get("ds3"))

public interface Connector {

    boolean canHandle(String identifier);

    Dataset getDataset(String identifier) throws ConnectorException;

    Dataset putDataset(String identifier, Dataset dataset) throws ConnectorException;

}

class StaticDataset implements Dataset {

    private final DataStructure structure = DataStructure.builder()
            .put("id", Role.IDENTIFIER, String.class)
            .put("period", Role.IDENTIFIER, Instant.class)
            .put("measure", Role.MEASURE, Long.class)
            .put("attribute", Role.ATTRIBUTE, String.class)
            .build();

    @Override
    public Stream<DataPoint> getData() {

        List<Map<String, Object>> data = new ArrayList<>();
        HashMap<String, Object> row = new HashMap<>();
        Instant period = Instant.now();
        for (int i = 0; i < 100; i++) {
            row.put("id", "id #" + i);
            row.put("period", period);
            row.put("measure", Long.valueOf(i));
            row.put("attribute", "attribute #" + i);
            data.add(row);
        }

        return data.stream().map(structure::wrap);
    }

    @Override
    public Optional<Map<String, Integer>> getDistinctValuesCount() {
        return Optional.empty();
    }

    @Override
    public Optional<Long> getSize() {
        return Optional.of(100L);
    }

    @Override
    public DataStructure getDataStructure() {
        return structure;
    }
}