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RDF List Creation Operations

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This guide covers operations for creating RDF lists and checking their state. These are fundamental operations you'll use when initializing lists or validating list contents.

What You'll Learn

  • How to create an empty RDF list
  • How to check if a list is empty
  • How to build lists manually using triples

Operations Summary

OperationDescriptionReturns
rdflist_empty(listVar)Create or bind to an empty listrdf:nil
rdflist_is_empty(consSubject)Check if a list is emptySucceeds if empty

rdflist_empty

Creates or binds a variable to an empty list (rdf:nil).

Syntax

Example: JavaScript
WOQL.lib().rdflist_empty(listVar)

Parameters

ParameterTypeDescription
listVarstringVariable to bind to the empty list

Example

Example: JavaScript
// Create an empty list
const query = WOQL.lib().rdflist_empty("v:empty_list");
const result = await client.query(query);

const emptyList = result.bindings[0]["empty_list"];
console.log(emptyList); // "rdf:nil"

Use Cases

  • Initialization: Set up a default empty list value
  • Default values: Provide empty lists when no data exists
  • Conditional logic: Use as a base case for list building

rdflist_is_empty

Checks if a list is empty. The query succeeds (returns bindings) if the list is empty, and fails (returns no bindings) if the list has elements.

Syntax

Example: JavaScript
WOQL.lib().rdflist_is_empty(consSubject)

Parameters

ParameterTypeDescription
consSubjectstringThe list head to check

Example

Example: JavaScript
// Check if a list is empty
const checkQuery = WOQL.lib().rdflist_is_empty("rdf:nil");
const checkResult = await client.query(checkQuery);

if (checkResult.bindings && checkResult.bindings.length > 0) {
  console.log("List is empty");
} else {
  console.log("List has elements");
}

Use Cases

  • Validation: Verify list state before operations
  • Conditional logic: Branch based on list contents
  • Guard clauses: Prevent operations on empty lists

Building Lists Manually

For complete control, you can build RDF lists using triples directly. This is useful for understanding the underlying structure.

Creating a Three-Element List

Example: JavaScript
// List: ["First", "Second", "Third"]
const createList = WOQL.and(
  // Generate random IDs for cons cells
  WOQL.idgen_random("terminusdb://data/Cons/", "v:cell1"),
  WOQL.idgen_random("terminusdb://data/Cons/", "v:cell2"),
  WOQL.idgen_random("terminusdb://data/Cons/", "v:cell3"),
  
  // Cell 1: first element
  WOQL.add_triple("v:cell1", "rdf:type", "rdf:List"),
  WOQL.add_triple("v:cell1", "rdf:first", WOQL.string("First")),
  WOQL.add_triple("v:cell1", "rdf:rest", "v:cell2"),
  
  // Cell 2: second element
  WOQL.add_triple("v:cell2", "rdf:type", "rdf:List"),
  WOQL.add_triple("v:cell2", "rdf:first", WOQL.string("Second")),
  WOQL.add_triple("v:cell2", "rdf:rest", "v:cell3"),
  
  // Cell 3: third element (points to nil)
  WOQL.add_triple("v:cell3", "rdf:type", "rdf:List"),
  WOQL.add_triple("v:cell3", "rdf:first", WOQL.string("Third")),
  WOQL.add_triple("v:cell3", "rdf:rest", "rdf:nil")
);

const result = await client.query(createList);
const listHead = result.bindings[0]["cell1"];

Creating a Single-Element List

Example: JavaScript
const createSingleElement = WOQL.and(
  WOQL.idgen_random("terminusdb://data/Cons/", "v:cell"),
  WOQL.add_triple("v:cell", "rdf:type", "rdf:List"),
  WOQL.add_triple("v:cell", "rdf:first", WOQL.string("Only Item")),
  WOQL.add_triple("v:cell", "rdf:rest", "rdf:nil")
);

Complete Example: Initialize and Validate

Example: JavaScript
import { WOQLClient, WOQL } from "@terminusdb/terminusdb-client";

const client = new WOQLClient("http://localhost:6363", {
  user: "admin",
  organization: "admin",
  key: "root"
});

async function initializeAndValidate() {
  // Create an empty list
  const emptyQuery = WOQL.lib().rdflist_empty("v:my_list");
  const emptyResult = await client.query(emptyQuery);
  const myList = emptyResult.bindings[0]["my_list"];
  
  console.log("Created list:", myList); // "rdf:nil"
  
  // Verify it's empty
  const checkQuery = WOQL.lib().rdflist_is_empty(myList);
  const checkResult = await client.query(checkQuery);
  
  if (checkResult.bindings?.length > 0) {
    console.log("✓ List is correctly empty");
  }
  
  // Now create a list with elements
  const createQuery = WOQL.and(
    WOQL.idgen_random("terminusdb://data/Cons/", "v:cell1"),
    WOQL.add_triple("v:cell1", "rdf:type", "rdf:List"),
    WOQL.add_triple("v:cell1", "rdf:first", WOQL.string("Item")),
    WOQL.add_triple("v:cell1", "rdf:rest", "rdf:nil")
  );
  
  const createResult = await client.query(createQuery);
  const nonEmptyList = createResult.bindings[0]["cell1"];
  
  // Verify it's NOT empty
  const checkNonEmpty = WOQL.lib().rdflist_is_empty(nonEmptyList);
  const nonEmptyResult = await client.query(checkNonEmpty);
  
  if (!nonEmptyResult.bindings?.length) {
    console.log("✓ List correctly has elements");
  }
}

initializeAndValidate();

Integration with Document Schema

Lists can be defined in your schema and populated automatically:

Example: JavaScript
// Schema definition
const schema = [
  {
    "@type": "Class",
    "@id": "TodoList",
    "@key": { "@type": "Random" },
    name: "xsd:string",
    items: { "@type": "List", "@class": "xsd:string" }
  }
];

await client.addDocument(schema, { graph_type: "schema" });

// Create a document with an empty list
const emptyTodoList = {
  "@type": "TodoList",
  name: "My Tasks",
  items: []  // Empty list in JSON becomes rdf:nil
};

await client.addDocument(emptyTodoList);

// Create a document with items
const populatedTodoList = {
  "@type": "TodoList",
  name: "Shopping",
  items: ["Milk", "Bread", "Eggs"]
};

await client.addDocument(populatedTodoList);

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