IRIs vs Literals

RDF objects come in two flavours:

• IRIs — globally unique identifiers for things you might want to say more about later (:Alice, :Tokyo, :Book123).
• Literals — raw data values you'll never need to identify on their own ("Alice", 42, "2024-01-01"^^xsd:date).

Rule of thumb: if you might attach more triples to it later, make it an IRI.

Quick decision table:

Use an IRI when…	Use a literal when…
The thing has its own identity	It's just a value carried along
You'll add more facts about it later	It's terminal data — nothing more
Two graphs might want to refer to it	Only this triple needs it
It's a person, place, document, event…	It's a name, number, date, flag…

A person's name is a literal — a string. But the person themself is an IRI: you can hire them, follow them, send them a postcard. You can't hire a string.

Same trick in software: "alice@example.com" is a literal (a delivery slot), whereas <https://alice.example/#me> is an IRI for the person who happens to use that mailbox. Confuse the two and your graph silently loses the ability to merge facts about Alice from a second source.

Worked example — the exact pattern you'll write below.

We want to say two facts about Alice in one compact statement:

1. Her name is the literal string "Alice Smith".
2. Her birth date is the literal date 1990-04-12.

Pulling on the vocabularies you met in Level 0:

• For the name of a person we reach for foaf:name.
• For the birth date we'll use our own predicate :birthDate (no need to borrow one — it's our model).
• The string is a plain literal — just wrap it in double quotes.
• The date needs the ^^xsd:date tag so the parser understands it as a calendar date rather than a string.
• The ; lets us avoid retyping :Alice twice.

:Alice  foaf:name   "Alice Smith" ;
        :birthDate  "1990-04-12"^^xsd:date .

Read it line-by-line:

Token	Why it's there
:Alice	the subject — our own IRI
foaf:name	the predicate — borrowed FOAF vocabulary for a person's name
"Alice Smith"	a literal string (default datatype xsd:string)
;	same subject, new predicate
:birthDate	our own predicate in the : namespace
"1990-04-12"^^xsd:date	a typed literal — the ^^ operator tags it as a date
.	end of the whole statement

That's everything the playground asks you to write.