Extensible Data Notation

In Datomic, a Datalog query is written in extensible data notation (edn). Edn is a data format similar to JSON, but it:

• is extensible with user defined value types,
• has more base types,
• is a subset of Clojure data.

Edn consists of:

• Numbers: 42, 3.14159
• Strings: "This is a string"
• Keywords: :kw, :namespaced/keyword, :foo.bar/baz
• Symbols: max, +, ?title
• Vectors: [1 2 3] [:find ?foo ...]
• Lists: (3.14 :foo [:bar :baz]), (+ 1 2 3 4)
• Instants: #inst "2013-02-26"
• .. and a few other things which we will not need in this tutorial.

Here is an example query that finds all movie titles in our example database:

[:find ?title
 :where 
 [_ :movie/title ?title]]

Note that the query is a vector with four elements:

• the keyword :find
• the symbol ?title
• the keyword :where
• the vector [_ :movie/title ?title]

We'll go over the specific parts of the query later, but for now you should simply type the above query verbatim into the textbox below, press Run Query, and then continue to the next part of the tutorial.