GNU ELPA - triples


A flexible triple-based database for us in apps.
triples-0.1.tar, 2022-Nov-07, 90.0 KiB
Andrew Hyatt <>
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To install this package, run in Emacs:

M-x package-install RET triples RET

Full description

The triples module is a standard database module designed for use in other emacs modules. It works with either the builtin sqlite in Emacs 29 or the emacsql module, and provides a simple way of storing entities and their associated schema. The triples module is well suited to graph-like applications, where links between entities are important. The module has wrappers for most common operations, but it is anticipated that occasionally client modules would need to make their own sqlite calls. Many different database instances can be handled by the triples module. It is expected that clients supply the database connection.

1 Installing

This module is available through GNU ELPA, and can be installed as normal. However, most of the time this module is only useful in concert with another module which uses it as a library and will declare it as a dependency, so unless you are planning on developing with it, there is usually no need to install it directly.

2 Maturity

This module is very new should be considered alpha quality.

While it has basic functionality, there are significant parts, such as a querying language, that are missing. Whether anyone needs such parts will determine the priority in which they get built.

3 Types and Schema

triples employs a design in which each entity can be a member of many types, and each type has multiple properties. The properties that a type has is defined by schema. Let's take an example:

;; We assume a database called db has already been set up.
(triples-add-schema db 'person
       '(name :base/unique t :base/type string)
       '(age :base/unique t :base/type integer))
(triples-add-schema db 'employee
       '(id :base/unique t :base/type integer)
       '(manager :base/unique t)
       '(reportees :base/virtual-reversed employee/manager))

This adds a type called person, which can be set on any entity. There's another type called employee, which can also be set, independently of other types. This schema is stored in the database itself, so the database can function properly regardless on what elisp has been loaded. The schema can be redefined multiple times without any issues.

The person has 2 properties, name, and age. They are both marked as unique, so they take a single value, not a list. If :base/unique was not true, the value would be a list. We also specify what type it is, which can be any elisp type. employee is similarly constructed, but has an interesting property, reportees, which is a base/virtual-reversed property, meaning that it is supplied with values, but rather can get them from the reversed relation of employee/manager.

We'll explore how these types are used can be used in the section after next.

4 The triples concept

A triple is a unit of data consisting of a subject, a predicate, an object, and, optionally, internal metadata about the unit. The triple can be thought of as a link between the subject and object via the predicate.

Let's say that, as in the example above, we want to store someone's name. The triples would be a subject that uniquely identifies the person, a predicate that indicates the link between subject and object is about a name, and the object, which is the name value.

The object can become the subject, and this explains how the base/virtual-reversed predicate works. If Bob is the manager of Alice, then there could be a triple with Alice as the subject, manager as the predicate, and Bob as the object. But we can also find the reversed links, and ask who all are all the people that Bob manages. In this case, Bob is the subject, and Alice is the object. However, we don't actually need to store this information and try to keep it in sync, we can just get it by querying for when the Bob is the object and manager is the predicate.

5 Setting and retrieving

A subject can be set all at once (everything about the subject), or dealt with per-type. For example, the following are equivalent:

(triples-delete-subject db "alice")
(triples-set-type db "alice" 'person :name "Alice Aardvark" :age 41)
(triples-set-type db "alice" 'employee :id 1901 :manager "bob")
(triples-set-subject db "alice" '(person :name "Alice Aardvark" :age 41)
		     '(employee :id 1901 :manager "bob"))

In the second, the setting of the entire subject implies deleting everything previously associated with it.

Here is how the data is retrieved:

(triples-get-subject db "alice")

Which returns, assuming we have "catherine" and "dennis" who have "alice" as their employee/manager:

'(:person/name "Alice Aardvark" :person/age 41 :employee/id 1901 :employee/manager "bob" :employee/reportees '("catherine" "dennis"))


(triples-get-type db "alice" 'employee)

Which returns

'(:manager "bob" :reportees '("catherine" "dennis"))

There are other useful functions, including:

  • triples-get-types, which gets all the types a subject has,
  • triples-delete-subject, which deletes all data associated with a subject,
  • triples-with-predicate, gets all triples that is about a specific property,
  • triples-with-predicate-object, get all subjects whose predicate is equal to object,
  • triples-subjects-of-type, get all subjects which have a particular type.

6 Predicates, with type and without

Sometimes the triples library will require predicates that are without type, and sometimes with type, or "combined predicates". The rule is that if the type is already specified in the function, it does not need to be respecified. If the type is not specified, it is included in the combined predicate.

When returning data, if data is from just one type, the type is not returned in the returned predicates. If the data is from multiple types, the type is returned as combined predicates.

7 Using direct SQL access

Sometimes clients of this library need to do something with the database, and the higher-level triples functionality doesn't help. If you would like lower-level functionality into handling triples, you can use the same low-level methods that the rest of this library uses. These start with triples-db-.

  • triples-db-insert: Add a triple. Uses SQL's REPLACE command, so there can't be completely duplicate triples (including the property, which often can serve as a disambiguation mechanism).
  • triples-db-delete: Delete triples matching the arguments. Empty arguments match everything, so (triples-db-delete db) will delete all triples.
  • triples-db-delete-subject-predicate-prefix: Delete triples matching subjects and with predicates with a certain prefix. This can't be done with triples-db-delete because that method uses exact matching for all arguments, and this uses prefix matching for the predicate.
  • triples-db-select-predicate-object-fragment: Select triples that contain an object partially in which the fragment appears.
  • triples-db-select: Select triples matching any of the parts of the triple. Like triples-db-delete, empty arguments match everything. You can specify exactly what to return with a selector.

Sometimes this still doesn't cover what you might want to do. In that case, you should write your own direct database access. However, please follow the coding patterns for the functions above in writing it, so that the code works with both Emacs 29's builtin sqlite, and emacsql.