Appendix: Result Formats

A DATALOG-TEXT processor may simply need to return query results via an API and so not deal with the display of data to human users. If the processor does display human-readable results it should be done according to the results formats defined in this section.

If a processor supports multiple result forms, including forms not defined in this specification, the programmer can select the preferred form via the results pragma § Pragma results or some other means such as processor command-line parameters, environment variables and so forth.

`native` Format

This format MUST be supported by a conforming DATALOG-TEXT processor that displays results in human-readable form

Example – Existential Query

.pragma results=native
?- mortal("Socrates").

In the native results form a processor MAY return the matching fact, or return a boolean response. The two results in the following example are therefore equivalent:

mortal("Socrates").
true

Example – Selection Query

.pragma results=native
?- mortal(X).

As above, but the output MUST be line-oriented with a single fact per line.

mortal("Socrates").
mortal("Plato").

Example – Projection Query

.pragma results=native
.assert car(make: string, model: string, age: integer).

car("ford", X, _)?

As above, but only variables are REQUIRED to be included in the response. Specifically, anonymous attributes MUST be removed from the results and the inclusion of constant values MAY be present, but it is RECOMMENDED to remove them.

In the native form the result car(edge). would be an error as the car relation has three attributes; similarly, car("ford", edge, _). is invalid as it includes an anonymous variable in a fact. The naïve approach is to return the entirety of the matching facts, as shown below. However, this results in a column that was specifically anonymous and really should not be present.

car("ford", edge, 22).
car("ford", escort, 12).
car("ford", fiesta, 8).
car("ford", focus, 19).
car("ford", fusion, 13).
car("ford", mustang, 3).
…

In this case the processor MAY choose to construct a new intensional relation representing the query and then return these simplified facts.

car_1 :- car("ford", X, _).

car_1(edge).
car_1(escort).
car_1(fiesta).
car_1(focus).
car_1(fusion).
car_1(mustang).
…

`tabular` Format

This format is optional but RECOMMENDED.

This specification does not require a specific set of characters for laying out tables. The following are all acceptable styles that use either ASCII characters or Unicode Box Drawing characters. Note that all the formats below do make a distinction between header and body rows.

+------------+------------+    ,------------+------------,    
| X: string  | Y: string  |    | X: string  | Y: string  |    X: string  | Y: string
+============+============+    |============+============|    -----------+-----------
| "Socrates" | "Plato"    |    | "Socrates" | "Plato"    |    "Socrates" | "Plato"    
+------------+------------+    '------------+------------'    

┌────────────┬────────────┐    ╭────────────┬────────────╮    
│ X: String  │ Y: String  │    │ X: String  │ Y: String  │    X: String  │ Y: String 
╞════════════╪════════════╡    ╞════════════╪════════════╡    ═══════════╪═══════════
│ "Socrates" │ "Plato"    │    │ "Socrates" │ "Plato"    │    "Socrates" │ "Plato"   
└────────────┴────────────┘    ╰────────────┴────────────╯

Example – Existential Query

In the fact result form a table has an unnamed column with the fact constants included.

+------------+
| _: string  |
+============+
| "Socrates" |
+------------+

In the boolean result form a table is written with an unlabeled column with the value true.

+------------+
| _: boolean |
+============+
| true       |
+------------+

Example – Selection Query

In the tabular results form a table is written with one column per variable in the goal and all matching facts as rows.

+------------+
| X: string  |
+============+
| "Socrates" |
| "Plato"    |
+------------+

Note that in naming a column it is appropriate to use either the name of the variable (as shown above) or the label of the attribute if known (below).

+---------------+
| name: string  |
+===============+
| "Socrates"    |
| "Plato"       |
+---------------+

Example – Projection Query

As above.

+------------+
| X: string  |
+============+
| edge       |
+------------+
| escort     |
+------------+
| fiesta     |
+------------+
| focus      |
+------------+
| fusion     |
+------------+
| mustang    |
+------------+
      …

Datalog Specifications