Value Types

As discussed in Facts, understanding value types is crucial for working with Logica. Logica supports two value types including basic types and composite types.

Basic Types

Logica handles numbers, strings, and booleans.

• Numbers: Examples include 1, 5, 10, 42, 3.1415, etc.
• Strings: Examples include "Hello World", "Ekaterinburg", "Athens", etc.
• Booleans: true, false

CAUTION

true and false must be lowercase

All data types are optional, meaning any variable or argument can take a special null value, indicating that the value is missing. The null value is also used in aggregation, which will be covered in the Aggregation section.

Composite Types

Logica includes two composite types: arrays and records.

TIP

Elements of composite types can also take the null value.

Arrays

In Logica, "list" and "array" are synonymous and can be used interchangeably. An array is an ordered sequence of elements of the same type. The syntax for lists is identical to Python: [element1, element2, …].

Example 1:

Person(name: "Victoria", children: ["Edward", "Leopold"]);
Person(name: "Edward", children: ["George V", "Maud"]);

The in operator allows a variable to iterate over elements of the list.

Example 2:

Centaur(x) :-
     x in ["Chiron", "Hylonome", "Dictys"];

This is equivalent to:

Centaur("Chiron");
Centaur("Hylonome");
Centaur("Dictys");

Records

A record is a data type consisting of one or more named fields, each storing an element of some data type. Records are similar to JSON objects, PostgreSQL composite types, and Google ProtoBuffers. The syntax for records is similar to JavaScript object syntax: {field_name_1: value_1, field_name_2: value_2, …}.

Fields are accessed using the syntax record_name.field_name.

Example 1:

Book(title: "To Kill a Mockingbird", 
           info: {author: "Harper Lee", publication_year: 1960});
Book(title: "1984", 
           info: {author: "George Orwell", publication_year: 1949});
Book(title: "The Great Gatsby", 
           info: {author: "F. Scott Fitzgerald", publication_year: 1925});

RecentBook(title:) :-
     Book(title:, info:),
     info.publication_year > 1950;