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books

This example illustrates how to use the optional terms library to decouple data acquisition, which must be able to represent optional values in the data, from data processing, which decides how to handle those values and their absence. The use of optionals terms avoids the often problematic solution of using special values to represent the absence of optionals values. The optional terms library predicates allow handling the optional terms without the need to use if-then-else or cut control constructs.

For a description of this example, please see the comments in the books.lgt source file.

See also the missing_data and cascade examples.

Print Logtalk, Prolog backend, and kernel versions (if running as a notebook):

%versions

Start by loading the example:

logtalk_load(books(loader)).

Print a list of books that have extras:

data_processing::print.

<!-- The Philosopher's Stone with free quidditch_set at 278 gr The Chamber of Secrets with free map The Prisoner of Azkaban The Goblet of Fire The Order of the Phoenix The Half-Blood Prince with free audio_cd The Deathly Hallows with free horcrux_set at 123 gr true. -->

Print variant using kilograms instead of grams by mapping optionals:

data_processing::print_kg.

<!-- The Philosopher's Stone with free quidditch_set at 0.278 kg The Chamber of Secrets with free map The Prisoner of Azkaban The Goblet of Fire The Order of the Phoenix The Half-Blood Prince with free audio_cd The Deathly Hallows with free horcrux_set at 0.123 kg true. -->

Print a list of extras with declared weight:

data_processing::print_heavy_extras.

<!-- quidditch_set at 278 gr horcrux_set at 123 gr

true. -->

Get a list of book titles that have extras:

data_processing::books_with_extras(Titles).

<!-- Titles = ['The Philosopher\'s Stone', 'The Chamber of Secrets', 'The Half-Blood Prince', 'The Deathly Hallows']. -->