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layout: default title: Tutorial - Constraints ---

Beyond form_time

form_time/2 supports many useful forms. No matter how powerful, specific applications will want to support additional, one-off constraints. Because Julian is built on top of library(clpfd), you can use its full power when working with dates.

For example, the Wikipedia article on US presidential elections says that they are held "quadrennially (the count beginning with the year 1792) on Election Day, the Tuesday between November 2 and 8". How would we select all election days in US history?

?- form_time([dow(tuesday), Y-11-D], _T),  % tuesday in November
|    Y #>= 1792,    % beginning with 1792
|    0 #= Y mod 4,  % quadrennially
|    D in 2..8,     % "between November 2 and 8"
|    date(_T).
Y = 1792,
D = 6 ;
Y = 1796,
D = 8 ;
Y = 1800,
D = 4 ;
...

Extending form_time

An application or a library might want to define its own form as a shortcut for a custom constraint. For example, we can imagine a hypothetical library(julian/us/politics). It wants to implement a form presidential_election which behaves like the example above. Can it be done?

Yes. Fortunately, form_time/2 is declared multifile. The following should do the trick:

:- multifile julian:form_time/2.
julian:form_time(presidential_election, Dt) :-
    datetime(Dt),
    !,  % help indexer: no other rules apply
    form_time([dow(tuesday), Y-11-D], Dt),
    Y #>= 1792,
    0 #= Y mod 4,
    D in 2..8.

Previous: [Time Sets](sets.html)