Stratified Negation

Flix supports stratified negation which allow restricted use of negation in rule bodies. For example:

def main(): Unit \ IO =
    let movies = #{
        Movie("The Hateful Eight").
    let actors = #{
        StarringIn("The Hateful Eight", "Samuel L. Jackson").
        StarringIn("The Hateful Eight", "Kurt Russel").
        StarringIn("The Hateful Eight", "Quentin Tarantino").
        StarringIn("Interstellar", "Matthew McConaughey").
        StarringIn("Interstellar", "Anne Hathaway").
    let directors = #{
        DirectedBy("The Hateful Eight", "Quentin Tarantino").
        DirectedBy("Interstellar", "Christopher Nolan").
    let rule = #{
        MovieWithoutDirector(title) :-
            DirectedBy(title, name),
            not StarringIn(title, name).
    query movies, actors, directors, rule
        select title from MovieWithoutDirector(title) |> println

The program defines three local variables that contain information about movies, actors, and directors. The local variable rule contains a rule that captures all movies where the director does not star in the movie. Note the use negation in this rule. The query returns an array with the string "Interstellar" because Christopher Nolan did not star in that movie.

Note: Flix enforces that programs are stratified, i.e. a program must not have recursive dependencies on which there is use of negation. If there is, the Flix compiler rejects the program.