There's a Prolog in your Scala!

Video is available here: http://skillsmatter.com/podcast/scala/theres-a-prolog-in-your-scala

Visualizations from slides 33-34 are available here:
slide 33 (scala -> prolog): https://db.tt/pjVwnQuj
slide 34 (prolog -> scala): https://db.tt/R5NSJF5g

document from slide 7: http://www.foia.cia.gov/search-results?search_api_views_fulltext=SW+90-10029X&field_collection=

paper from slide 9: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.34.3972

DEMO repository: https://github.com/folone/scalaeXchange

Copyrights:
The photo of Battersea Power station: http://www.flickr.com/photos/stephenwalford/8420082569/
The photo of hanoi towers: http://www.flickr.com/photos/46268742@N00/8455144986/
The photo of Buran is taken from the following article: http://englishrussia.com/2006/09/14/buran-the-first-russian-shuttle/
Photo of Spock, played by Zachary Quinto, is presumably copyrighted by Paramount Pictures, Skydance Productions, and Bad Robot.

Transcript

1. There is a Prolog
   in your Scala!
   @folone Scala eXchange ‘13
   

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2. One particular
   approach
   

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3. 8+ implicits
   FnHipsterAux
   

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4. Logic
   Programming
   

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5. programmation en
   logique, 1972, 5GL
   Lingua franca
   

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6. A language that doesn't
   affect the way you think
   about programming,
   is not worth knowing.
   — Alan J. Perlis
   

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7. Buran
   

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8. IBM Watson
   

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9. View Slide

10. What > How
    Relations > Functions
    Facts
    Rules (to generate more facts)
    Main principles
    It's all about formulating the question.
    Expressed with
    

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11. Predicates > Functions
    Map inputs to outputs
    Run
    Return some output
    Define constraints
    Match
    Only return yes or no*
    *If "yes", they may add bindings to variables.
    log(2, 16, 4). log(2, 16) = 4
    

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12. — How many Prolog
    programmers does it take
    to change a lightbulb?
    

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13. — How many Prolog
    programmers does it take
    to change a lightbulb?
    NO.
    

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14. Scala eXchange
    tracks planning
    https://github.com/folone/scalaeXchange/
    

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15. Facts
    talk_about(dpp, webdev).
    talk_about(milessabin, typelevel).
    talk_about(larsr_h, typelevel).
    talk_about(xeno_by, macros).
    talk_about(milessabin, macros).
    talk_about(sirthias, webdev).
    

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16. Questions
    ?- talk_about(milessabin, webdev).
    no
    ?- talk_about(larsr_h, typelevel).
    yes
    ?- talk_about(dpp, X).
    X = webdev.
    ?- talk_about(milessabin, X).
    X = typelevel ;
    X = macros.
    

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17. Questions
    ?- talk_about(Who, What).
    Who = dpp, What = webdev ;
    Who = milessabin, What = typelevel ;
    Who = milessabin, What = macros ;
    Who = larsr_h, What = typelevel ;
    Who = xeno_by, What = macros ;
    Who = sirthias, What = webdev.
    

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18. Rules
    same_topic(Person1, Person2) :-
    talk_about(Person1, Topic),
    talk_about(Person2, Topic),
    Person1 \== Person2.
    ?- same_topic(milessabin, Who).
    Who = larsr_h ;
    Who = xeno_by.
    

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19. Facts
    works_in(dpp, industry).
    works_in(milessabin, industry).
    works_in(milessabin, academia).
    works_in(larsr_h, academia).
    works_in(xeno_by, academia).
    works_in(sirthias, industry).
    

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20. same_topic(Person1, Person2) :-
    talk_about(Person1, Topic),
    talk_about(Person2, Topic),
    Person1 \== Person2.
    same_topic(Person1, Person2) :-
    works_in(Person1, Area),
    works_in(Person2, Area),
    Person1 \== Person2.
    

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21. Questions
    ?- same_topic(dpp, Who).
    Who = sirthias ;
    Who = milessabin ;
    Who = sirthias.
    

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22. Rules
    exactly_same_topic(Person1, Person2) :-
    talk_about(Person1, Topic),
    talk_about(Person2, Topic),
    works_in(Person1, Area),
    works_in(Person2, Area),
    Person1 \== Person2.
    

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23. Questions
    ?- exactly_same_topic(dpp, Who).
    Who = sirthias.
    ?- exactly_same_topic(milessabin, Who).
    Who = larsr_h ;
    Who = xeno_by.
    

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24. topic(Topic, Res) :-
    findall(Person, talk_about(Person, Topic), L1),
    Res = (Topic, L1).
    environment(Area, Res) :-
    findall(Person, works_in(Person, Area), L1),
    Res = (Area, L1).
    ?- topic(webdev, List).
    List = (webdev, [dpp, sirthias]).
    ?- environment(academia, List).
    List = (academia, [milessabin, larsr_h, xeno_by]).
    

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25. topics(L) :-
    findall(Topic, talk_about(_, Topic), L1),
    list_to_set(L1, L).
    tracks(L) :-
    topics(L1),
    member(Topic, L1),
    topic(Topic, L).
    ?- topics(L).
    L = [webdev, typelevel, macros].
    ?- tracks(L).
    L = (webdev, [dpp, sirthias]) ;
    L = (typelevel, [milessabin, larsr_h]) ;
    L = (macros, [xeno_by, milessabin]).
    

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26. Functional is imperative without state.
    Logic is functional without manual search*.
    * DFS
    

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27. Scala is a logic
    programming language...
    

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28. in type system!
    Scala is a logic
    programming language...
    

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29. TYPELEVEL
    programming in Scala is...
    in type system!
    programming language...
    

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30. logic programming
    TYPELEVEL
    programming in Scala is...
    in type system!
    in Scala!
    

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31. Prolog
    gcd(X, X, X).
    gcd(X, Y, Out) :-
    X < Y
    Z is Y - X
    gcd(X, Z, Out).
    gcd(X, Y, Out) :-
    Y < X,
    gcd(Y, X, Out).
    

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32. trait GCD[X <: Nat, Y <: Nat] { type Out <: Nat }
    object GCD
    def gcd[N<:Nat](x:Nat,y:Nat)(implicit gcd:Aux[x.N,y.N,N],wn:Witness.Aux[N]):N =
    wn.value
    type Aux[X <: Nat, Y <: Nat, Z <: Nat] = GCD[X, Y] { type Out = Z }
    Scala
    {
    implicit def gcd0[X <: Nat]:
    Aux[X, X, X] =
    new GCD[X, X] { type Out = X }
    implicit def gcd1[X <: Nat, Y <: Nat, Z <: Nat,
    Out0 <: Nat]
    (implicit ev0 : LT[X, Y],
    ev1 : Diff.Aux[Y, X, Z],
    ev2 : Aux[X, Z, Out0]):
    Aux[X, Y, Out0] =
    new GCD[X, Y] { type Out = Out0 }
    implicit def gcd2[X <: Nat, Y <: Nat, Out0 <: Nat]
    (implicit ev0 : LT[Y, X],
    ev1 : Aux[Y, X, Out0]):
    Aux[X, Y, Out0] =
    new GCD[X, Y] { type Out = Out0}
    }
    

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35. Facts — implicit vals.
    Rules — implicit defs taking implicit
    vals as parameters.
    Implication is the other way around.
    

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36. Gotchas:
    • Scala does not perform the DFS. In case of
    multiple implicits, it does not compile. Prioritizing
    implicits via inheritance.
    • Diverging implicit expansion -Xlog-implicits.
    • Extremely hard to debug (pen and paper style).
    • Peculiar exceptions (SO in compiler, Method too
    large, etc.)
    

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37. — So now I need to redefine all the
    goodness from prolog stdlib?! Lists,
    naturals, etc.
    Shapeless: stdlib for logic programming
    in scala.
    

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38. DEMO
    https://github.com/folone/scalaeXchange/
    

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39. Functional in the small,
    OO in the large,
    logic in the type system!
    

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40. http://mitpress.mit.edu/books/art-prolog
    shapeless (and all the typelevel.scala libraries)
    metascala https://www.assembla.com/spaces/metascala/wiki
    Links
    

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41. @milessabin
    @xeno_by
    @travisbrown
    @larsr_h
    @killnicole
    Thanks
    

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42. ?-
    (λx.folonexlambda-calcul.us)@
    

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