There's a Prolog in your Scala!
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The document discusses logic programming in Scala, emphasizing its type system and potential applications. It explores concepts like predicates, functions, and implicit definitions, providing examples of how they can generate facts and rules. Additionally, it highlights challenges related to debugging and implicit expansions in Scala compared to Prolog.
![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]).](https://image.slidesharecdn.com/presentation-140327152910-phpapp02/85/There-s-a-Prolog-in-your-Scala-24-320.jpg)
![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]).](https://image.slidesharecdn.com/presentation-140327152910-phpapp02/85/There-s-a-Prolog-in-your-Scala-25-320.jpg)
![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}
}](https://image.slidesharecdn.com/presentation-140327152910-phpapp02/85/There-s-a-Prolog-in-your-Scala-32-320.jpg)
There's a Prolog in your Scala!
- 1. There is a Prolog in your Scala! @folone Scala eXchange ‘13
- 5. programmation en logique, 1972, 5GL Lingua franca
- 6. A language that doesn't affect the way you think about programming, is not worth knowing. — Alan J. Perlis
- 7. Buran
- 8. IBM Watson
- 10. What > How Relations > Functions Facts Rules (to generate more facts) Main principles It's all about formulating the question. Expressed with
- 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
- 12. — How many Prolog programmers does it take to change a lightbulb?
- 13. — How many Prolog programmers does it take to change a lightbulb? NO.
- 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).
- 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.
- 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.
- 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.
- 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).
- 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.
- 21. Questions ?- same_topic(dpp, Who). Who = sirthias ; Who = milessabin ; Who = sirthias.
- 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.
- 23. Questions ?- exactly_same_topic(dpp, Who). Who = sirthias. ?- exactly_same_topic(milessabin, Who). Who = larsr_h ; Who = xeno_by.
- 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]).
- 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]).
- 26. Functional is imperative without state. Logic is functional without manual search*. * DFS
- 27. Scala is a logic programming language...
- 28. in type system! Scala is a logic programming language...
- 29. TYPELEVEL programming in Scala is... in type system! programming language...
- 30. logic programming TYPELEVEL programming in Scala is... in type system! in Scala!
- 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).
- 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} }
- 35. Facts — implicit vals. Rules — implicit defs taking implicit vals as parameters. Implication is the other way around.
- 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.)
- 37. — So now I need to redefine all the goodness from prolog stdlib?! Lists, naturals, etc. Shapeless: stdlib for logic programming in scala.
- 39. Functional in the small, OO in the large, logic in the type system!
- 40. http://mitpress.mit.edu/books/art-prolog shapeless (and all the typelevel.scala libraries) metascala https://www.assembla.com/spaces/metascala/wiki Links