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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.
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There is a Prolog in your Scala!
@folone Scala eXchange ‘13
One particular approach
8+ implicits FnHipsterAux
Logic Programming
programmation en
logique, 1972, 5GL
Lingua franca
A language that doesn't affect the way you think
about programming, is not worth knowing.
— Alan J. Perlis
Buran
IBM Watson
What > HowRelations > Functions
FactsRules (to generate more facts)
Main principles
It's all about formulating the question.
Expressed with
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
— How many Prolog
programmers does it take
to change a lightbulb?
— How many Prolog
programmers does it take
to change a lightbulb?NO.
Scala eXchange tracks planning
https://github.com/folone/scalaeXchange/
Factstalk_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).
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.
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.
Rulessame_topic(Person1, Person2) :-
talk_about(Person1, Topic),
talk_about(Person2, Topic),
Person1 \== Person2.
?- same_topic(milessabin, Who).
Who = larsr_h ;
Who = xeno_by.
Factsworks_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).
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.
Questions?- same_topic(dpp, Who).Who = sirthias ;Who = milessabin ;Who = sirthias.
Rulesexactly_same_topic(Person1, Person2) :-
talk_about(Person1, Topic),
talk_about(Person2, Topic),
works_in(Person1, Area),
works_in(Person2, Area),
Person1 \== Person2.
Questions?- exactly_same_topic(dpp, Who).
Who = sirthias.
?- exactly_same_topic(milessabin, Who).
Who = larsr_h ;
Who = xeno_by.
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]).
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]).
Functional is imperative without state.Logic is functional without manual search*.
* DFS
Scala is a logicprogramming language...
in type system!
Scala is a logicprogramming language...
TYPELEVELprogramming in Scala is...
in type system!programming language...
logic programming
TYPELEVELprogramming in Scala is...
in type system!
in Scala!
Prologgcd(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).
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} }
Facts — implicit vals.
Rules — implicit defs taking implicit vals as parameters.
Implication is the other way around.
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.)
— So now I need to redefine all the goodness from prolog stdlib?! Lists, naturals, etc.
Shapeless: stdlib for logic programming in scala.
DEMOhttps://github.com/folone/scalaeXchange/
Functional in the small, OO in the large, logic in the type system!
http://mitpress.mit.edu/books/art-prolog shapeless (and all the typelevel.scala libraries) metascala https://www.assembla.com/spaces/metascala/wiki
Links
@milessabin@xeno_by@travisbrown@larsr_h@killnicole
Thanks
