Immutability Against The Machine

Immutability has taken over the software world. Programmers are using immutable values to make their products more maintainable. They are able to focus on higher-level architectural problems instead of hard-to-debug accidental mutations.

However, the software we write often needs to run on a real machine. It needs state, it needs to do many things at once using multiple threads, and it needs to acquire some resources like sockets or files. Moreover, it needs to clean after itself in any condition, even when things go awry, making sure it never leaks memory or resources. We may be tempted to say that solving these concerns requires some old-school mutations. But it doesn’t!

In this talk we will show a real-world application that uses state, multiple threads, and resources. We will use real data from an external Wikidata service, make sure we conform to the API limits, implement a cache, and make sure we release all unneeded connections along the way. Most importantly, all this is going to be modeled as immutable values!

Transcript

1. @miciek
   Michał Płachta
   www.michalplachta.com
   IMMUTABILITY
   
   
   AGAINST
   
   
   THE MACHINE
   

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2. @miciek
   Back to 2001
   

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3. @miciek
   2001
   Let’s replace
   Haskell with
   Java
   University of
   
   
   Texas
   

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4. @miciek
   2001
   Let’s replace
   Haskell with
   Java
   University of
   
   
   Texas
   

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5. @miciek
   Dijkstra writes to University of Texas
   https://www.cs.utexas.edu/users/EWD/transcriptions/OtherDocs/Haskell.html
   Facing students with
   the novelty of
   functional programming
   immediately drives
   home the message
   that there is more to
   programming than they
   thought.
   “
   ”
   

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6. @miciek
   Dijkstra writes to University of Texas
   https://www.cs.utexas.edu/users/EWD/transcriptions/OtherDocs/Haskell.html
   Facing students with
   the novelty of
   functional programming
   immediately drives
   home the message
   that there is more to
   programming than they
   thought.
   “
   ”
   

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7. @miciek
   - declarative, functional
   
   
   - relationships between values
   
   
   - rigorous reasoning
   - imperative
   
   
   - step-by-step statements
   
   
   - operational reasoning
   Learning from other paradigms
   Our Paradigm
   Other Paradigms
   

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8. @miciek
   DEMO: printing stuff in the REPL
   

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9. @miciek
   What is it? vs What does it do?
   declarative vs imperative
   The tools we use shape our thinking!
   

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10. @miciek
    The tools we use shape our thinking
    What is it?
    
    
    An immutable value!
    What does it do?
    
    
    Eeee… It prints ‘4’.
    

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11. @miciek
    - declarative, functional
    
    
    - relationships between values
    
    
    - rigorous reasoning
    - imperative
    
    
    - step-by-step statements
    
    
    - operational reasoning
    Learning from other paradigms
    What Does It Do?
    What is It?
    We can learn a lot from there!
    

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12. @miciek
    What does it do?
    List xs = Arrays.asList(1, 2, 3, 4, 5);
    
    List result = new ArrayList
    < >
    ();
    //
    empty
    
    for (Integer x: xs) {
    
    result.add(x * 2);
    
    }
    
    //
    now result is a list [2, 4, 6, 8, 10]
    
    Java
    

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13. @miciek
    What is it?
    val numbers = List(1, 2, 3, 4, 5)
    
    val doubles = numbers.map(i
    =>
    i * 2)
    / /
    List(2, 4, 6, 8, 10)
    
    

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14. @miciek
    What is it?
    val numbers = List(1, 2, 3, 4, 5)
    
    val doubles = numbers.map(i
    =>
    i * 2)
    / /
    List(2, 4, 6, 8, 10)
    
    Scala
    numbers
    List[Int]
    doubles
    List[Int]
    .map
    

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15. @miciek
    What does it do?
    String tvShow = "The Wire (2002-2008)";
    
    int extractYearStart(String show) throws Exception {…}
    
    int extractYearEnd(String show) throws Exception {…}
    
    int start = extractYearStart(tvShow);
    / /
    2002
    
    int end = extractYearEnd(tvShow);
    //
    2008
    
    Java
    

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16. @miciek
    What does it do?
    String tvShow = "The Wire (2002-2008)";
    
    int extractYearStart(String show) throws Exception {…}
    
    int extractYearEnd(String show) throws Exception {…}
    
    int start = extractYearStart(tvShow);
    / /
    2002
    
    int end = extractYearEnd(tvShow);
    //
    2008
    
    extractYearStart("Chernobyl (2019)");
    / /
    ???
    
    Java
    

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17. @miciek
    What does it do?
    String tvShow = "The Wire (2002-2008)";
    
    int extractYearStart(String show) throws Exception {…}
    
    int extractYearEnd(String show) throws Exception {…}
    
    int start = extractYearStart(tvShow);
    / /
    2002
    
    int end = extractYearEnd(tvShow);
    //
    2008
    
    extractYearStart("Chernobyl (2019)");
    / /
    ???
    
    Java
    

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18. @miciek
    What does it do?
    int extractYearStart(String show) throws Exception {…}
    
    int extractYearEnd(String show) throws Exception {…}
    
    int extractSingleYear(String show) throws Exception {…}
    
    Integer year = null;
    
    try {
    
    year = extractYearStart(tvShow);
    
    } catch(Exception e) {
    
    year = extractSingleYear(tvShow);
    
    }
    
    Java
    

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19. @miciek
    What is it?
    def extractYearStart(show: String): Option[Int] = ???
    
    def extractYearEnd(show: String): Option[Int] = ???
    
    def extractSingleYear(show: String): Option[Int] = ???
    Scala
    None
    Option[A]
    Some[A]
    val value: A
    

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20. @miciek
    What is it?
    def extractYearStart(show: String): Option[Int] = ???
    
    def extractYearEnd(show: String): Option[Int] = ???
    
    def extractSingleYear(show: String): Option[Int] = ???
    
    val year = extractYearStart(tvShow).orElse(extractSingleYear(tvShow))
    Scala
    extractYearStart
    Option[Int]
    extractSingleYear
    Option[Int]
    year
    Option[Int]
    .orElse
    

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21. @miciek
    - declarative, functional
    
    
    - relationships between values
    
    
    - rigorous reasoning
    
    
    - what is it?
    - imperative
    
    
    - step-by-step statements
    
    
    - operational reasoning
    
    
    - what does it do?
    We don’t have to think like a machine
    The Machine
    Immutability
    

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22. @miciek
    Shaping our thinking habits
    It is not only
    the violin that
    shapes the
    violinist, we are
    all shaped by
    the tools we
    train ourselves
    to use…
    “
    ”
    https://www.cs.utexas.edu/users/EWD/transcriptions/OtherDocs/Haskell.html
    

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23. @miciek
    Plus… FP ideas aren’t going away
    https://www.infoq.com/articles/data-oriented-programming-java/
    

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24. @miciek
    Passing immutable values around
    numbers
    List[Int]
    extractYearStart
    Option[Int]
    extractSingleYear
    Option[Int]
    doubles
    List[Int]
    year
    Option[Int]
    .map
    .orElse
    

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25. @miciek
    What about talking to the outside world?
    

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26. @miciek
    IMMUTABILITY AGAINST THE MACHINE
    Using S
    tri
    n
    g
    s, L
    i
    s
    t
    s, Op
    ti
    ons
    Talking to the outside world
    Threads, resource safety, and state
    

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27. @miciek
    -User can search for an attraction name
    
    
    -The app finds the attraction and its location
    
    
    -It returns movies that are set in this location
    A peculiar travel guide
    

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28. @miciek
    A peculiar travel guide
    > “Bridge of Sighs”
    You searched for “Bridge of
    Sighs”, which is located in Venice.
    Before visiting, you can watch:
    Casino Royale
    

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29. @miciek
    The model
    case class LocationId(value: String)
    
    case class Location(id: LocationId, name: String)
    
    case class Attraction(name: String, location: Location)
    
    case class Movie(name: String, boxOffice: Long)
    
    case class TravelGuide(attraction: Attraction, movies: List[Movie])
    
    

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30. @miciek
    We will use Wikidata
    https://query.wikidata.org/
    
    
    

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31. @miciek
    Thinking Like A Machine
    https://jena.apache.org/
    

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32. @miciek
    String query = "SELECT DISTINCT ?attraction ?attractionLabel \n" +
    
    " ?location ?locationLabel WHERE {\n" +
    
    " ?attraction wdt:P31 wd:Q570116;\n" +
    
    " rdfs:label ?attractionLabel;\n" +
    
    " wdt:P131 ?location.\n" +
    
    "} LIMIT 3";
    
    RDFConnection connection = RDFConnectionRemote.create()
    
    .destination("https:
    //
    query.wikidata.org/")
    
    .queryEndpoint("sparql")
    
    .build();
    
    QueryExecution execution = connection.query(QueryFactory.create(query));
    
    Java
    Thinking Like A Machine
    https://jena.apache.org/
    

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33. @miciek
    Iterator solutions = execution.execSelect();
    
    while(solutions.hasNext()) {
    
    QuerySolution solution = solutions.next();
    
    String id = solution.getResource("attraction").getLocalName();
    
    String label = solution.getLiteral("label").getString();
    
    System.out.printf("Got attraction %s (id = %s)%n", label, id);
    
    }
    
    Java
    Thinking Like A Machine
    Got attraction Yellowstone National Park (id = Q351)
    
    Got attraction Eiffel Tower (id = Q243)
    
    Got attraction Table Mountain (id = Q213360)
    Output
    

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34. @miciek
    def execQuery(query: String): IO[List[QuerySolution]] = { … }
    
    Scala
    Immutability
    https://typelevel.org/cats-effect/
    

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35. @miciek
    def execQuery(query: String): IO[List[QuerySolution]] = { … }
    
    Scala
    Immutability
    https://typelevel.org/cats-effect/
    IO[List[QuerySolution]]
    Immutable value representing a
    potentially side-effectful program that,
    once executed successfully, will produce
    a list of query solutions.
    

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36. @miciek
    def execQuery(query: String): IO[List[QuerySolution]] = {
    
    IO.delay(connection.query(QueryFactory.create(query)).execSelect())
    
    }
    
    Scala
    Immutability
    IO.delay( … )
    Creates an immutable value
    representing a potentially side-effectful
    program that will execute the given
    block of code and return the result.
    

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37. @miciek
    def execQuery(query: String): IO[List[QuerySolution]] = {
    
    IO.delay(connection.query(QueryFactory.create(query)).execSelect())
    
    }
    
    val solutionsProgram: IO[List[QuerySolution]] = execQuery(query)
    
    println(solutionsProgram)
    
    Scala
    Immutability
    

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38. @miciek
    def execQuery(query: String): IO[List[QuerySolution]] = {
    
    IO.delay(connection.query(QueryFactory.create(query)).execSelect())
    
    }
    
    val solutionsProgram: IO[List[QuerySolution]] = execQuery(query)
    
    println(solutionsProgram)
    //
    IO(
    .. .
    )
    
    Scala
    Immutability
    

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39. @miciek
    def execQuery(query: String): IO[List[QuerySolution]] = {
    
    IO.delay(connection.query(QueryFactory.create(query)).execSelect())
    
    }
    
    val solutionsProgram: IO[List[QuerySolution]] = execQuery(query)
    
    val attractionsProgram: IO[List[Attraction]] =
    
    Scala
    Immutability
    We don’t have to run anything,
    
    
    let’s work with values first!
    

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40. @miciek
    def execQuery(query: String): IO[List[QuerySolution]] = {
    
    IO.delay(connection.query(QueryFactory.create(query)).execSelect())
    
    }
    
    val solutionsProgram: IO[List[QuerySolution]] = execQuery(query)
    
    def parseAttraction(s: QuerySolution): Option[Attraction] = { … }
    
    val attractionsProgram: IO[List[Attraction]] =
    
    solutionsProgram.map(solutions
    = >
    solutions.flatMap(parseAttraction))
    Scala
    Immutability
    

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41. @miciek
    val solutionsProgram: IO[List[QuerySolution]] = execQuery(query)
    
    val attractionsProgram: IO[List[Attraction]] =
    
    solutionsProgram.map(solutions
    = >
    solutions.flatMap(parseAttraction))
    Scala
    Immutability
    List[QuerySolution]
    =>
    List[Attraction]
    
    We transformed results without having them!
    

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42. @miciek
    Immutability
    def execQuery(query: String): IO[List[QuerySolution]] = {
    
    IO.delay(connection.query(QueryFactory.create(query)).execSelect())
    
    }
    
    val solutionsProgram: IO[List[QuerySolution]] = execQuery(query)
    
    def parseAttraction(s: QuerySolution): Option[Attraction] = { … }
    
    val attractionsProgram: IO[List[Attraction]] =
    
    solutionsProgram.map(solutions
    = >
    solutions.flatMap(parseAttraction))
    Scala
    solutionsProgram
    IO[List[QuerySolution]]
    attractionsProgram
    IO[List[Attraction]]
    .map
    

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43. @miciek
    DEMO: Let’s code the travel guide!
    > “Bridge of Sighs”
    You searched for “Bridge of
    Sighs”, which is located in Venice.
    Before visiting, you can watch:
    Casino Royale
    

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44. @miciek
    IMMUTABILITY AGAINST THE MACHINE
    Using S
    tri
    n
    g
    s, L
    i
    s
    t
    s, Op
    ti
    ons
    Talking to the outside world
    Threads, resource safety, and state
    

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45. @miciek
    IMMUTABILITY AGAINST THE MACHINE
    IO
    List
    Option
    String
    Ref
    Resource
    

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46. @miciek
    - declarative, functional
    
    
    - relationships between values
    
    
    - rigorous reasoning
    
    
    - what is it?
    - imperative
    
    
    - step-by-step statements
    
    
    - operational reasoning
    
    
    - what does it do?
    We don’t have to think like a machine
    The Machine
    Immutability
    

    View Slide

47. @miciek
    Shaping our thinking habits
    It is not only
    the violin that
    shapes the
    violinist, we are
    all shaped by the
    tools we train
    ourselves to
    use…
    “
    ”
    https://www.cs.utexas.edu/users/EWD/transcriptions/OtherDocs/Haskell.html
    

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48. @miciek
    Let’s go back to 2023
    

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49. @miciek
    Java (will) have it, too!
    FP
    Future
    
    
    Java
    

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50. @miciek
    https://www.manning.com/plachta
    
    
    40% off code:
    
    
    watchplachta40
    www.michalplachta.com
    

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51. @miciek
    Michał Płachta
    www.michalplachta.com
    IMMUTABILITY
    
    
    AGAINST
    
    
    THE MACHINE
    code, links,
    slides,
    book info
    

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