{"id":572,"date":"2015-08-07T07:44:11","date_gmt":"2015-08-07T07:44:11","guid":{"rendered":"http:\/\/blog.stratio.com\/?p=572"},"modified":"2023-09-20T13:37:59","modified_gmt":"2023-09-20T13:37:59","slug":"variance-in-scala-luke-he-is-your-father-too","status":"publish","type":"post","link":"https:\/\/www.stratio.com\/blog\/variance-in-scala-luke-he-is-your-father-too\/","title":{"rendered":"Variance in Scala (\u201cLuke, he is your father too\u201d)"},"content":{"rendered":"

When working with Big Data<\/strong>, sometimes it\u2019s useful to remember that powerful products wouldn’t work properly without the tools that build them. It\u2019s possible to start programming in Scala<\/strong> with a few case classes and a bunch of for-comprehensions, but those are only little scratches in a huge ice surface like Scala is. It may not be enough to make your code clean and comprehensible.  I\u2019ve been developing with this programming language for almost 4 years, and every day I discover a new feature that surprises me. That acknowledgement, in the end, is the main reason to keep digging deeper into Scala.<\/p>\n

For instance, working with generics can be sometimes really messy, especially when we want to play a little bit with inheritance<\/strong> (which turns out to be very common).<\/p>\n

It’s very usual to clash with variance<\/strong> issues in that case, but not as usual as getting a clear idea about what it really implies.<\/p>\n

Just to set the tone, we will propose a simple example.<\/p>\n

Invariance <\/strong><\/h2>\n

Let’s suppose we’re defining the behavior of some servers\u2019 connection handler…<\/p>\n

case class ConnectionHandler[C](connections: List[C])<\/pre>\n
…and connection’s definition (we could have two types of connections: user and system connections):<\/p>\n
trait Connection\n\nclass UserConnection extends Connection\n\nclass SystemConnection extends Connection<\/pre>\n
If we want to list all connections given a connection handler, we probably need a logger to print them all and a function for using this logger for each connection:<\/p>\n
case class FunctionalLogger(traces: Seq[String]= Seq()) {\n\n  def print(trace: String): FunctionalLogger =\n    this.copy(traces :+ trace)\n\n}<\/pre>\n
Why this freak logger? It\u2019s quite common to invoke \u2018println\u2019 and similar methods in Java, and it can be done as well in Scala, but functional purists don\u2019t support this idea.<\/p>\n
Let\u2019s focus for a moment on the main ideas of functional programming:<\/p>\n
    \n
  • a function may have one (or more) input parameters and an only parameter type<\/li>\n
  • same input will always generate the same output.<\/li>\n
  • instances shall not be modified: state mutation is provided by a new instance of same type.<\/li>\n<\/ul>\n
    Having this in mind, what we have provided here is a pretty functional logger that doesn’t perform IO operations that would break the pure nature of functions.<\/p>\n
    If we continue implementing our method…<\/p>\n
    def listConnections(\n  handler: ConnectionHandler[Connection])(\n  logger: FunctionalLogger): FunctionalLogger = {\n\n  (logger \/: handler.connections)(\n    (logger,connection) => logger.print(connection.toString))\n\n}<\/pre>\n
    …what we have is a method that performs a foldLeft<\/code> over the logger, printing on it all connections that belong to given handler.<\/p>\n
    Easy peasy, but what happens if we try to list the connections of the following connection handler?<\/p>\n
    val handler: ConnectionHandler[UserConnection] = \n  ConnectionHandler(List(new UserConnection))\n\nlistConnections(handler)(FunctionalLogger())<\/pre>\n
    The compiler will complain with the following \u201cwookie-incomprehensible\u201d error:<\/p>\n
    <console>:19: error: type mismatch;\n\nfound   : ConnectionHandler[UserConnection]\n\nrequired: ConnectionHandler[Connection]\n\nNote: UserConnection <: Connection, but class ConnectionHandler is invariant in type C.\n\nYou may wish to define C as +C instead. (SLS 4.5)<\/pre>\n
    If we translate this into human, it means that UserConnection<\/code> is a subtype of Connection<\/code>, but we cannot make this condition extensive to connection handler (ConnectionHandler[UserConnection]<:ConnectionHandler[Connection]<\/code>). How do we fix this?<\/p>\n
    Covariance<\/h2>\n
    In order to make this inheritance relation flexible, we have to slightly modify the signature of ConnectionHandler<\/code>:<\/p>\n
    case class ConnectionHandler[+C](connections: List[C])<\/pre>\n
    Note the plus sign: it means that any ConnectionHandler[T]<\/code> will be a subtype of ConnectionHandler[C]<\/code> if T<\/code> is a subtype of C<\/code>.<\/p>\n
    Now ConnectionHandler<\/code> is covariant in C<\/code> and the code that wasn\u2019t working before should work now. We can prove it by the following simple line<\/p>\n
    implicitly[<:<[ConnectionHandler[UserConnection],ConnectionHandler[Connection]]]<\/pre>\n
    which tries to find an implicit evidence of first type is a subtype of the second.<\/p>\n
    If we tried this with unrelated types like Int<\/code> and String<\/code>\u2026.<\/p>\n
    implicitly[<:<[ConnectionHandler[String],ConnectionHandler[Int]]]<\/pre>\n
    we would get a compilation error like:<\/p>\n
    Error: Cannot prove thatConnectionHandler[String] <:<ConnectionHandler[Int].\n\nimplicitly[<:<[ConnectionHandler[String],ConnectionHandler[Int]]]\n\n^<\/pre>\n
    Contravariance<\/h2>\n
    On the other hand, we could define a contravariant class C[_]<\/code> in its parameter type using the minus sign, just in front of parameter type.<\/p>\n
    This means, given two types T<\/code> and V<\/code>, if T<\/code> is a supertype of V<\/code>, then C[T]<\/code> will be subtype of C[V]<\/code>.<\/p>\n
    Confusing? The real question here is, who in his right mind would need this? Maybe the following example is self-explanatory.<\/p>\n
    Without going too far…Scala\u2019s functions are defined like:<\/p>\n
    trait Function1[-Parameter,+Result] extends AnyRef<\/pre>\n
    Not so inspiring so far, but think about the following class hierarchy and the following higher-order function:<\/p>\n
    trait Vehicle { def wheels: Int }\n\nclass Car extends Vehicle { val wheels = 4 }\n\nclass Truck extends Car { override val wheels = 6 }\n\nval myFunction: Car => Any = ???<\/pre>\n
    Remember that myFunction<\/code> signature is syntactic sugar for<\/p>\n
    val myFunction: Function1[Car,Any] = ???<\/pre>\n
    So if you had a Truck<\/code>, that is a subtype of Car<\/code>, you couldn’t define your function as follows:<\/p>\n
    val myFunction: Function1[Car,Any] = \n  (t: Truck) => println(t.wheels)<\/pre>\n
    because it is contravariant in Parameter<\/code>, Function1<\/code> expects a supertype of Car<\/code>, that’s why you could define your function using Vehicle<\/code> instead:<\/p>\n
    val myFunction: Function1[Car,Any] = (t: Vehicle) => println(t.wheels)<\/pre>\n
    And also don’t forget that covariance in type Result<\/code> is being satisfied due to Unit<\/code> (the result type of println<\/code>) is a subtype of Any<\/code>. A little bit tricky in the end, but essentially, that was the main idea \ud83d\ude42<\/p>\n
    \u201cMay the functional programming be with you<\/i>\u201d…<\/strong><\/p>\n","protected":false},"excerpt":{"rendered":"
    When working with\u00a0Big Data, sometimes it\u2019s useful to remember that powerful products wouldn’t work properly without the tools that build them. <\/p>\n","protected":false},"author":1,"featured_media":632,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[686],"tags":[],"ppma_author":[795],"yoast_head":"\nVariance in Scala (\u201cLuke, he is your father too\u201d) - Stratio Blog<\/title>\n<meta name=\"description\" content=\"When programming in Scala it's usual to clash with variance issues, but it's not always easy to know what it implies: invariance, covariance, contravariance\" \/>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.stratio.com\/blog\/variance-in-scala-luke-he-is-your-father-too\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Variance in Scala (\u201cLuke, he is your father too\u201d)\" \/>\n<meta property=\"og:description\" content=\"When programming in Scala it's usual to clash with variance issues, but it's not always easy to know what it implies: invariance, covariance, contravariance\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.stratio.com\/blog\/variance-in-scala-luke-he-is-your-father-too\/\" \/>\n<meta property=\"og:site_name\" content=\"Stratio\" \/>\n<meta property=\"article:published_time\" content=\"2015-08-07T07:44:11+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2023-09-20T13:37:59+00:00\" \/>\n<meta property=\"og:image\" content=\"https:\/\/www.stratio.com\/blog\/wp-content\/uploads\/2015\/08\/ScalaVarianceInheritance.png\" \/>\n\t<meta property=\"og:image:width\" content=\"730\" \/>\n\t<meta property=\"og:image:height\" content=\"312\" \/>\n\t<meta property=\"og:image:type\" content=\"image\/png\" \/>\n<meta name=\"author\" content=\"Stratio\" \/>\n<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n<meta name=\"twitter:creator\" content=\"@stratiobd\" \/>\n<meta name=\"twitter:site\" content=\"@stratiobd\" \/>\n<meta name=\"twitter:label1\" content=\"Written by\" \/>\n\t<meta name=\"twitter:data1\" content=\"Stratio\" \/>\n\t<meta name=\"twitter:label2\" content=\"Est. reading time\" \/>\n\t<meta name=\"twitter:data2\" content=\"4 minutes\" \/>\n<script type=\"application\/ld+json\" class=\"yoast-schema-graph\">{\"@context\":\"https:\/\/schema.org\",\"@graph\":[{\"@type\":\"Article\",\"@id\":\"https:\/\/www.stratio.com\/blog\/variance-in-scala-luke-he-is-your-father-too\/#article\",\"isPartOf\":{\"@id\":\"https:\/\/www.stratio.com\/blog\/variance-in-scala-luke-he-is-your-father-too\/\"},\"author\":{\"name\":\"Stratio\",\"@id\":\"https:\/\/www.stratio.com\/blog\/#\/schema\/person\/d0377b199cd052b17e15c9ba44c45ab7\"},\"headline\":\"Variance in Scala (\u201cLuke, he is your father too\u201d)\",\"datePublished\":\"2015-08-07T07:44:11+00:00\",\"dateModified\":\"2023-09-20T13:37:59+00:00\",\"mainEntityOfPage\":{\"@id\":\"https:\/\/www.stratio.com\/blog\/variance-in-scala-luke-he-is-your-father-too\/\"},\"wordCount\":755,\"publisher\":{\"@id\":\"https:\/\/www.stratio.com\/blog\/#organization\"},\"image\":{\"@id\":\"https:\/\/www.stratio.com\/blog\/variance-in-scala-luke-he-is-your-father-too\/#primaryimage\"},\"thumbnailUrl\":\"https:\/\/www.stratio.com\/blog\/wp-content\/uploads\/2015\/08\/ScalaVarianceInheritance.png\",\"articleSection\":[\"Product\"],\"inLanguage\":\"en-US\"},{\"@type\":\"WebPage\",\"@id\":\"https:\/\/www.stratio.com\/blog\/variance-in-scala-luke-he-is-your-father-too\/\",\"url\":\"https:\/\/www.stratio.com\/blog\/variance-in-scala-luke-he-is-your-father-too\/\",\"name\":\"Variance in Scala (\u201cLuke, he is your father too\u201d) - 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