Implicits inspected and explained

Talk by Tim Soethout. The average age among attendees seem to be higher than other talks, maybe we elders are looking for understanding finally what the hell are implicits (then we would need something similar for monads…). Here we go.
Implicit enables you to use values without explicit reference. Implicit enables the relationship “is viewable as”.
As a parameter the value is taken from the calling context (eg akka sender).
Incomprehensible examples follow.
Implicits are used for DSL, type evidence (whatever this be), reduce verbosity… Other stuff I didn’t make to copy.
Caveats – resolution can be difficult to understand. Automatic conversions. For these reasons better to not overuse.
Implicit parameter just takes the only variable with a matching type as argument.
If two or more variables with a matching types are available an error is reported by the compiler.
Implicit conversion (please avoid them) allows to automatically convert from a type A to a type B every time B is required. This comes handy to convert from and to Java collection types, but it is deprecated. The new approach is to add the asScala method to the Java types.
Implicit view is not understood by yours truly, sorry.
Implicit class. This is used when a class is missing a method. You can write a wrapper to add what you need. If this is declared implicit then when the compiler finds an invocation to the new method the wrapper class is constructed and the method is invoked.
Scoping. Some explanation I didn’t understand. Interactive moment turned quite to an epic failure with most of the attendees having no idea on what was going to happen in the example shown.
Type classes. This concept is used for ad-hoc polymorphism and to extend libraries. A naive approach to serialize to json uses subclasses. All the classes need to inherit from a JsonSerializable and to define the serialization abstract method.
With type class the class to serialize do not need to derive from the same base. A generic trait is defined for the serialization method. By defining implicit variables and assigning them the class which defines the serialization for the specific type then type serializers will be uses when needed.
Other examples just make things worse for me, I though I got it, but now I’m no longer sure (also, now, rereading what I wrote, I’m unsure I understood anything at all).
Wrap up time. I still don’t get this stuff, at least not completely and only slightly more than “danger – do not use”. Type class seems to be really powerful in the sense that they lift Java from its pond to the static polymorphism of C++. But the mechanics that power this specific kind of implicit are still obscure to me. Likely nothing that a good book and some months of hard study couldn’t solve. Just it seems not to be some knowledge you can transfer in a conference.

1 thought on “Implicits inspected and explained

  1. Rather than “danger – do not use” it should be “danger – do not abuse”… As any powerful tool, implicits should be handle with care. But they can be useful in many cases.

    You don’t need to understand all the use cases where implicits are useful, before using them. For example, you can skip all the use cases for library developers (e.g. type classes).

    When I first stumbled upon implicits, I admit I was rather confused. But then I realised they could be useful to extend classes behaviour (“implicit class” use case). In cases when you can’t derive a class, with an implicit you can “add” methods to it in an elegant way.
    This is a simple use case that comes in handy in many cases and let you start appreciating implicits.

    Then, with more experience, you can learn how to exploit other use cases and so on. In short, do not be frightened by a presentation that overwhelms you with a lot of information, but start with small steps with your real problems to solve.

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