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From A to L: Designing Scala Libraries

From A to L: Designing Scala Libraries

This is a talk I gave for the Dutch Scala Enthusiasts meetup on April 25th 2013. It describes some intermediate-level Scala tips and tricks that I learned while developing an open source Scala library.

Age Mooij

April 26, 2013

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  1. From A to L Designing Scala Libraries Scala features, patterns,

    idioms, tips and tricks I picked up while reading other people’s library source code and writing my own SCALAPENOS
  2. Where do I get these skillz? Writing lots of application

    code Making lots of mistakes Reading LOTS of Scala library source code Mostly: Spray source code
  3. Why? Most of the existing Scala Riak client libraries were

    dead projects None of them were compatible with Scala 2.10 or Akka 2.1.x None of them were non-blocking The official Java client library has a very (ugly) Java-ish API The official Java client library is blocking It was the Christmas holidays and I needed a new hobby project
  4. Design Goals: It should be completely non-blocking It should integrate

    well with Akka It should be simple and easy to use It should be easy to extend The API should be idiomatic Scala
  5. Using types external to your library will force you and

    your users to import those types whenever they need to be instantiated... The Problem:
  6. Wikipedia Definition: "... a type system construct that supports ad-hoc

    polymorphism. This is achieved by adding constraints to type variables in parametrically polymorphic types" Ehm...?
  7. Other Definition: "A type of Adapter that uses Scala’s implicits

    to add some extra capabilities to an existing type without direct coupling" Ehm...?
  8. The Problem: It would be nice if there were an

    easy way to constrain T to something that can be serialized and indexed without forcing users to extend their domain classes from my traits How do I turn a T into a RiakValue?
  9. The Solution: Add a Serializer[T] implicit parameter Making the parameter

    implicit is the big trick that turns RiakSerializer into a type class
  10. Context Bounds: Can also be written using a Context Bound:

    And if you do need a reference to the unnamed implicit parameter, you can get one using implicitly(...)
  11. A Nicer Solution: Use implicitly(...) to get a reference to

    any unnamed implicit value or parameter
  12. Some Type Classes They are regular traits. It’s how you

    use them that makes them type classes.
  13. “But how do I provide default implementations of these type

    classes without preventing custom implementations?” - Me, two months ago
  14. The Problem: error: ambiguous implicit values: both ... and ...

    match expected ... When the Scala compiler tells you: When two implicits get resolved at the same level the compiler cannot choose between them
  15. Implicit Resolution Rules: Implicits Defined in Current Scope Explicit Imports

    Wildcard Imports Same Scope in Other Files Companion Objects of a Type Implicit scope of an argument’s type Implicit scope of type arguments Outer Objects for Nested Types Other Dimensions Don’t worry, it’s easier than it sounds!
  16. When [T] doesn’t make sense: When information about a type

    is only useful on the inside, don’t bother your users with a type parameter and use abstract types instead