Configuring generic derivation

This post is a kind of sequel to my previous article on type classes and generic derivation, although unfortunately there's a lot of intermediate content that should go between there and here that I haven't written yet. This post introduces a new feature in circe that I'm pretty excited about, though, so I'm not going to worry about skipping over that stuff for now.

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Type classes and generic derivation

Yesterday I wrote a Stack Overflow answer about using Shapeless for generic derivation of type class instances, and this morning I started putting together some new documentation for circe's generic derivation, and after a few paragraphs I decided that it might make sense to write a blog post that could serve as a bridge between the two—between simple examples like the one in my answer (which doesn't really go into motivation, etc.) and the kinds of things we're doing in circe. I'll start more or less from scratch, assuming only some basic familiarity with Scala syntax.

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Deriving incomplete type class instances

Suppose we've got a simple representation of a user:

case class User(id: Long, name: String, email: String)

Now suppose we're writing a web service where we allow clients to post some JSON to a resource to create a new user. We get to pick the id, not the client, so we might accept something like this:

  "name": "Foo McBar",
  "email": "foo@mcbar.com"

If we're using a type class-based JSON library like Argonaut, we'll probably have written a codec instance for User (or we may be using a library like argonaut-shapeless that derives instances for our case classes automatically).

The problem is that our User codec won't work on JSON like the example above (since it's missing the id field).

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Partitioning by constructor

It's not unusual in Scala to want to take a collection with items of some algebraic data type and partition its elements by their constructors. In this Stack Overflow question, for example, we're given a type for fruits:

sealed trait Fruit

case class Apple(id: Int, sweetness: Int) extends Fruit
case class Pear(id: Int, color: String) extends Fruit

The goal is to be able to take a collection of fruits and split it into two collections—one of apples and one of pairs.

def partitionFruits(fruits: List[Fruit]): (List[Apple], List[Pear]) = ???

It's pretty easy to use collect to solve this problem for particular cases. It's a little trickier when we start thinking about what a more generic version of such a method would look like—we want to take a collection of items of some arbitrary algebraic data type and return a n-tuple whose elements are collections of items of each of that ADT's constructors (and let's require them to be typed as specifically as possible, since this is Scala). It's not too hard to imagine how you could write a macro that would perform this operation, but it'd be messy and would probably end up feeling kind of ad-hoc (at least without a lot of additional work and thinking).

Fortunately we've got Shapeless 2.0, where Miles Sabin and co. have written the macros for us so we can keep our hands clean.

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Ordering case classes

Scala provides lexicographic Ordering instances for TupleN types when all of the element types have Ordering instances. It doesn't provide the same instances for case classes, however—probably just because lexicographic order isn't what you want for case classes as often as it is for tuples.

Sometimes you actually do want a lexicographic ordering for your case classes, though, and Scala unfortunately doesn't provide any nice boilerplate-free way to create such orderings. This post will provide a quick sketch of two approaches to filling this gap: one using macros, and one using Shapeless 2.0's new Generic machinery and the TypeClass type class.

First for a case class to use as a running example, along with some instances:

case class Foo(x: Int, y: String)

val a = Foo(9, "x")
val b = Foo(1, "z")
val c = Foo(9, "w")

val foos = List(a, b, c)

Let's quickly confirm that there's no Ordering[Foo] already sitting around:

scala> foos.sorted
<console>:14: error: No implicit Ordering defined for Foo.

Yep, we're going to have to take care of this ourselves.

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Learning Shapeless

I began writing this post as an answer to this Stack Overflow question about learning how to use Shapeless, but it ended up a little long for a Stack Overflow answer, so I'm posting it here instead.

When I first started teaching myself type-level programming in the context of Shapeless last year, I spent a lot of time working through simple problems with heterogeneous lists of type-level natural numbers. One fairly straightforward (but still non-trivial) example of such a problem is the second user story of this Coding Dojo kata, which is also outlined by Paul Snively in this email to the Shapeless mailing list.

The goal is to determine whether a list of numbers is the appropriate length (nine) and has a valid checksum, which is calculated by taking the sum of each element multiplied by its distance (plus one) from the end of the list, modulo eleven.

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Applicative validation syntax

People say that Validation is Scalaz's gateway drug, which might be accurate if you ignore the suggestion that there's anything even remotely fun about validation. In my book, making sure that your program doesn't choke on bad input is always a chore.

Applicative validation is at least a step in the right direction—it makes it easier to write less code, introduce fewer bugs, and draw clearer lines between data models and validation logic. Suppose for example that we have the following case class in Scala:

case class Foo(a: Int, b: Char, c: String)

Suppose also that we have a form with three fields that we want to use to create instances of Foo. We receive input from this form as strings, and we want to be sure that these strings have certain properties.

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