API Documentation: Show

Show is an alternative to the Java toString method. It is defined by a single function show:

def show(a: A): String

You might be wondering why you would want to use this, considering toString already serves the same purpose and case classes already provide sensible implementations for toString. The difference is that toString is defined on Any(Java's Object) and can therefore be called on anything, not just case classes. Most often, this is unwanted behaviour, as the standard implementation of toString on non case classes is mostly gibberish. Consider the following example:

(new {}).toString
// res0: String = "repl.MdocSession$MdocApp$$anon$1@5fbef2d7"

The fact that this code compiles is a design flaw of the Java API. We want to make things like this impossible, by offering the toString equivalent as a type class, instead of the root of the class hierarchy. In short, Show allows us to only have String-conversions defined for the data types we actually want.

To make things easier, Cats defines a few helper functions to make creating Show instances easier.

/** creates an instance of Show using the provided function */
def show[A](f: A => String): Show[A]

/** creates an instance of Show using object toString */
def fromToString[A]: Show[A]

These can be used like this:

import cats.Show

case class Person(name: String, age: Int)

implicit val showPerson: Show[Person] = =>
// showPerson: Show[Person] = cats.Show$$$Lambda$15446/0x00007f59db9b3690@60abafe0

case class Department(id: Int, name: String)

implicit val showDep: Show[Department] = Show.fromToString
// showDep: Show[Department] = cats.Show$$$Lambda$14496/0x00007f59db66b6a8@1883595c

This still may not seem useful to you, because case classes already automatically implement toString, while show would have to be implemented manually for each case class. Thankfully with the help of a small library called kittens a lot of type class instances including Show can be derived automatically!

Cats also offers Show syntax to make working with it easier. This includes the show method which can be called on anything with a Show instance in scope:

import cats.syntax.all._

val john = Person("John", 31)
// john: Person = Person(name = "John", age = 31)
// res1: String = "John"

It also includes a String interpolator, which works just like the standard s"..." interpolator, but uses Show instead of toString:

val engineering = Department(2, "Engineering")
// engineering: Department = Department(id = 2, name = "Engineering")
show"$john works at $engineering"
// res2: String = "John works at Department(2,Engineering)"