use-custom-types-in-assertions.md

Use Custom Types in Assertions

To use boon with your own custom types, you need three functions:

1. (T, T) => Boolean - defines how two values of a type T are equated. This is modelled by the Equality typeclass. It is similar to the Cats Eq typeclass.
2. T => String - defines how an instance of type T is displayed. This is modelled by the StringRep typeclass. It is similar to the Cats Show typeclass
3. (T, T, EqualityType) => NonEmptySeq[String] - defines how the differences between two instances of type T are displayed on failure. This is modelled by the Difference trait. EqualityType specifies how the values differ from each other.

These three functions are bundled into the BoonType typeclass.

For instance, given a Person class:

final case class Name(value: String)
final case class Age(value: Int)
final case class Person(name: Name, age: Age)

you could use one of the helper functions on BoonType to generate a default instance:

implicit val personBoonType = BoonType.defaults[Person]

What BoonType.defaults does is to use scala's == for equality, .toString for display Strings and default difference of t1 != t2.

After defining the above you can make Assertions on Person instances:

val p1 = Person(Name("Royd Eris"), Age(30))
val p2 = Person(Name("Royd Eris"), Age(30))
val p3 = Person(Name("Melantha Jhirl"), Age(26))

p1 =?= p2 | "Person instances with same data are equal" and
p2 =/= p3 | "Person instances with different data are not equal" and

If we change p1 =?= p3 we get:

[info]    - Person instances with same data are equal [✗]
[info]      => Person(Name(Royd Eris),Age(30)) != Person(Name(Melantha Jhirl),Age(26))

If you don't want to use default instances, you can use one of the many methods on BoonType to create BoonType instances.