Scala With Cats

Question 1

A Type Class is…

Answer 1

Interface or API that represents some functionality we want to implement.

Represented by a Trait with at least One Type Parameter.

Question 2

Type Class Instances

Answer 2

Provide implementations for the Types

Question 3

Type Class Interface

Answer 3

Functionality exposed to users

Generic methods that accept instances of the Type Class as implicit parameters

• Interface Objects: place methods in a singleton object
• Interface Syntax: use extension methods to extend
  exiting Types with interface methods

Question 4

The ‘implicitly’ Method

Answer 4

Scala generic Type Class Interface

Summon any value from Implicit Scope

Note:
- Good fallback for debugging purposes, the compiler will ensure it can find an instance of the type and that there are not ambiguous

• Type Classes in Scala means working with Implicit Values and Implicit Parameters
• Implicit in Scala must be placed inside an Object or Trait

Question 5

Type Class Definitions are only included in Implicit Scope if …

Answer 5

They are tagged with the ‘implicit’ keyword

Note:

• the compiler searches for candidate Type Class Instances by Type in the Implicit Scope at the call site
  
  • in local or inherited Definitions
  • in imported Definitions
  • in the Companion Object of the Type Class or the Parameter Type
• if the compiler sees multiple candidate Definitions, it fails with an ambiguous implicit values error
• The power of Type Classes and Implicit lies in the compiler’s ability to Combine Implicit Definitions when searching for candidate Instances
• We can define Instances in two ways
  
  • By defining concrete Instances as Implicit Vals of the required Type
  • By defining Implicit Methods to construct Instances from other Type Class instances
    example: JsonWriter[A] and JsonWriter[Option[A]]
  • Be sure to mark the parameters to the method as implicit parameters.

Question 6

Implicit methods with non-implicit parameters form a different Scala pattern called an …

Answer 6

Implicit Conversion (frowned upon)

Question 7

Cats is written using a …

Answer 7

Modular Structure that allows us to choose which Type Classes, Instances, and Interface Methods we want to use.

Note:

• import cats._ imports all of Cats’ Type Classes
• import cats.instances.all._ imports all of the Type Class Instances for the Standard Pibrary
• import cats.syntax.all._ imports all of the Syntax
• import cats.implicits._ imports all of the Standard Type Class Instances and all of the Syntax

Question 8

When working with Type Classes we must consider two issues that control instance selection

Answer 8

The relationship between an Instance defined on a Type and its Subtypes

How do we choose between Type Class Instances when there are many available