Lesson 27 of 55
10 mins readDefining Custom Typeclasses & Instances
In Plain English
Create your own typeclasses using the class keyword and implement them for concrete types using instance declarations.
Deep Dive: How It Works
class Declaration: class Describable a where describe :: a -> String.
instance Implementation: instance Describable Bool where describe True = "Yes"; describe False = "No".
Default Implementations: Typeclasses can provide default method implementations that instances can optionally override.
Core Rules to Remember
class Keyword: Defines a new typeclass interface and required method signatures.
instance Keyword: Provides the concrete implementation of the typeclass for a specific type.
Live Interactive Example
Hit Run Code to see it liveCustom Describable Typeclass
Python 3.12
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Output Console
Click "Run Code" to view the rendered output.
How it works: describe dynamically resolves to the appropriate instance implementation.
Your Turn: Micro Challenge
No pressure! Edit the starter code below and test your solution with instant feedback.
Micro Exercise
Implement Sizeable Instance
- Define `class Sizeable a where size :: a -> Int`.
- Implement `instance Sizeable Int where size _ = 1`.
- Print `"Size: "` followed by `show (size (99 :: Int))`.
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Sandbox Output
Click "Run & Check" to test your solution.
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