Lesson 34 of 55
10 mins readPython Domain Errors & Validation Pipelines
In Plain English
Create custom Algebraic Data Types representing specific error conditions (e.g. InvalidEmail, PasswordTooShort) and compose validation pipelines cleanly.
Deep Dive: How It Works
Custom Error ADT: data AuthError = UserNotFound | InvalidPassword | AccountLocked deriving (Show, Eq).
Validation Pipeline: Chain validation steps using Either so execution stops on the first Left error.
Core Rules to Remember
Structured Domain Errors: Model distinct error types instead of generic error strings.
Early Exit Failure: Short-circuit immediately upon encountering a validation error.
Live Interactive Example
Hit Run Code to see it liveUser Input Validation Pipeline
Python 3.12
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Output Console
Click "Run Code" to view the rendered output.
How it works: validateUsername returns specific structured errors for each constraint failure.
Your Turn: Micro Challenge
No pressure! Edit the starter code below and test your solution with instant feedback.
Micro Exercise
Validate Positive Balance
- Define `validateBalance b = if b >= 0 then Right b else Left "Negative balance"`.
- Print `"Balance check: "` followed by `show (validateBalance 250.0)`.
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Sandbox Output
Click "Run & Check" to test your solution.
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