Advanced · Module 9 · Lesson 5 of 7

Project: A Generic Result Type for the Library

Build a constrained Result for catalogue lookups that carries either a value or an error, composes without exceptions, and reports misuse at compile time.

ProAbout 12 minutes

By the end you can

Build a constrained Result class template holding value or error
Apply map-style composition to chain fallible catalogue queries
Measure clarity and cost versus exceptions and error codes

Builds on Concepts and Constraints: Requirements That Read Like English.

01 · A question first

Catalogue lookup can succeed with a record, fail with a missing id, or fail with a corrupt entry. Exceptions hide paths, integer codes lose context, and each query repeats the same branch logic. How does one generic type carry either outcome, chain the next query safely, and prove the cost?

Make a guess before you read on.

02 · The idea

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What's inside

Start with the problem
How the idea works
A second runnable example
Why it is correct
What it costs
Pitfalls and variations
Complete solution
Where it is used