The document discusses making bad data impossible by creating custom data types that implement business rule constraints. It suggests that custom types could ensure data is always correct, without needing to worry about edits or meeting database constraints. The custom types could have meaningful names and support features like extension methods to improve usability.

1. Make Bad Data Unrepresentable
Jack Fox
@foxyjackfox
Engineer at
Visual Studio and Development Technologies MVP

2. Ideally…
Front
End Business Logic
Incoming data
properly edited
Database
imposes
constraints

3. Perspective of the Applications Programmer
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4. Static Types
• String
• Int
• Float
• Datetime

5. What if you could easily create custom types
implementing the required business rule constraints?

6. What if you could easily create custom types
implementing the required business rule constraints?
…and give the type a meaningful name.

7. What if you could easily create custom types
implementing the required business rule constraints?
…and give the type a meaningful name.
• Assurance the data is always correct

8. What if you could easily create custom types
implementing the required business rule constraints?
…and give the type a meaningful name.
• Assurance the data is always correct
• No worries about whether edits were performed

9. What if you could easily create custom types
implementing the required business rule constraints?
…and give the type a meaningful name.
• Assurance the data is always correct
• No worries about whether edits were performed
• No worries about meeting DB constraints (if they exist)

10. Dependent Types
https://jackfoxy.github.io/DependentTypes

11. Dependent Types – Future Directions
https://jackfoxy.github.io/DependentTypes
• Support extension methods
• C# / VB.NET support
• Literal type parameters
• Add to F# core library