Parse, don’t validate

TL;DR:

• parsers take less-structured data and try to turn it into more-structured data. They don’t always succeed, so all parsers need to handle failure.
• validating throws away information as exceptions/errors, while parsing holds onto that information
• parsing inputs as early as possible helps you trust the validity of data once it reaches your business logic.
• Beware of “shotgun parsing”: validation checks scattered throughout a codebase. “Shotgun parsing necessarily deprives the program of the ability to reject invalid input instead of processing it.”

Or, more succinctly,