GoKawiilPublished April 17, 2026
Alexis King’s Parse, don’t validate had a huge impact on how I write code, particularly my stance toward Python type annotations in production. However, as someone who has written practically zero Haskell, the idea didn’t click for me until I started seeing other examples, like this one in Rust. This post explores how we can take this paradigm and apply it to a simple date-parsing problem in C++98, C++11, C++17, and finally C++23.
If you haven’t read the original essay, it is a great explainer for a genuinely tricky topic. Here’s my attempt at summarizing the key idea:
Use your language’s type system to parse unstructured inputs. A successful instantiation of that type should mean the data is valid, and you eliminate the need for validation control flow down the line.
I’ve revised this summary a few times and I still don’t love it. Likely a mix of skill issue and the fact that the idea is very hard to stuff into the small box of two sentences without relying on more jargon, which I am still using.
Timestamp and date parsing are notoriously riddled with edge cases. That being said, much like validating email addresses, it’s a perfect example of a problem where a programmer has to make a contextually optimal tradeoff between correctness and readability.
In this case, let’s imagine we’re building something that consumes unstructured data from a file, and we need to extract personally identifiable information. For the purposes of this post, I’ll be ingesting a raw string, raw_input , that needs to be used by the rest of the code.
Starting point
Let’s start with a dead simple implementation that is mostly apathetic to the idea of type-driven design.
See it in Compiler Explorer
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