Parse, Don't Validate

Parse, Don't Validate, written by Alexis King, talks about the distinction between parsing and validation. These two approaches are a means to reject invalid data in two distinct ways. Validators are pretty simple, you validate the data after you have constructed it. This is not necessarily wrong, but it can be. Parsers, on the other hand, take full advantage of type systems to ensure that they cannot be wrong. They do this by designing a type which can only be instantiated through a function (or functions) that validates the data. This might not seem like much, but having distinct types for validated and non-validated data facilitates development by encoding more information about our data into the type-system. From the article:

[...] parsers are an incredibly powerful tool: they allow discharging checks on input up-front, right on the boundary between a program and the outside world, and once those checks have been performed, they never need to be checked again!