Parse, don’t type-check

There’s a fantastic article from last year titled Parse, don’t validate. I’d highly recommend it to any programmer (along with the more recent follow up Names are not type safety). The basic idea is that there are two ways to check that some input to a function is valid:

  • A validator checks that the input is valid and throws an error if not. It doesn’t return anything. For example, checking that a list is not empty.
  • A parser does the same as a validator, but returns a more specific representation of the input that ensures that the required property is satisfied. For example, checking that a list is not empty and returning a NonEmptyList type.

The thesis of the article is that parsers are preferable to validators. If you’ve not read the original article, please do so – it’s very well written and makes the case much better than I can summarise it. The essential message is to make illegal states unrepresentable. In the article, this is done by making use of the type system. This is a philosophy I entirely agree with, but I want to point out and expand upon one ironic aspect of the argument:

A type checker is a paradigmatic example of a validator!

Continue reading “Parse, don’t type-check”