Parse, don't validate (2019)

   parseNonEmpty [] = throwIO $ userError "list cannot be empty"

How would that interact with a scenario where we want a specific error message if a specific list is empty? Eg, "you want to build the list using listBuilder()". Making illegal states unrepresentable is good advice but I don't think that escapes the value of good validation.

It is a mistake to do ad-hoc validation. But it makes a lot of sense to have a validation phase, a parse phase then an execution phase when dealing with untrusted data. The validation phase gives context-aware feedback, the parse phase catches what is left and then execution happens.

A type system doesn't seem like a good defence against end user error. The error messages in practice are mystic. I think the complaint here is if people are trying to implement a type system using ad-hoc validation which is a bad idea.

When you’re building a ‘parser’ (in this broad, type-narrowing sense) to handle user-supplied data, the result type really needs to be a rich mix of

- successfully parsed data objects

- error objects

- warning objects

That way your consumers can themselves decide what to do in the face of errors and warnings.

(Of course one ugly old fashioned way to add optional ‘error’ types to your return signature is checked exceptions, but we don’t talk about that model any more.)

Certainly I don’t think `parseNonEmpty` would be especially useful in a real program, it’s only there as an example to provide a particularly simple contrasting example against `validateNonEmpty`. The example earlier in the blog post using the `nonEmpty` function (which returns an optional result) is a more realistic example of how such things are actually used in practice, since that allows you to raise a domain-appropriate error message.

Tangentially, in Haskell specifically, I have actually written a library specifically designed for checking the structure of input data and raising useful error messages, which is somewhat ironically named `monad-validate` (https://hackage.haskell.org/package/monad-validate). But it has that name because similar types have historically been named `Validation` within the Haskell community; using the library properly involves doing “parsing” in the way this blog post advocates.