[lock1]

Coming from a more "average imperative" background like C and Java, outside of compiler or serde context, I don't think "parse" is a frequently used term there. The idea of "checking values to see whether they fulfill our expectations or not" is often called "validating" there.

So I believe the "Parse, Don't Validate" catchphrase means nothing, if not confusing, to most developers. "Does it mean this 'parse' operation doesn't 'validate' their input? How do you even perform 'validation' then?" is one of several questions that popped up in my head the first time I read the catchphrase prior to Haskell exposure.

Something like "Utilize your type system" probably makes much more sense for them. Then just show the difference between `ValidatedType validate(RawType)` vs `void RawType::validate() throws ParseError`.

[tialaramex]

The crucial design choice is that you can't get a Doodad by just saying oh, I'm sure this is a Doodad, I will validate later. You have to parse the thing you've got to get a Doodad if that's what you meant, and the parsing can fail because maybe it isn't one.

    let almost_pi: Rational = "22/7".parse().unwrap();

Here the example is my realistic::Rational. The actual Pi isn't a Rational number so we can't represent it, but 22 divided by 7 is a pretty good approximation considering.

I agree that many languages don't provide a nice API for this, but what I don't see (and maybe you have examples) is languages which do provide a nice API but call it validate. To me that naming would make no sense, but if you've got examples I'll look at them.

[whalesalad]

The point is parse and validate are interchangeable words for the most part. If you’re parsing something you expect to be an int, but it’s a float or the letter “a” is that not invalid? Is this assessment a form of validating expectations? The line between parsing and validating doesn’t exist.