Joël shares his experience with the dry-rb suite of gems, focusing on how he's been using contracts to validate input data. Stephanie relates to Joël's insights with her preparation for RailsConf, discussing her methods for presenting code in slides and weighing the aesthetics and functionality of different tools like VS Code and Carbon.sh. She also encounters a CI test failure that prompts her to consider the implications of enforcing specific coding standards through CI processes.

The conversation turns into a discussion on managing coding standards and tools effectively, ensuring that automated systems help rather than hinder development. Joël and Stephanie ponder the balance between enforcing strict coding standards through CI and allowing developers the flexibility to bypass specific rules when necessary, ensuring tools provide valuable feedback without becoming obstructions.

• dry-rb


• A broader take on parsing


• Parse; don’t validate


• Debugging at the boundaries


• Specialized vocabulary


• Carbon


• RailsConf 2024


• Moving errors to the left


• Contracts


• Linters shouldn’t be optional


• Linter rules to avoid focused tests


• Thoughtbot Rails guides


• Danger

Transcript:

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STEPHANIE: Hello and welcome to another episode of the Bike Shed, a weekly podcast from your friends at thoughtbot about developing great software. I'm Stephanie Minn.

JOËL: And I'm Joël Quenneville. And together, we're here to share a bit of what we've learned along the way.

STEPHANIE: So, Joël, what's new in your world?

JOËL: I've been working on a project that uses the dry-rb suite of gems. And one of the things we're doing there is we're validating inputs using this concept of a contract. So, you sort of describe the shape and requirements of this, like hash of attributes that you get, and it will then tell you whether it's valid or not, along with error messages. We then want to use those to eventually build some other sort of value object type things that we use in the app. And because there's, like, failure points at multiple places that you have to track, it gets a little bit clunky.

And I got to thinking a little bit about, like, forget about the internal machinery. What is it that I would actually like to happen here? And really, what I want is to say, I've got this, like, bunch of attributes, which may or may not be correct. I want to pass them into a method, and then either get back a value object that I was hoping to construct or some kind of error.

STEPHANIE: That sounds reasonable to me.

JOËL: And then, thinking about it just a little bit longer, I was like, wait a minute, this idea of, like, unstructured input goes into a method, you get back something more structured or an error, that's kind of the broad definition of parsing. I think what I'm looking for is a parser object. And this really fits well with a style of processing popularized in the functional programming community called parse, don't validate the idea that you use a parser like this to sort of transform data from more loose to more strict values, values where you can have more assumptions.

And so, I create an object, and I can take a contract. I can take a class and say, "Attempt to take the following attributes. If they're valid according to the construct, create this classroom." And it, you know, does a bunch of error handling and some...under the hood, dry-rb does all this monad stuff. So, I handled that all inside of the object, but it's actually really nice.

STEPHANIE: Cool. Yeah, I had a feeling that was where you were going to go. A while back, we had talked about really impactful articles that we had read over the course of the year, and you had shared one called Parse, Don't Validate. And that heuristic has actually been stuck in my head a little bit. And that was really cool that you found an opportunity to use it in, you know, previously trying to make something work that, like, you weren't really sure kind of how you wanted to implement that.

JOËL: I think I had a bit of a light bulb moment as I was trying to figure this out because, in my mind, there are sort of two broad approaches. There's the parse, don't validate where you have some inputs, and then you transform them into something stricter. Or there's more of that validation approach where you have inputs, you verify that they're correct, and then you pass them on to someone else. And you just say, "Trust me, I verified they're in the right shape." Dry-rb sort of contracts feel like they fit more under that validation approach rather than the parse, don't validate.

Where I think the kind of the light bulb turned on for me is the idea that if you pair a validation step and an object construction step, you've effectively approximated the idea of parse, don't validate. So, if I create a parser object that says, in sort of one step, I'm going to validate some inputs and then immediately use them if they're valid to construct an object, then I've kind of done a parse don't validate, even though the individual building blocks don't follow that pattern.

STEPHANIE: More like a parse and validate, if you will [laughs]. I have a question for you. Like, do you own those inputs kind of in your domain?

JOËL: In this particular case, sort of. They're coming from a form, so yes. But it's user input, so never trust that.

STEPHANIE: Gotcha.

JOËL: I think you can take this idea and go a little bit broader as well. It doesn't have to be, like, the dry-rb-related stuff. You could do, for example, a JSON schema, right? You're dealing with the input from a third-party API, and you say, "Okay, well, I'm going to have a sort of validation JSON schema." It will just tell you, "Is this data valid or not?" and give you some errors.

But what if you paired that with construction and you could create a little parser object, if you wanted to, that says, "Hey, I've got a payload coming in from a third-party API, validate it against this JSON schema, and attempt to construct this shopping cart object, and give me an error otherwise." And now you've sort of created a nice, little parse, don't validate pipeline which I find a really nice way to deal with data like that.

STEPHANIE: From a user perspective, I'm curious: Does this also improve the user experience? I'm kind of wondering about that. It seems like it could. But have you explored that?

JOËL: This is more about the developer experience.

STEPHANIE: Got it.

JOËL: The user experience, I think, would be either identical or, you know, you can play around with things to display better errors. But this is more about the ergonomics on the development side of things. It was a little bit clunky to sort of assemble all the parts together. And sometimes we didn't immediately do both steps together at the same time. So, you might sort of have parameters that we're like, oh, these are totally good, we promise. And we pass them on to someone else, who passes them on to someone else. And then, they might try to do something with them and hope that they've got the data in the right shape.

And so, saying, let's co-locate these two things. Let's say the validation of the inputs and then the creation of some richer object happen immediately one after another. We're always going to bundle them together. And then, in this particular case, because we're using dry-rb, there's all this monad stuff that has to happen. That was a little bit clunky. We've sort of hidden that in one object, and then nobody else ever has to deal with that.

So, it's easier for developers in terms of just, if you want to turn inputs into objects, now you're just passing them into one object, into one, like, parser, and it works. But it's a nicer developer experience, but also there's a little bit more safety in that because now you're sort of always working with these richer objects that have been validated.

STEPHANIE: Yeah, that makes sense. It sounds very cohesive because you've determined that these are two things that should always happen together. The problems arise when they start to actually get separated, and you don't have what you need in terms of using your interfaces. And that's very nice that you were able to bundle that in