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Stop writing CLI validation. Parse it right the first time

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Re: Stop writing CLI validation. Parse it right the first time

#81

Post by jpc0 »

pmarreck wrote: post #56
ambicapter wrote: post #32

Earlier quoted context omitted.

Most validation libraries worth their salt give you options to deal with this sort of thing? They'll hand you an aggregate error with an 'errors' array, or they'll let you write an error message "prettify-er" to make a particular validation error easier to read.

Right, but that's validation, and this article is talking about parsing (not validating) into an already-correct structure by making invalid inputs unrepresentable. So maybe the reason why they were able to reduce the code is because they lost the ability to do good error reporting.

How is getting an error array not making invalid input unrepresentable.

You either get the correctly parsed data or you get an error array. The incorrect input was never represented in code, vs a 0 value being returned or even worse random gibberish.

A trivial example: 1/0 should return DivisionByZero not 0 or infinity or NaN or whatever else. You can then decide in your UI whether that is a case you want to handle as an error or as an edge case but the parser knows that is not possible to represent.

Re: Stop writing CLI validation. Parse it right the first time

#83

Post by bradrn »

mark38848 wrote: post #76
12_throw_away wrote: post #30

I like this advice, and yeah, I always try to make illegal states unrepresentable, possibly even to a fault. The problem I run into here is - how do you create good error messages when you do this? If the user has passed you input with multiple problems, how do you build a list of everything that's wrong with it if the parser crashes out halfway through?

Just use optparse-applicative in PureScript. Applicatives are great for this and the library gives it to you for free.

> Just use optparse-applicative in PureScript.

Or in Haskell!

Re: Stop writing CLI validation. Parse it right the first time

#84

Post by majorbugger »

bschwindHN wrote: post #82

Rust with Clap solved this forever ago. Also - don't write CLI programs in languages that don't compile to native binaries. I don't want to have to drag around your runtime just to execute a command line tool.

I will keep writing my CLI programs in the languages I want, thanks. Have it crossed your mind that these programs might be for yourself or for internal consumption? When you know runtime will be installed anyway?

Re: Stop writing CLI validation. Parse it right the first time

#85

Post by dcminter »

bschwindHN wrote: post #82

Rust with Clap solved this forever ago. Also - don't write CLI programs in languages that don't compile to native binaries. I don't want to have to drag around your runtime just to execute a command line tool.

The declarative form of clap is not quite as well documented as the programmatic approach (but it's not too bad to figure out usually).

One of the things I love about clap is that you can configure it to automatically spit out --help info, and you can even get it to generate shell autocompletions for you!

I think there are some other libraries that are challenging it now (fewer dependencies or something?) but clap sets the standard to beat.

Re: Stop writing CLI validation. Parse it right the first time

#86

Post by dcminter »

majorbugger wrote: post #84
bschwindHN wrote: post #82

Rust with Clap solved this forever ago. Also - don't write CLI programs in languages that don't compile to native binaries. I don't want to have to drag around your runtime just to execute a command line tool.

I will keep writing my CLI programs in the languages I want, thanks. Have it crossed your mind that these programs might be for yourself or for internal consumption? When you know runtime will be installed anyway?

You do you, obviously, but "now let npm work its wicked way" is an offputting step for some of us when narrowing down which tool to use.

My most comfortable tool is Java, but I'm not going to persuade most of the HN crowd to install a JVM unless the software I'm offering is unbearably compelling.

Internal to work? Yeah, Java's going to be an easy sell.

I don't think OP necessarily meant it as a political statement.

Re: Stop writing CLI validation. Parse it right the first time

#87

Post by [deleted] »

jmull wrote: post #9

> Think about it. When you get JSON from an API, you don't just parse it as any and then write a bunch of if-statements. You use something like Zod to parse it directly into the shape you want. Invalid data? The parser rejects it. Done. Isn’t writing code and using zod the same thing? The difference being who wrote the code. Of course, you hope zod is robust, tested, supported, extensible, and has docs so you can und…

[deleted]

Re: Stop writing CLI validation. Parse it right the first time

#89

Post by goku12 »

8n4vidtmkvmk wrote: post #65
ffsm8 wrote: post #57

Earlier quoted context omitted.

I think you're looking at it too literally - what people usually mean with"making invalid state unrepresentable" is in the main application which has your domain code - which should be separate from your inputs He even gives the example of zod, which is a validation library he defines to be a parser. What he wants to say : "I don't want to write my own validation in a CLI, give me a good API already that first valida…

Zod might be a validation library, but it also does type coercion and transforms. I believe that's what the author means by a parser.

Apparently not. The author cites the example of json parsing for APIs. You usually don't split it into a generic parsing into native data types and then validate the result in memory (unless you're on a dynamically typed language and don't use a validation schema). Instead, the expected native data type of the result (composed using structs, enums, unions, vectors, etc) is defined first and then you try to parse the json into that data type. Any json errors and schema violations will error out in a single step.
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