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Re: [Json] Another problematic JSON Schema use-case

Tim Bray <tbray@textuality.com> Tue, 24 May 2016 03:54 UTC

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From: Tim Bray <tbray@textuality.com>
Date: Mon, 23 May 2016 20:54:33 -0700
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To: Austin William Wright <aaa@bzfx.net>
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Subject: Re: [Json] Another problematic JSON Schema use-case
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Hmm, I have to excerpt one line here:

On Mon, May 23, 2016 at 6:30 PM, Austin William Wright <aaa@bzfx.net> wrote:

>
> In general, JSON Schema and many other schemas are designed to ensure
> things are valid. They're not well engineered to help people fix things
> when they're invalid.
>

​I find this troubling.  I expect modern compilers to give me highly
actionable errors, with line numbers. Is the nature of JSON DSL’s and
programming languages so deeply different that this is not a reasonable
thing to ask a schema processor to do?

If so, then what are schemas for?  I’ve seen them as having two uses:

1. Specification aids
2. Debugging aids

For #1, I see schemas as a poor second-best to clear human-readable
description.
For #2, Schemas are appealingly declarative and presumably much more
compact than the equivalent declarative code.  But if they can’t tell me
what the problem is, why bother?  And furthermore, I’ve never thought
validators could rely on schemas alone, because schemas can’t check
semantic constraints (“is that a valid part number?”)

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