May I also add this ain't a mere one liner. It's a masterclass!

Hmm. I stick to jq for basically any JSON -> JSON transformation or summarization (field extraction, renaming, etc.). Perhaps I should switch to scripts more. uv is... such a game changer for Python, I don't think I've internalized it yet!

But as an example of about where I'd stop using jq/shell scripting and switch to an actual program... we have a service that has task queues. The number of queues for an endpoint is variable, but enumerable via `GET /queues` (I'm simplifying here of course), which returns e.g. `[0, 1, 2]`. There was a bug where certain tasks would get stuck in a non-terminal state, blocking one of those queues. So, I wanted a simple little snippet to find, for each queue, (1) which task is currently executing and (2) how many tasks are enqueued. It ended up vaguely looking like:

    for q in $(curl -s "$endpoint/queues" | jq -r '.[]'); do
        curl -s "$endpoint/queues/$q" \
        | jq --arg q "$q" '
            {
                "queue": $q,
                "executing": .currently_executing_tasks,
                "num_enqueued": (.enqueued_tasks | length)
            }'
    done | jq -s

which ends up producing output like (assuming queue 0 was blocked)

    [
        {
            "queue": 0,
            "executing": [],
            "num_enqueued": 100
        },
        ...
    ]

I think this is roughly where I'd start to consider "hmm, maybe a proper script would do this better". I bet the equivalent Python is much easier to read and probably not much longer.

Although, I think this example demonstrates how I typically use jq, which is like a little multitool. I don't usually write really complicated jq.

I could Google it, but tell a bit more about uv scripts. Isn't uv a package manager like pip?

Oh wow, that's fantastic. I love that it includes real values while still summarizing the doc's structure. I'm going to steal that. I'll probably keep jq-structure around because it's so easy to copy/paste paths I'm looking for, but yours is definitely better for understanding what the JSON doc actually contains.

Got a bit nerd-sniped here, but first of all we can reduce if A then B else . end === if A then B end since jq 1.7:

    jq 'walk(if type == "array" then (if length > 0 then [.[0]] end) end)'

Now we could contract those conditionals:

    jq 'walk(if type == "array" and length > 0 then [.[0]] end)'

but it turns out we can even more usefully express if length > 0 then [.[0]] end === [limit(1; .[])] == .[:1]:

    jq 'walk(if type == "array" then .[:1] end)'

From here, we can golf it a little further (this is kind of a generic type-matching pattern):

    jq 'walk(arrays[:1] // .)'

although this does incur a bit more overhead than checking type directly.

Speaking of overhead, though, it turns out that the implementation of walk/1 (https://github.com/jqlang/jq/blob/master/src/builtin.jq#L212) will actually run the filter on every element of an array, even though we're about to throw most of them out, which we can eliminate by writing the recursion explicitly:

    jq 'def w: if type=="array" then [limit(1; .[]|w)] elif type=="object" then .[] |= w end; w'

which gets the operation down from ~200 ms on my machine (not long enough to really get distracted, but enough to feel the wait) to a visually instant ~40 ms (which is mostly just the cost of reading the input). Now we can golf it down a little more:

    jq 'def w: if type=="array" then [limit(1; .[]|w)] else objects[] |= w end; w'
    jq 'def w: (arrays[:1]|map(w)) // (objects[] |= w); w'

(the precedence here actually allows us to eliminate the parens here...)

    jq 'def w: arrays |= .[:1]|iterables[] |= w; w'

And, inaccessibility of the syntax aside, I think this does an incredible job of expressing the essence of what we're trying to do: we trim any array down to its first element, and then recursively apply the same transformation throughout the structure. jq is a very expressive language, it just looks like line noise...

A standard as in there is a cottage industry of tools and websites built around it now, like this one.

Sic semper :)