feat: callable forms

[Rich-Harris]

I found myself wanting to call a form directly, without actually using it on a <form>. In my case specifically it was for a recursive operation — I wanted the user to invoke deleteFile as a traditional <form>, but inside the handler I wanted to be able to call deleteFile as a function, which is currently impossible:

Obviously you can work around this by yoinking the logic out into a separate function, but it's not as nice — it's less self-contained, and you need to come up with two names instead of one. I gather I'm not the only person who has encountered cases like this.

The implementation is pleasingly simple — we just call command under the hood, remove the is_form_content_type check and it basically Just Works AFAICT? There is one change: myform.pending needs to become myform.submitting so that we can distinguish between form submissions (myform.submitting) and direct calls (myform.pending).

Note that the semantics of direct calls are exactly the same as for command:

• things like myform.fields.foo.issues() and myform.result do not get populated — these only happen for form submissions
• form submissions invalidate everything on the page by default (we still need to come up with a sensible API for preventing this within the form handler...), whereas direct calls do not

Of course, the obvious follow-up question: if we can do this, is there any point in keeping command around? Two possible arguments:

• if you only use command in your app and not form, you will have a very slightly smaller JS bundle
• command can have any schema, whereas form always has to be a v.object({...}) or the non-Valibot equivalent

I'm personally not sure those are compelling enough reasons, but this is a conversation to have as a follow-up.

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[phi-bre]

I've found myself wishing for this feature but not to replace commands but rather because only form properly handles streaming of file uploads. Using command under the hood in combination with #15978 will buffer the whole file for its JSON body.

I can open a new issue for that tho (I'm aware this addition is non-trivial, devalue does not support streaming, and the form uploads use a custom protocol afaik)

[Rich-Harris]

Yeah, would love to be able to do that. Mostly we just haven't figured out a good use-case-agnostic API for devalue to deal with async/streaming values (but then again we haven't spent a lot of time thinking about it)