Docs on “Changed?” inaccurate? about reagent HOT 5 CLOSED

Here’s an OrgPad for tracing this: https://orgpad.com/s/n3W2BotZfA_?open=all

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@shaunlebron
I'm struggling to remember the flow of control right now (I knew it well in 2015!) , but from memory, I feel this is a key line of code:

https://github.com/reagent-project/reagent/blob/master/src/reagent/ratom.cljs#L466

Note the = check.

Given your investigation (it looks like you have your head wrapped around the code) does that make any sense?

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I wrote the docs which you are questioning, so I'm feeling like I should be able to answer you definitively, BUT it is all a long time ago ... so all I can add for the moment is a further fact I can remember ... there was a time when identical? was used but that lead to programmers being "surprised", so we switched to = (which actually does an identical? check internally anyway but moves on to deeper, recursive comparison on false)

This breaking change is documented in the change log for v6.0.0. Note the line:

Reactions now only trigger updates of dependent components if their value change, as reported by = (previously, identical? was used).

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There is also = check on Reaction _run method: https://github.com/reagent-project/reagent/blob/master/src/reagent/ratom.cljs#L436 This is called either directly from handle-change or through the queue.

But yes, looks like if the component is directly dereffing a Ratom, the component will be rendered even if reset/swap doesn't change the value.

Component dereffing a RAtom:
ratom reset/swap -> notify the render reaction -> queue-render

Component dereffing a Reaction depending on a RAtom:
ratom reset/swap -> notify reaction -> run -> IF result changed -> notify the render reaction -> queue-render

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@mike-thompson-day8 Not a bad memory for seven years ago! I currently understand only how ratoms trigger a specific kind of reaction— the one created for a component’s render function. Looking at your reference, I think that = is related to a lazily evaluated reaction (?), wherein a Deref notifies its watchers, but only if it has changed in value.

I don’t think the reaction created purely for a component’s render function is ever deref’d like that. The only path to its auto-run function (i.e. batch/queue-render) is in handle-change, which I think only fires in response to:

• a dependent ratom being reset to a non-identical? value or,
• a dependent reaction being reset to a non-= value

NEW EDIT FOR THE DOCS?
“The ‘changed?’ semantics that make a component re-render are: identical? for ratoms, and = for reactions.

I don’t know yet if this is accurate or desirable, but that’s how I currently understand it.

@Deraen Thank you, I think you summarized it well, except for the last “reaction reset” is supposed to be “ratom reset”?— since the atom triggers the first reaction, which triggers the render reaction.

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