Scan with halt

[gamb]

Description

If the reducer function doesn't change the value, then don't call the dependant streams.

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly. (awaiting feedback)
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have updated docs/change-log.md (awaiting feedback)

---

Does this seem like reasonable expectation for scan? Feedback greatly appreciated.

Test case provided, and style guide followed :)

[Gandalf-the-Bot]

	Warnings
⚠️	Please add an entry to docs/change-log.md.

Generated by 🚫 dangerJS

[dead-claudia]

From an API standpoint, I'm not so sure. I'd rather not prevent stateful reducers like these:

function update(store, {type, value}) {
    switch (type) {
        case UNDO: store.undo(); return store
        case REDO: store.redo(); return store
        case INSERT: store.insert(value); return store
        case FLUSH: return history()
        default: return stream.HALT
    }
}

Stream.scan(update, new Store(), dispatch)

I personally hardly ever use streams, though (since Mithril core doesn't have any idea of streams), so I'm not super invested in anything.

[gamb]

Thanks @isiahmeadows, that seems reasonable. So perhaps scan could be implemented like this:

function scan(reducer, seed, stream) {
	var newStream = combine(function (s) {
		var next = reducer(seed, s._state.value)
		if (next !== HALT) return seed = next
		return HALT
	}, [stream])

	if (newStream._state.state === 0) newStream(seed)

	return newStream
}

Then behaviour is the same, except reducers can return HALT if nothing happens.

[dead-claudia]

That seems better to me.

[gamb]

Agreed. Updated as discussed, and rebased against the latest.

[gamb]

Anything I can do to get this one moved along? cc. @tivac @pygy

[pygy]

LGTM

[gamb]

Updated docs with a few notes on this change