Fix #1714 conditionally halting stream (#2200)

* Fix #1714 conditionally halting stream

* Add note in changelog

docs/change-log.md

@@ -26,6 +26,7 @@
 - API: `m.request` will no longer reject the Promise on server errors (eg. status >= 400) if the caller supplies an `extract` callback. This gives applications more control over handling server responses.
 - hyperscript: when attributes have a `null` or `undefined` value, they are treated as if they were absent. [#1773](https://github.com/MithrilJS/mithril.js/issues/1773) ([#2174](https://github.com/MithrilJS/mithril.js/pull/2174))
 - hyperscript: when an attribute is defined on both the first and second argument (as a CSS selector and an `attrs` field, respectively), the latter takes precedence, except for `class` attributes that are still added together. [#2172](https://github.com/MithrilJS/mithril.js/issues/2172) ([#2174](https://github.com/MithrilJS/mithril.js/pull/2174))
+- stream: when a stream conditionally returns HALT, dependant stream will also end ([#2200](https://github.com/MithrilJS/mithril.js/pull/2200))
 
 #### News
 

stream/stream.js

@@ -53,7 +53,7 @@ function updateDependency(stream, mustSync) {
 	var state = stream._state, parents = state.parents
 	if (parents.length > 0 && parents.every(active) && (mustSync || parents.some(changed))) {
 		var value = stream._state.derive()
-		if (value === HALT) return false
+		if (value === HALT) return unregisterStream(stream)
 		updateState(stream, value)
 	}
 }

stream/tests/test-stream.js

@@ -164,6 +164,27 @@ o.spec("stream", function() {
 			o(b()).equals(undefined)
 			o(count).equals(0)
 		})
+		o("combine can conditionaly halt", function() {
+			var count = 0
+			var halt = false
+			var a = Stream(1)
+			var b = Stream.combine(function(a) {
+				if (halt) {
+					return Stream.HALT
+				}
+				return a()
+			}, [a])["fantasy-land/map"](function(a) {
+				count++
+				return a
+			})
+			o(b()).equals(1)
+			o(count).equals(1)
+			halt = true
+			count = 0
+			a(2)
+			o(b()).equals(1)
+			o(count).equals(0)
+		})
 		o("combine will throw with a helpful error if given non-stream values", function () {
 			var spy = o.spy()
 			var a = Stream(1)