Stop ignoring null nodes in crawlAsync (#59)

Closes #58

Leave the decision about whether `null` is meaningful in another way to
the caller.

.github/workflows/ci.yml

@@ -20,7 +20,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        sdk: [beta]
+        sdk: [dev]
     steps:
       - uses: actions/checkout@v2
       - uses: dart-lang/setup-dart@v0.3
@@ -47,7 +47,7 @@ jobs:
       matrix:
         # Add macos-latest and/or windows-latest if relevant for this package.
         os: [ubuntu-latest]
-        sdk: [beta]
+        sdk: [2.12.0, dev]
     steps:
       - uses: actions/checkout@v2
       - uses: dart-lang/setup-dart@v0.3

CHANGELOG.md

@@ -1,5 +1,9 @@
-# 1.0.0
+# 2.0.0-dev
+
+- **Breaking**: `crawlAsync` will no longer ignore a node from the graph if the
+  `readNode` callback returns null.
 
+# 1.0.0
 
 - Migrate to null safety.
 - **Breaking**: Paths from `shortestPath[s]` are now returned as iterables to

lib/src/crawl_async.dart

@@ -5,7 +5,7 @@
 import 'dart:async';
 import 'dart:collection';
 
-final _empty = Future<Null>.value(null);
+final _empty = Future<void>.value();
 
 /// Finds and returns every node in a graph who's nodes and edges are
 /// asynchronously resolved.
@@ -22,10 +22,6 @@ final _empty = Future<Null>.value(null);
 /// performed at every node in an asynchronous graph, but does not give
 /// guarantees that the work is done in topological order.
 ///
-/// If [readNode] returns null for any key it will be ignored from the rest of
-/// the graph. If missing nodes are important they should be tracked within the
-/// [readNode] callback.
-///
 /// If either [readNode] or [edges] throws the error will be forwarded
 /// through the result stream and no further nodes will be crawled, though some
 /// work may have already been started.
@@ -54,7 +50,7 @@ class _CrawlAsync<K, V> {
 
   /// Add all nodes in the graph to [result] and return a Future which fires
   /// after all nodes have been seen.
-  Future<Null> run() async {
+  Future<void> run() async {
     try {
       await Future.wait(roots.map(_visit), eagerError: true);
       await result.close();
@@ -66,9 +62,8 @@ class _CrawlAsync<K, V> {
 
   /// Resolve the node at [key] and output it, then start crawling all of it's
   /// edges.
-  Future<Null> _crawlFrom(K key) async {
+  Future<void> _crawlFrom(K key) async {
     var value = await readNode(key);
-    if (value == null) return;
     if (result.isClosed) return;
     result.add(value);
     var next = await edges(key, value);
@@ -81,7 +76,7 @@ class _CrawlAsync<K, V> {
   /// The returned Future will complete only after the work for [key] and all
   /// transitively reachable nodes has either been finished, or will be finished
   /// by some other Future in [_seen].
-  Future<Null> _visit(K key) {
+  Future<void> _visit(K key) {
     if (_seen.contains(key)) return _empty;
     _seen.add(key);
     return _crawlFrom(key);

pubspec.yaml

@@ -1,10 +1,10 @@
 name: graphs
-version: 1.0.0
+version: 2.0.0-dev
 description: Graph algorithms that operate on graphs in any representation
-homepage: https://github.com/dart-lang/graphs
+repository: https://github.com/dart-lang/graphs
 
 environment:
-  sdk: '>=2.12.0-0 <3.0.0'
+  sdk: '>=2.12.0 <3.0.0'
 
 dev_dependencies:
   pedantic: ^1.10.0

test/crawl_async_test.dart

@@ -73,13 +73,13 @@ void main() {
       expect(result, allOf(contains('a'), contains('b')));
     });
 
-    test('ignores null nodes', () async {
+    test('allows null nodes', () async {
       var result = await crawl({
         'a': ['b'],
       }, [
         'a'
       ]);
-      expect(result, ['a']);
+      expect(result, ['a', null]);
     });
 
     test('surfaces exceptions for crawling edges', () {