taskgroup: add sketch of Gatherer type

This is basically a Collector that can be bolted to a Group or to a start
function extracted from a Group using a Throttle.

collector.go

@@ -50,3 +50,60 @@ func (c *Collector[T]) Report(f func(report func(T)) error) Task {
 func (c *Collector[T]) Run(f func() T) Task {
 	return NoError(func() { c.report(f()) })
 }
+
+// A Gatherer manages a group of [Task] functions that report values, and
+// gathers the values they return.
+type Gatherer[T any] struct {
+	run func(Task) // start the task in a goroutine
+
+	μ      sync.Mutex
+	gather func(T) // handle values reported by tasks
+}
+
+func (g *Gatherer[T]) report(v T) {
+	g.μ.Lock()
+	defer g.μ.Unlock()
+	g.gather(v)
+}
+
+// Gather creates a new empty gatherer that uses run to execute tasks returning
+// values of type T.
+//
+// If gather != nil, values reported by successful tasks are passed to the
+// function, otherwise such values are discarded. Calls to gather are
+// synchronized to a single goroutine.
+//
+// If run == nil, Gather will panic.
+func Gather[T any](run func(Task), gather func(T)) *Gatherer[T] {
+	if run == nil {
+		panic("run function is nil")
+	}
+	if gather == nil {
+		gather = func(T) {}
+	}
+	return &Gatherer[T]{run: run, gather: gather}
+}
+
+// Call runs f in g. If f reports an error, the error is propagated to the
+// runner; otherwise the non-error value reported by f is gathered.
+func (g *Gatherer[T]) Call(f func() (T, error)) {
+	g.run(func() error {
+		v, err := f()
+		if err == nil {
+			g.report(v)
+		}
+		return err
+	})
+}
+
+// Run runs f in g, and gathers the value it reports.
+func (g *Gatherer[T]) Run(f func() T) {
+	g.run(func() error { g.report(f()); return nil })
+}
+
+// Report runs f in g. Any values passed to report are gathered.  If f reports
+// an error, that error is propagated to the runner.  Any values sent before f
+// returns are still gathered, even if f reports an error.
+func (g *Gatherer[T]) Report(f func(report func(T)) error) {
+	g.run(func() error { return f(g.report) })
+}