feat: export BatchMsg

There's no good reason to keep it private. Exporting it helps
testability, debugging, and allows for a few special model.Update
implementations.

commands.go

@@ -24,13 +24,13 @@ func Batch(cmds ...Cmd) Cmd {
 		return nil
 	}
 	return func() Msg {
-		return batchMsg(validCmds)
+		return BatchMsg(validCmds)
 	}
 }
 
-// batchMsg is the internal message used to perform a bunch of commands. You
-// can send a batchMsg with Batch.
-type batchMsg []Cmd
+// BatchMsg is a message used to perform a bunch of commands concurrently with
+// no ordering guarantees. You can send a BatchMsg with Batch.
+type BatchMsg []Cmd
 
 // Sequence runs the given commands one at a time, in order. Contrast this with
 // Batch, which runs commands concurrently.

commands_test.go

@@ -94,13 +94,13 @@ func TestBatch(t *testing.T) {
 	})
 	t.Run("single cmd", func(t *testing.T) {
 		b := Batch(Quit)()
-		if l := len(b.(batchMsg)); l != 1 {
+		if l := len(b.(BatchMsg)); l != 1 {
 			t.Fatalf("expected a []Cmd with len 1, got %d", l)
 		}
 	})
 	t.Run("mixed nil cmds", func(t *testing.T) {
 		b := Batch(nil, Quit, nil, Quit, nil, nil)()
-		if l := len(b.(batchMsg)); l != 2 {
+		if l := len(b.(BatchMsg)); l != 2 {
 			t.Fatalf("expected a []Cmd with len 2, got %d", l)
 		}
 	})

tea.go

@@ -295,7 +295,7 @@ func (p *Program) eventLoop(model Model, cmds chan Cmd) (Model, error) {
 				// NB: this blocks.
 				p.exec(msg.cmd, msg.fn)
 
-			case batchMsg:
+			case BatchMsg:
 				for _, cmd := range msg {
 					cmds <- cmd
 				}

tea_test.go

@@ -108,7 +108,7 @@ func TestTeaBatchMsg(t *testing.T) {
 	m := &testModel{}
 	p := NewProgram(m, WithInput(&in), WithOutput(&buf))
 	go func() {
-		p.Send(batchMsg{inc, inc})
+		p.Send(BatchMsg{inc, inc})
 
 		for {
 			time.Sleep(time.Millisecond)