Add Func type

`Func` is a function that returns a `Node` that is itself also a `Node`.

Used with `If`, it enables lazy evaluation of the node to return, depending on the condition passed to `If`.

See also #99 for a different approach explored.

gomponents.go

@@ -235,3 +235,12 @@ func If(condition bool, n Node) Node {
 	}
 	return nil
 }
+
+// Func is a function that returns a Node that is itself also a Node.
+// Used with If, it enables lazy evaluation of the node to return, depending on the condition passed to If.
+type Func func() Node
+
+// Render implements Node.
+func (f Func) Render(w io.Writer) error {
+	return f().Render(w)
+}

gomponents_test.go

@@ -291,3 +291,23 @@ func ExampleIf() {
 	_ = e.Render(os.Stdout)
 	// Output: <div><span>You lost your hat!</span></div>
 }
+
+func TestFunc(t *testing.T) {
+	t.Run("Func is a function type that is also a Node", func(t *testing.T) {
+		n := g.Func(func() g.Node {
+			return g.El("div")
+		})
+		assert.Equal(t, "<div></div>", n)
+	})
+}
+
+func ExampleIf_func() {
+	var message *string
+	e := g.El("div",
+		g.If(message != nil, g.Func(func() g.Node {
+			return g.El("span", g.Text(*message))
+		})),
+	)
+	_ = e.Render(os.Stdout)
+	// Output: <div></div>
+}