updates.count++ in render function is occured rendering loop.

the below rendering flow:

- render root 
- render child  (HelloWorld)
- root watcher detect props changing
- render root 
- render child 
- root watcher detect props changing
- ...

I think that render function of functional component is updating the parent props is bad practice in Vue 2.0

@yyx990803 If you have my mistake comment or any comments, please tell us.

@kazupon Sorry, counter was here for showing purposes only. I didn't realize that it will affect to rendering flow of child component.
So, updated repro: http://codepen.io/OEvgeny/pen/qNpBqX?editors=1010
Warning message is equal with previous:

[Vue warn]: You may have an infinite update loop for watcher with expression "function () {
        vm._update(vm._render(), hydrating);
      }" (found in root instance)

Thanks!!

I found your mistake at render function.

components: {
    HelloWorld: {
      props: ['params'],
      functional: true,
      render (h, ctx) {
        console.log(ctx.props.params.style.background)
        //return h('div', ctx.props.params, ctx.children)
        return h('div', ctx.data, ctx.children) // correct usage
      }
    }
  }

About h, See the more detail:
https://github.com/vuejs/vue/releases/tag/v2.0.0-alpha.5

@kazupon Hmm ctx.data contains only raw attributes, I need to pass to h function component property called params as a tag props.

Okay.
In this case, You need to pass '{ props: ctx.props }' to 'h' function.

@kazupon No, again. By props i meant tag attributes not component properties. Sorry for mistake.
Provided repro works as expected except warning and extra render calls.

So, params from component properties should be applied as tag attributes. For example for params:

params: {
  style: { backgroundColor: "#ccc" }
}

I need to create node which looks like:

h('div', {style: { backgroundColor: "#ccc" }}, ctx.children)

My component gets params as its property and passes it down to h function as node attributes.

      props: ['params'],
      functional: true,
      render (h, ctx) {
        console.log(ctx.props.params) //{style: { backgroundColor: "#ccc" }}
        return h('div', ctx.props.params, ctx.children)
      }
    }

This is somewhat expected because vnode data should be fresh objects on each render. I've added a warning for using observed objects as vnode data.

To get around this, simply shallow copy the object before sending it in:

render (h, ctx) {
   return h('div', Vue.util.extend({}, ctx.props.params), ctx.children)
}

@yyx990803 Thanks!