render-prop-function.jsx

const Input = (props, state, validate) => {
  return (
    <div>
      <input {...props} onChange={e => props.onChange(e).then(validate)}/>
      <span>{state.message}</span>
    </div>
  );
}

class App extends Component {
  handleSubmit(form) {
    console.log(form.values);
  }

  render() {
    return (
      <Form onSubmit={form => this.handleSubmit(form)}>
        <Field name="username" validators={[minLength(3)]}>
          {Input}
        </Field>
        <Field name="password" type="password" validators={[minLength(6)]}>
          {Input}
        </Field>
        <button>Submit</button>
      </Form>
    );
  }
}