Terraform sample provider

main.go

package main

import (
	"github.com/hashicorp/terraform/builtin/providers/test"
	"github.com/hashicorp/terraform/plugin"
)

func main() {
	plugin.Serve(&plugin.ServeOpts{
		ProviderFunc: test.Provider,
	})
}

provider.go

package test

import (
	"math/rand"
	"log"

	"github.com/hashicorp/terraform/helper/hashcode"
	"github.com/hashicorp/terraform/helper/schema"
	"github.com/hashicorp/terraform/terraform"
)

var letters = []rune("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ")

func generateRandomId() string {
	b := make([]rune, 30)
	for i := range b {
		b[i] = letters[rand.Intn(len(letters))]
	}
	return string(b)
}

func Provider() terraform.ResourceProvider {
	return &schema.Provider{
		ResourcesMap: map[string]*schema.Resource{
			 "test_container": resourceContainer(),
			 "test_element":   resourceElement(),
		},
	}
}

func resourceContainer() *schema.Resource {
	return &schema.Resource{
		Create: func (d *schema.ResourceData, meta interface{}) error {
			id := generateRandomId()
			log.Printf("[DEBUG] Create container with id %s and elements %#v", id, d.Get("elements").(*schema.Set))
			d.SetId(id)
			return nil
		},
		Schema: map[string]*schema.Schema{
			"elements": &schema.Schema{
				Type:     schema.TypeSet,
				Optional: true,
				Elem:     &schema.Schema{Type: schema.TypeString},
				Set: func(v interface{}) int {
					return hashcode.String(v.(string))
				},
			},
		},
	}
}

func resourceElement() *schema.Resource {
	return &schema.Resource{
		Create: func (d *schema.ResourceData, meta interface{}) error {
			id := generateRandomId()
			log.Printf("[DEBUG] Create element with id %s", id)
			d.SetId(id)
			return nil
		},
	}
}

test.tf

provider "test" {
}

resource "test_container" "ctn001" {
    elements = [
        "${test_element.elt001.id}",
        "${test_element.elt002.id}",
        "${test_element.elt003.id}"
    ]
}

resource "test_element" "elt001" {
}

resource "test_element" "elt002" {
}

resource "test_element" "elt003" {
}