diff --git a/locals.tf b/locals.tf
index 1db87dd..bb15dbb 100644
--- a/locals.tf
+++ b/locals.tf
@@ -4,7 +4,7 @@ resource "random_id" "name" {
 }
 
 locals {
-  name = "${var.name == "" ? random_id.name.hex : var.name}"
+  name = var.name == "" ? random_id.name.hex : var.name
 
   frontend_ip_configuration_name = "fe-ipconfig"
   https_port_name                = "https"
diff --git a/main.tf b/main.tf
deleted file mode 100644
index 9661d67..0000000
--- a/main.tf
+++ /dev/null
@@ -1,7 +0,0 @@
-provider "azurerm" {
-  version = "~> 1.23"
-}
-
-provider "random" {
-  version = "~> 2.1"
-}
diff --git a/outputs.tf b/outputs.tf
index 9cfd65e..9444f98 100644
--- a/outputs.tf
+++ b/outputs.tf
@@ -1,14 +1,14 @@
 output "id" {
   description = "The ID of the application gateway"
-  value       = "${azurerm_application_gateway.this.id}"
+  value       = azurerm_application_gateway.this.id
 }
 
 output "backend_address_pool_id" {
   description = "The ID of the application gateway backend address pool"
-  value       = "${azurerm_application_gateway.this.backend_address_pool.0.id}"
+  value       = azurerm_application_gateway.this.backend_address_pool[0].id
 }
 
 output "public_ip" {
   description = "The public IP of the application gateway"
-  value       = "${azurerm_public_ip.this.ip_address}"
+  value       = azurerm_public_ip.this.ip_address
 }
diff --git a/resources.tf b/resources.tf
index 74cce1f..4b9dee5 100644
--- a/resources.tf
+++ b/resources.tf
@@ -1,19 +1,19 @@
 resource "azurerm_public_ip" "this" {
   name                = "${local.name}-vip"
-  resource_group_name = "${var.resource_group_name}"
-  location            = "${var.location}"
+  resource_group_name = var.resource_group_name
+  location            = var.location
   sku                 = "Standard"
   allocation_method   = "Static"
 
-  tags = "${var.tags}"
+  tags = var.tags
 }
 
 resource "azurerm_application_gateway" "this" {
-  name                   = "${local.name}"
-  resource_group_name    = "${var.resource_group_name}"
-  location               = "${var.location}"
-  enable_http2           = "${var.enable_http2}"
-  disabled_ssl_protocols = ["${var.disabled_ssl_protocols}"]
+  name                   = local.name
+  resource_group_name    = var.resource_group_name
+  location               = var.location
+  enable_http2           = var.enable_http2
+  disabled_ssl_protocols = var.disabled_ssl_protocols
 
   sku {
     name     = "Standard_v2"
@@ -23,103 +23,106 @@ resource "azurerm_application_gateway" "this" {
 
   gateway_ip_configuration {
     name      = "gateway-ipconfig"
-    subnet_id = "${var.subnet_id}"
+    subnet_id = var.subnet_id
   }
 
   frontend_ip_configuration {
-    name                 = "${local.frontend_ip_configuration_name}"
-    public_ip_address_id = "${azurerm_public_ip.this.id}"
+    name                 = local.frontend_ip_configuration_name
+    public_ip_address_id = azurerm_public_ip.this.id
   }
 
   frontend_port {
-    name = "${local.https_port_name}"
+    name = local.https_port_name
     port = 443
   }
 
   frontend_port {
-    name = "${local.http_port_name}"
+    name = local.http_port_name
     port = 80
   }
 
   ssl_certificate {
-    name     = "${local.certificate_name}"
-    data     = "${data.azurerm_key_vault_secret.cert.value}"
+    name     = local.certificate_name
+    data     = data.azurerm_key_vault_secret.cert.value
     password = ""
   }
 
   http_listener {
-    name                           = "${local.https_listener_name}"
-    frontend_ip_configuration_name = "${local.frontend_ip_configuration_name}"
-    frontend_port_name             = "${local.https_port_name}"
-    ssl_certificate_name           = "${local.certificate_name}"
+    name                           = local.https_listener_name
+    frontend_ip_configuration_name = local.frontend_ip_configuration_name
+    frontend_port_name             = local.https_port_name
+    ssl_certificate_name           = local.certificate_name
     protocol                       = "Https"
   }
 
   http_listener {
-    name                           = "${local.http_listener_name}"
-    frontend_ip_configuration_name = "${local.frontend_ip_configuration_name}"
-    frontend_port_name             = "${local.http_port_name}"
+    name                           = local.http_listener_name
+    frontend_ip_configuration_name = local.frontend_ip_configuration_name
+    frontend_port_name             = local.http_port_name
     protocol                       = "Http"
   }
 
   backend_address_pool {
-    name = "${local.backend_address_pool_name}"
+    name = local.backend_address_pool_name
   }
 
   probe {
-    name                = "${local.health_probe_name}"
-    protocol            = "${var.health_probe_protocol}"
-    interval            = "${var.health_probe_interval}"
-    timeout             = "${var.health_probe_timeout}"
-    unhealthy_threshold = "${var.health_probe_threshold}"
-    path                = "${var.health_probe_path}"
+    name                = local.health_probe_name
+    protocol            = var.health_probe_protocol
+    interval            = var.health_probe_interval
+    timeout             = var.health_probe_timeout
+    unhealthy_threshold = var.health_probe_threshold
+    path                = var.health_probe_path
     host                = "127.0.0.1"
   }
 
   backend_http_settings {
-    name                  = "${local.backend_http_settings_name}"
-    cookie_based_affinity = "${var.cookie_based_affinity}"
+    name                  = local.backend_http_settings_name
+    cookie_based_affinity = var.cookie_based_affinity
     port                  = 80
     protocol              = "Http"
-    request_timeout       = "${var.backend_request_timeout}"
-    probe_name            = "${local.health_probe_name}"
+    request_timeout       = var.backend_request_timeout
+    probe_name            = local.health_probe_name
 
     connection_draining {
-      enabled           = "${var.enable_connection_draining}"
-      drain_timeout_sec = "${var.connection_drain_timeout}"
+      enabled           = var.enable_connection_draining
+      drain_timeout_sec = var.connection_drain_timeout
     }
   }
 
   request_routing_rule {
     name                       = "https-routing"
     rule_type                  = "Basic"
-    http_listener_name         = "${local.https_listener_name}"
-    backend_address_pool_name  = "${local.backend_address_pool_name}"
-    backend_http_settings_name = "${local.backend_http_settings_name}"
+    http_listener_name         = local.https_listener_name
+    backend_address_pool_name  = local.backend_address_pool_name
+    backend_http_settings_name = local.backend_http_settings_name
   }
 
   request_routing_rule {
     name                       = "http-routing"
     rule_type                  = "Basic"
-    http_listener_name         = "${local.http_listener_name}"
-    backend_address_pool_name  = "${local.backend_address_pool_name}"
-    backend_http_settings_name = "${local.backend_http_settings_name}"
+    http_listener_name         = local.http_listener_name
+    backend_address_pool_name  = local.backend_address_pool_name
+    backend_http_settings_name = local.backend_http_settings_name
   }
 
-  tags = "${var.tags}"
+  tags = var.tags
 }
 
 data "azurerm_network_interface" "targets" {
-  count = "${length(var.targets)}"
+  count = length(var.targets)
 
-  name                = "${var.targets[count.index]}"
-  resource_group_name = "${var.resource_group_name}"
+  name                = var.targets[count.index]
+  resource_group_name = var.resource_group_name
 }
 
 resource "azurerm_network_interface_application_gateway_backend_address_pool_association" "this" {
-  count = "${length(var.targets)}"
-
-  network_interface_id    = "${element(data.azurerm_network_interface.targets.*.id, count.index)}"
-  ip_configuration_name   = "${element(data.azurerm_network_interface.targets.*.ip_configuration.0.name, count.index)}"
-  backend_address_pool_id = "${azurerm_application_gateway.this.backend_address_pool.0.id}"
+  count = length(var.targets)
+
+  network_interface_id = element(data.azurerm_network_interface.targets.*.id, count.index)
+  ip_configuration_name = element(
+    data.azurerm_network_interface.targets.*.ip_configuration.0.name,
+    count.index,
+  )
+  backend_address_pool_id = azurerm_application_gateway.this.backend_address_pool[0].id
 }
diff --git a/ssl.tf b/ssl.tf
index 1af2ed9..fd40426 100644
--- a/ssl.tf
+++ b/ssl.tf
@@ -1,18 +1,19 @@
-data "azurerm_client_config" "current" {}
+data "azurerm_client_config" "current" {
+}
 
 resource "azurerm_key_vault" "this" {
-  name                = "${local.name}"
-  location            = "${var.location}"
-  resource_group_name = "${var.resource_group_name}"
-  tenant_id           = "${data.azurerm_client_config.current.tenant_id}"
+  name                = local.name
+  location            = var.location
+  resource_group_name = var.resource_group_name
+  tenant_id           = data.azurerm_client_config.current.tenant_id
 
   sku {
     name = "standard"
   }
 
   access_policy {
-    tenant_id = "${data.azurerm_client_config.current.tenant_id}"
-    object_id = "${var.object_id}"
+    tenant_id = data.azurerm_client_config.current.tenant_id
+    object_id = var.object_id
 
     certificate_permissions = [
       "create",
@@ -60,12 +61,12 @@ resource "azurerm_key_vault" "this" {
     ]
   }
 
-  tags = "${var.tags}"
+  tags = var.tags
 }
 
 resource "azurerm_key_vault_certificate" "this" {
-  name         = "${local.certificate_name}"
-  key_vault_id = "${azurerm_key_vault.this.id}"
+  name         = local.certificate_name
+  key_vault_id = azurerm_key_vault.this.id
 
   certificate_policy {
     issuer_parameters {
@@ -106,7 +107,7 @@ resource "azurerm_key_vault_certificate" "this" {
       ]
 
       subject_alternative_names {
-        dns_names = ["${var.ssl_sans}"]
+        dns_names = var.ssl_sans
       }
 
       subject            = "CN=${var.ssl_cn}"
@@ -114,12 +115,12 @@ resource "azurerm_key_vault_certificate" "this" {
     }
   }
 
-  tags = "${var.tags}"
+  tags = var.tags
 }
 
 data "azurerm_key_vault_secret" "cert" {
-  name      = "${local.certificate_name}"
-  vault_uri = "${azurerm_key_vault.this.vault_uri}"
+  name      = local.certificate_name
+  vault_uri = azurerm_key_vault.this.vault_uri
 
-  depends_on = ["azurerm_key_vault_certificate.this"]
+  depends_on = [azurerm_key_vault_certificate.this]
 }
diff --git a/versions.tf b/versions.tf
new file mode 100644
index 0000000..b636bd0
--- /dev/null
+++ b/versions.tf
@@ -0,0 +1,11 @@
+terraform {
+  required_version = ">= 0.12"
+}
+
+provider "azurerm" {
+  version = "~> 1.33.0"
+}
+
+provider "random" {
+  version = "~> 2.1"
+}