From 81eb6d72e48c22cf0ab9c88344e88faf978c7477 Mon Sep 17 00:00:00 2001
From: Kyle Kotowick <kyle@kotowick.ca>
Date: Fri, 30 Jul 2021 12:21:59 -0400
Subject: [PATCH] Switch method to not use external data source

---
 README.md  | 4 ++--
 main.tf    | 7 ++-----
 outputs.tf | 3 +--
 3 files changed, 5 insertions(+), 9 deletions(-)

diff --git a/README.md b/README.md
index d333bce..3864de3 100644
--- a/README.md
+++ b/README.md
@@ -1,7 +1,7 @@
 # Terraform Assertion
 A simple module that asserts that a condition is true. The assertion is checked during planning, unless the condition depends on a value that will only be known during apply.
 
-The module forces Terraform to fail if the condition is false by attempting to call a non-existant external command. It uses the error message in the command name so that the resulting Terraform failure message is descriptive (although it is slightly confusing in the format; see the example below).
+The module forces Terraform to fail if the condition is false by providing an invalid input to a Terraform function. It uses the error message in the invalid input so that the resulting Terraform failure message is descriptive (although it is slightly confusing in the format; see the example below).
 
 ## Usage
 ```
@@ -24,5 +24,5 @@ No changes. Your infrastructure matches the configuration.
 
 Output on Windows machines:
 ```
-Error: can't find external program "ERROR: This Terraform configuration can only be run on Unix-based machines."
+Invalid value for "number" parameter: cannot parse "ERROR: This Terraform configuration can only be run on Unix-based machines." as a base 2 integer.
 ```
diff --git a/main.tf b/main.tf
index 1298a95..c769a5a 100644
--- a/main.tf
+++ b/main.tf
@@ -2,9 +2,6 @@ terraform {
   required_version = ">= 0.13.0"
 }
 
-// Terraform has no built-in customizable error system, so instead we try to run a non-existant program with the error message in the name.
-// It's not pretty, but it works.
-data "external" "assertion" {
-  count   = var.condition ? 0 : 1
-  program = ["ERROR: ${var.error_message}"]
+locals {
+  content = var.condition ? "" : parseint("ERROR: ${var.error_message}", 2)
 }
diff --git a/outputs.tf b/outputs.tf
index 3027681..013c901 100644
--- a/outputs.tf
+++ b/outputs.tf
@@ -1,5 +1,4 @@
 output "checked" {
   description = "Whether the condition has been checked (used for assertion dependencies)."
-  depends_on  = [data.external.assertion]
-  value       = true
+  value       = local.content == "" ? true : true
 }