Switch method to not use external data source

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.
 ```

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)
 }

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
 }