Liberally use try, document idea

2024/bonus/README.md

@@ -0,0 +1,49 @@
+# Bonus Challenge
+
+This part of the repo holds the bonus challenge for 2024: implement as much as possible in pure
+Terraform.
+
+## Usage
+
+The infrastructure definitions expect the input files to be located at `../inputs/[\d]{2}.txt`.
+After storing the input files there, the code should be runnable as follows:
+
+```console
+$ terraform init
+< a lot of modules being installed >
+$ terraform plan
+
+Changes to Outputs:
+  + day01_1 = 42
+  + day01_2 = 12
+  + day02_1 = …
+
+You can apply this plan to save these new output values to the Terraform state, without changing any real infrastructure.
+```
+
+Note that Terraform may freeze for tens of seconds while it's running the computations. This is
+normal, and all Terraform code ought to think a little before doing anything. It would save people
+some bad rollbacks.
+
+## Development
+
+I use [Terraform tests](https://developer.hashicorp.com/terraform/language/tests) to automatically
+run my terraform code on the sample inputs. It's almost a normal development workflow.
+
+The only rule is that the code should be all terraform, no cheating by shelling out to external
+programs. Using providers is allowed, as long as the providers don't actually interact with external
+systems and are reasonably self-contained. I will try to limit my use of those regardless.
+
+## Why
+
+DevOps will continue until morale improves. But really, a friend remarked that my Python solutions
+were strangely normal for me, so I opted to use a language that I do use professionally.
+
+Terraform is a unique beast. It can do a lot, but it is also very limited, and intentionally so.
+There's a standard library of functions you might want to use, but all of them work in strange ways
+and there isn't that much to begin with. You can never mutate any variables, you can only declare
+new ones. You don't have recursion and your only source of loops are list- and map comprehensions,
+or multiple instantiations or a module.
+
+These make for a very constrained programming environment, and constrained programming is fun. It
+makes you think outside the box.

2024/bonus/day01/main.tf

@@ -16,7 +16,7 @@ locals {
 
   counts = { for num in local.right : num => num... }
 
-  matching = [for left in local.left : left * length(lookup(local.counts, left, []))]
+  matching = [for left in local.left : left * try(length(local.counts[left]), 0)]
 }
 
 output "part1" {

2024/bonus/day02/is_savable/main.tf

@@ -8,7 +8,7 @@ module "is_valid" {
 
   report = concat(
     count.index > 0 ? slice(var.report, 0, count.index) : [],
-    count.index < length(var.report) - 1 ? slice(var.report, count.index + 1, length(var.report)) : []
+    try(slice(var.report, count.index + 1, length(var.report)), [])
   )
 }
 

2024/bonus/day03/should_execute/main.tf

@@ -10,8 +10,8 @@ locals {
   is_mul   = startswith(var.ops[var.index][0], "mul")
   subslice = reverse(slice(var.ops[*][0], 0, var.index))
 
-  do_pos   = contains(local.subslice, "do()") ? index(local.subslice, "do()") : var.index
-  dont_pos = contains(local.subslice, "don't()") ? index(local.subslice, "don't()") : var.index + 1
+  do_pos   = try(index(local.subslice, "do()"), var.index)
+  dont_pos = try(index(local.subslice, "don't()"), var.index + 1)
 }
 
 output "value" {