hunfabric/.agents/rules/context-interpolation.md

trigger

trigger
always_on

Context-Based Interpolation

When designing factory patterns or datasets, you MUST leverage the context-based interpolation system.

A context object variable is used to hold maps of known, existing resource IDs (such as project_ids, folder_ids, networks, iam_principals).

Usage Pattern

1. Variables: Add a context variable in variables.tf.

   variable "context" {
     description = "Context-specific interpolations."
     type = object({
       project_ids = optional(map(string), {})
       folder_ids  = optional(map(string), {})
     })
     default = {}
   }
   

2. Locals: Build ctx and ctx_p local variables in main.tf by flattening the var.context object.

   locals {
     ctx = {
       for k, v in var.context : k => {
         for kk, vv in v : "${local.ctx_p}${k}:${kk}" => vv
       }
     }
     ctx_p = "$"
   }
   

3. Lookups: Apply lookups inside resources.

   project = lookup(local.ctx.project_ids, var.project_id, var.project_id)
   

4. YAML Interpolation: In factory YAML files, use the $ prefix convention to reference the lookup map keys.

   # Instead of hardcoding the folder ID: parent: folders/1234567890
   parent: $folder_ids:teams/team-a
   

This pattern makes YAML configuration files highly portable across installations and environments by substituting mnemonic keys for hardcoded values.