Lua Academy · Lesson

Building a Config DSL

Create a declarative config language that loads via dofile() in a sandbox.

Building a Config DSL is a free Lua Academy lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Lua Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Goal: Declarative Config

A config DSL lets users express configuration in a readable, declarative style while using Lua's full power (conditionals, loops, functions) behind the scenes.

Config Function Pattern

Expose a config(tbl) function that validates and stores configuration. Call it with a table literal for clean syntax.

local function config(tbl)
  -- validate and store
  return tbl
end
-- config.lua:
return config {
  host = "localhost",
  port = 8080,
  debug = os.getenv("DEBUG") == "1",
}

Nested Config with Sections

Support nested sections for organized configuration.

return config {
  database = { host="db.local", port=5432 },
  cache    = { host="cache.local", ttl=300 },
  logging  = { level="INFO", file="/var/log/app.log" },
}

Helper Functions in Config

Allow calling helper functions within the config for computed values.

local function env(name, default)
  return os.getenv(name) or default
end
return config {
  host  = env("DB_HOST", "localhost"),
  port  = tonumber(env("DB_PORT", "5432")),
}

Sandboxed Config Loading

For security, load the config in a restricted environment that allows only safe helpers.

local function loadConfig(path)
  local safeEnv = {
    config = config,
    env    = env,
    math   = math,
    table  = table,
  }
  local fn, err = loadfile(path, "t", safeEnv)
  if not fn then error(err) end
  return fn()
end

Schema Validation

Validate required fields and types after loading.

local function validate(cfg)
  assert(type(cfg.host) == "string", "host must be string")
  assert(type(cfg.port) == "number", "port must be number")
  assert(cfg.port > 0 and cfg.port < 65536, "port out of range")
end
validate(loadConfig("config.lua"))

Default Values

Merge loaded config with defaults to ensure all required keys have values.

local defaults = {host="localhost", port=8080, debug=false}
local function loadWithDefaults(path)
  local cfg = loadConfig(path)
  for k, v in pairs(defaults) do
    if cfg[k] == nil then cfg[k] = v end
  end
  return cfg
end

Environment Override

Allow environment variables to override config file values — essential for 12-factor app design.

Config Inheritance

Load a base config then overlay environment-specific config (development, staging, production).

Immutable Config

After loading, freeze the config table with a __newindex that raises an error, preventing accidental runtime mutation.

Documentation in Config

Use comments and structured key names as self-documentation. A well-designed config DSL is self-explanatory to DevOps teams.

Config DSL Question

Why load a config file with a sandboxed _ENV?

Recap: Building a Config DSL

Expose a config(tbl) function. Support nested sections and helper functions. Load in a sandboxed environment. Validate required fields and types. Provide defaults and allow environment variable overrides for 12-factor compliance.

Frequently asked questions

Is the “Building a Config DSL” lesson free?

Yes — the full text of “Building a Config DSL” is free to read here on the web, and the Lua Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Lua Academy course, upgrade to CoddyKit PRO.

What will I learn in “Building a Config DSL”?

Create a declarative config language that loads via dofile() in a sandbox. You practise Lua Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Lua Academy?

No prior experience is required. Lua Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Building a Config DSL” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Lua Academy lesson?

Yes. Every Lua Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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