Guarded Struct Extensions

Custom derive extensions

Reference: usage-rules/extensions.md.

Declaring an extension module

defmodule MyApp.Derives do
  use GuardedStruct.Derive.Extension

  derives do
    validator :slug, fn input ->
      is_binary(input) and Regex.match?(~r/^[a-z0-9-]+$/, input)
    end

    sanitizer :slugify, fn input when is_binary(input) ->
      input |> String.downcase() |> String.replace(~r/[^a-z0-9-]+/u, "-")
    end
  end
end

The Spark transformer generates:

• __validators__/0, __sanitizers__/0 — atom lists
• __validate__(op, input, field) — runtime dispatcher; :__not_found__ on miss
• __sanitize__(input, op) — runtime dispatcher (pipe-friendly arg order)
• __derive_extension__?/0 — marker

Validator return contract

The validator :name, fn input -> ... end callback may return:

Sanitizer return contract

Whatever the function returns replaces the input. Sanitizers run before validators.

Registering

Global

# config/config.exs
config :guarded_struct, derive_extensions: [MyApp.Derives]

Cached in :persistent_term keyed by raw config. Auto-invalidates when the config changes. Manual reset: GuardedStruct.Derive.Extension.clear_cache/0.

Per-module

use GuardedStruct, derive_extensions: [MyApp.Derives]            # this module only
use GuardedStruct, derive_extensions: [MyApp.Derives, :config]   # merge with global

The :config sentinel expands to the global list at the position it appears. See usage-rules/extensions.md for full precedence rules.

Shadow warning

Declaring validator :string or sanitizer :trim shadows a built-in op. The Codegen transformer emits a Spark.Warning.warn/3 pointing at the entity's source line. GuardedStruct.Derive.Registry.known_validate?/1 / known_sanitize?/1 decide what counts as built-in.

Resolution in nested sub_field submodules

Auto-generated sub_field submodules inherit the root module's per-module opt via the process-dict :guarded_struct_current_module flag. They never declare their own __guarded_derive_extensions_opt__/0 (nil from codegen). Don't try to override per-submodule — set the opt on the root user module.