A transformer manipulates and/or validates the entire DSL state of a resource.

It's transform/2 takes a map, which is just the values/configurations at each point of the DSL. Don't manipulate it directly, if possible, instead use functions like get_entities/3 and replace_entity/5 to manipulate it.

Use the after?/1 and before?/1 callbacks to ensure that your transformer runs either before or after some other transformer.

Return true in after_compile/0 to have the transformer run in an after_compile hook, but keep in mind that no modifications to the dsl structure will be retained, so there is no point in returning a new dsl structure from transform/2 if after_compile/0 is defined. Instead, simply return :ok or {:error, error}