Update: The fate of Transformer feature has been decided, and it is now considered deprecated except for internal Caster use eg.
Simplified Transformers view words.txt. See Issue #43. No new transformers can be defined by the end user.
Complex Transformers - Feature
In pre-1.0.0 Caster, there was a badly named feature called "filter functions". The idea of these "filter functions" was that Caster would define several times at which rules were being merged together, and would give the user access to make changes to the merged rules at these times. Due primarily to difficulty of use, "filter functions" did not get used much.
What "filter functions" ultimately sought to provide was a way for the user to change parts of Caster rules without needing to make changes to the source code, so that personal customizations wouldn't affect everyone else.
Transformers are a simplified, safer, and more descriptively named version of filter functions. Rather than providing the user with a pair of rules and a metadata object, transformers simply provide the user with a rule instance, with the intent that the user can then modify the rule instance and expect their modifications to be applied to whichever rules they've defined the transformer as being applicable to. See:
See also section 4A, "The fate of transformers" for comments on transformer vs user directory usage/applicability.
Some time after filter functions were added to Caster, the Caster user directory was added. This directory was a user-defined location from which user rules and filter functions would be loaded.
The fate of transformers
The creation of the Caster user directory largely denecessited filter functions. If users now had a place to put their custom rules, why create functions to modify existing rules? After all, the process of copying a file out of Caster, changing the pronunciation, and making whatever other modifications were desired was much simpler than learning the filter function API, and just as effective in most cases.
There were potential edge cases for filter functions which both the Caster user directory and the new transformers solution do not cover, but since no one ever found those edge cases, nothing is lost by simplifying the system, and in fact, much is gained.
The question remains though: if I have the user directory, and its content overrides Caster "starter" content, why do I need transformers? There are some prospective projects in the planning stages which might make use of them, and also, it was the most natural way to preserve words.txt functionality.