14. Alternative design choices and avenues for improvement

Pretty much every newtype or single-constructor declaration that is involved in a foreign language call needs a corresponding %dis definition. Maybe this %dis definition should be automated. On the other hand, there are many fewer data types than procedures, so perhaps it isn't too big a burden to define a %dis for each.

Alternatively, go the other way and have Green Card generate a Haskell type from a %dis definition, along the lines of what %enum already does.

The error handling provided by %fail is fairly rudimentary. It isn't obvious how to improve it in a systematic manner though.