Abstract types for external modules #847
Description
External modules can be used to declare function and scope prototypes without a Catala implementation, but these prototypes must for now refer only to types declared explicitly in Catala modules. This prevents external modules from exposing functions that manipulate data whose type cannot be expressed in Catala, for instance arbitrary recursive algebraic data types.
The syntax for declaring these external types could be :
> Module Foo external
```catala-metadata
declaration external type bar
declaration baz content decimal depends on arg content bar
```