- Worse type error messages, as the type system has to be more complicated to handle the sorts of patterns that are common in untyped code.
- You can still get type errors at runtime in your typed code, if it interacts with untyped code, because it isn't feasible to validate all types on the boundary. You're only guaranteed no runtime type errors if all of your code is typed.
- The migration path from untyped to typed isn't always easy. Depending on how your code is organized, it's possible to have correct untyped code such that there do not exist type annotations you could add to it that would make it type check.
Comments
Some downsides of gradual typing:
- Worse type error messages, as the type system has to be more complicated to handle the sorts of patterns that are common in untyped code.
- You can still get type errors at runtime in your typed code, if it interacts with untyped code, because it isn't feasible to validate all types on the boundary. You're only guaranteed no runtime type errors if all of your code is typed.
- The migration path from untyped to typed isn't always easy. Depending on how your code is organized, it's possible to have correct untyped code such that there do not exist type annotations you could add to it that would make it type check.