In Haskell as in C++, they serve as an escape valve where the formal type system is too constraining. In C++, we see std::optional, std::variant, and boost::result, and don't find we need many more.
Product types are ad-hoc structs, analogous to lambdas as ad-hoc functions. They are important to enable compounding of libraries from different sources, aside from their frequent convenience. Compounding libraries is still hard for other reasons, and so not useful as often as might be hoped.
In Python you have the opposite problem of C++ and Haskell: everything could be anything, and you would like them to be guaranteed to be only one or a few things.
Naturally by "sum types" I mean "sum types plus the associated language-level conveniences that make them reasonable to work with". Perhaps case statements may be enough. Perhaps arbitrary lambdas are needed. Anyway, they're not reasonable to work with in Python. I'm not particularly familiar with C++ but I doubt they're reasonable to work with in that language either.
In Haskell as in C++, they serve as an escape valve where the formal type system is too constraining. In C++, we see std::optional, std::variant, and boost::result, and don't find we need many more.
Product types are ad-hoc structs, analogous to lambdas as ad-hoc functions. They are important to enable compounding of libraries from different sources, aside from their frequent convenience. Compounding libraries is still hard for other reasons, and so not useful as often as might be hoped.
In Python you have the opposite problem of C++ and Haskell: everything could be anything, and you would like them to be guaranteed to be only one or a few things.