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Comment by fooker | Hacker News Reader

Return type polymorphism and inheritance doesn't mix very well.

Swift got into this mess early in it's lifecycle and it's type checking is still more expensive than the rest of the compiler combined, and unpredictable on top of that.

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Yeah if you ask me, inheritance is the one to go. Every time. Inheritance just makes things more complicated. It’s not a great tool of abstraction.

C++ has both return type polymorphism and inheritance. What it doesn't have is the kind of type inference that Haskell has. Its type inference is very limited.

You can't have two functions that differ in just the return type in C++

You can sort of do it so long as the return type is a template parameter.

    template<typename T>
    T my_construct() { T result; return result; }

That's not polymorphism as that template parameter won't be deduced in any context and you will always have to explicitly instantiate the template.

That is still polymorphic, but as I mentioned C++ does not do the kind of type deduction that Haskell supports so you do have to explicitly instantiate the template. However, you can instantiate it based on context, for example using decltype.

challenge accepted: https://gcc.godbolt.org/z/bGdP79aEj

edit: you get pseudo call-by-name as a bonus.

You can, if you abuse C++ enough. I needed this very thing for a custom DSL.

    struct PolyReturn {
      const int value;
      operator int() const { return value; }
      operator bool() const { return value > 0; }
    };

https://cppinsights.io/s/f0b9976f

You certainly can and I know there are several JSON libraries that do it. The gist of it is to write a struct and overload the conversion operator for the set of return types you want to support, for example overload the conversion operator to int and std::string. As soon as the instance of that struct is used in a context that needs an std::string, then it calls the string overload.

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