[pjmlp]

There is no way to enforce that + actually implements the concept of adding two values.

[hood_syntax]

With dependent types you can. I'm not aware of whether that proof is possible in Haskell, however.

Besides, that's not what I was saying. What I was saying it that + in Haskell means more than an overloaded operator. It means the type has access to additional methods. This gives you more information.

[pjmlp]

In that case you get the same information on any usable C++ IDE.