On 20/08/2014 12:36, Jeremy Maitin-Shepard wrote:
Why isn't a 2-element List, a 2-element std::tuple, etc. an instance?
`hana::list`, `std::tuple` and friends can't be made an instance of the `Product` type class because they would not satisfy its laws.
From my reading of the description of the Product type class laws, the problem you are referring to is the uniqueness requirement on the function "make": for a tuple, you could add more than two elements, with the extra elements containing arbitrary unused values.
However, I don't see any practical reason why it is useful to have a separate Product type from a Tuple type. Potentially, you could document that first and second are only valid for 2-element tuples, and make them fail for tuples with more than 2 elements. However, particularly since we are just talking about metaprogramming, it seems letting them work for any tuple would be better.
I might be taking something out of context here, but if you want to have
arbitrary-length tuples, I think it's usually more typical to define
head and tail operations:
tuple<>::head => undefined
tuple<A>::head => A
tuple