TODO: GroupBy & GroupJoin

[luvies]

i have never fully understood group-by, even in SQL, so i'll need to get on that an implement these methods

• GroupBy
• GroupJoin

[luvies]

I haven't mimicked the GroupBy method exactly, there's 1 overload that isn't implemented, and that's the one that takes in the key & result selector. This is because the only distinguishing property between a lambda that selects the element to use and the one that creates the result element is that the former has 1 parameter, and the second 2.

Having the groupBy function do different behaviour on this factor alone feels like it is very much open to weird behaviour if someone puts in a function with an unexpected number of arguments, so I have not done it. Instead, you should do:

Lazy.from(iterable).groupBy(e => e.key, e => e, e => e.elements);

The only difference to the Linq method is that you explicitly pass e => e instead of it being implicit, which I think is better for the library (since the explicit behaviour difference is clearer overall).