tails, inits, initTails example implementation

[BebeSparkelSparkel]

Here is my example implementation. I have only added:

• tails, inits, initTails to class IsSequence
• their default implementations
• added non-default implementations to the List instance

Can you review for correctness and then I will add it to the other instances?

I am unsure how to add this to tests. How should I add the tests?

closes #213

[BebeSparkelSparkel]

I'll try to add some tests this week

[BebeSparkelSparkel]

Added tests and fixed error for list instance.

Question: Should initTails [] == [([],[])] or initTails == []?

[snoyberg]

I honestly have no idea, I've never used that function. Is there any prior art you're aware of for a definition?

[tomjaguarpaw]

One invariant of this function seems to be that

length xs + 1 == length (initTails xs)

therefore initTails [] == [([], [])] is the correct choice.

[BebeSparkelSparkel]

I have received comment from @tomjaguarpaw above and Atijohn.

Both recommend initTails [] == [([], [])], as now implemented, with good reasoning.

So, this is ready to go.

[snoyberg]

Thanks!