On Fri, Jun 19, 2009 at 6:17 AM, Matthew Brecknell
On Thu, 2009-06-18 at 23:57 +0800, Lee Duhem wrote:
[...] I have prepared a blog post for how I worked out some of these answers, here is the draft of it, I hope it can help you too.
Nice post! Certainly, pen-and-paper reasoning like this is a very good way to develop deeper intuitions.
Answer 1 (by Matthew Brecknell):
concat $ tail $ iterate (map (:) ['a' .. 'z'] <*>) [[]]
I actually said "tail $ concat $ iterate ...", because I think the initial empty string is logically part of the sequence. Tacking "tail" on the front then produces the subsequence requested by the OP.
Yes, I changed your solution from "tail $ concat $ iterate ..." to "concat $ tail $ iterate ...", because I think cut useless part out early is good idea, forgot to mention that, sorry.
I should have given more credit to Reid for this solution. I'm always delighted to see people using monadic combinators (like replicateM) in the list monad, because I so rarely think to use them this way. Sadly, my understanding of these combinators is still somewhat stuck in IO, where I first learned them. I never would have thought to use <*> this way if I had not seen Reid's solution first.
Actually, I first figure out how Reid's solution works, then figure out yours. After that, I found, for me, your solution's logic is easier to understand, so I take it as my first example. As I said at the end, or as I'll said at the end, Reid' solution and yours are the same (except effective) lee