This could be improved by defaulting args to [ :to_f ], thus eliminating the need for the ternary operator.

I couldn't really see a way to handle in_range in the same way as the others, without cracking open Integer as well, which I was trying to avoid. However, I've just realised my in_range method is broken.

Not sure about the range.to_a. I like way to dried up the method defs.

I should try harder to avoid method_missing.

+1 modules

I like the way you've extended Integer so that you can generalise the filter, but don't like the indiscriminate use of method_missing. Also, you could DRY up under and over.

This is the right idea, but has side effects, so can't be counted as "best practice".

that turned out to be a bug/ruby version difference about whether you can capitalize 'matrix' when requiring it.

Where does it say that the Matrix library is off-limits?

What if your hash looks like this?

{ :a => nil, 'a' => 'something' }

Brilliant! Wish I could mark this is "Best Practices" AND "Clever".

Doesn't this achieve the same thig?

(Matrix[[0,1],[1,1]] ** n)[1,0]

Your solution looks great. Very consise. Mine was very naive.

Nice trick to use a lambda, but using a recursive function to iterate over Fibonacci takes O(n^2) time.