[pacaro]

Or to take his doubling example...

    double([], []).
    double(H | T, H2 | T2) :- H2 is H * 2, double(T, T2).

[masklinn]

That's a lower-level version of map in a language with destructuring: Haskell would let you say

    double numbers = map (\x -> x * 2) numbers

or

    double [] = []
    double (x:xs) = x * 2 : double xs

or

    double = map (*2)

I don't think #2 is more declarative than #1, let alone #3. Then again, I don't think any of these versions is very declarative.

Declarative would be numpy:

    doubled = numbers * 2