Hi Achim,
On Sun, Aug 31, 2008 at 1:58 PM, Achim Passen <[EMAIL PROTECTED]> wrote:
> Now suppose you want to define a function which returns the nth
> fibonacci number, but you down want "fibs" around globally. My guess
> was:
>
> (defn fib [n]
> (let [fibs2 (lazy-cat '(0 1) (map + fibs2 (drop 1 fibs2)))]
> (nth fibs2 n)))
>
> But this yields "Unable to resolve symbol: fibs2 in this context", so
> let doesn't allow being self-referential the way def does. I don't
> quite understand why the second arg to lazy-cat is being evaluated in
> the first place. Shouldn't evaluation be delayed until needed?
>
> Does anyone know how to get around this?
You can create locally recursive functions by writing an inline (fn
..) form with a name. So your example above becomes:
(defn fib [n]
(let [fibs2 (fn fibs2 [] (lazy-cat '(0 1) (map + (fibs2) (drop 1 (fibs2)))))]
(nth (fibs2) n)))
So instead of defining fibs2 as a lazy-cat directly, we wrap it in a
function that when called returns the lazy-cat you are trying to
define. Note that the inline (fn) form has a symbol inserted before
its argument list:
(fn fibs2 [] (lazy-cat ...)))
This allows an anonymous function to refer to itself. Then we just
need to replace each use of fibs2 with a call to the wrapper instead
of a direct reference to the lazy-cat--each reference to fibs2 becomes
a call to (fibs2).
Results:
user> (fib 0)
0
user> (fib 1)
1
user> (fib 2)
1
user> (fib 3)
2
user> (fib 4)
3
user> (fib 5)
5
There might be a cleaner way to do this, I'm not sure. This was the
first idea that popped into my head :).
/mike.
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