You could define a naive 'foldr' like this:
(defn foldr [f coll]
(if (empty? (rest coll))
(first coll)
(f (first coll) (foldr f (rest coll)))) )
(foldr list '(1 2 3 4)) => (1 (2 (3 4)))
(foldr - [1 2 3 4]) => -2
However, this is not a tail-recursive function and will be limited by the size
of the stack.
An alternative involves reversing the sequence and applying the function with
its arguments reversed. Then we can just pretend that we are doing a left fold
and use 'reduce':
(defn foldr [f coll]
(reduce (fn [x y] (f y x)) (reverse coll)))
Or for those of you who prefer that other people won't be able to read your
code:
(defn foldr [f coll]
(reduce #(f %2 %1) (reverse coll)))
Have all good days,
David Sletten
On Nov 5, 2010, at 10:03 PM, Yang Dong wrote:
> Maybe because Clojure has a vector, and conj conjoins new elements to
> the end of the vector, so there's mere little use of fold-right. But,
> fold-right is an abstraction tool, missing it in the core is kind of
> pity.
>
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