foldr does not involve an accumulator. Only left folds involve an accumulator. Specifically, the expression: foldl f z list can be though of as this loop: var r = z foreach(element in list) { r = f(r, element) } return r That is why, for example, foldl (\r element -> element : r) [] will reverse a list. The accumulator here is the (r) value in the loop. The foldr function does constructor replacement, and in no prescribed order. The expression foldr f z list will replace every (:) with (f) and [] with (z) in list. So the expression foldr (<=<) return will take a list, such as thing one: let list = a : b : c : d : e : [] and turn it into this value: foldr (<=<) return list = a <=< b <=< c <=< d <=< e <=< return This will occur in no prescribed order, although the replacement is right-associative. It is often said that foldr "starts at the right-most", however, this is untrue, since foldr works on infinite list, which has no notion of right-most. Importantly, there is no notion of "accumulator" here, only constructor replacement. The right-associativity simply means that the parentheses are to the right: a <=< (b <=< (c <=< (d <=< (e <=< return)))) However, this does not necessarily impose an execution order, or accumulator. Consider this expression: foldr const (repeat 1) This will produce the value: 1 `const` (1 `const` (1 `const` 1 ... The result of normalising this expression produces the value 1. There is no notion of the "right-most" to "accumulate" anything. It simply evaluates and the answer is 1. On 07/26/2018 11:44 AM, Olumide wrote:
Dear List,
Chapter 13 of LYAH (http://learnyouahaskell.com/for-a-few-monads-more#useful-monadic-functions) has the following code block
import Data.List
inMany :: Int -> KnightPos -> [KnightPos] inMany x start = return start >>= foldr (<=<) return (replicate x oveKnight)
What I'd like to know is where the accumulator of foldr is in this example.
Regards,
- Olumide
_______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners