On 2015-12-05 at 19:20, Raja
foldr is supposed to start folding from the right side (as the name suggests). and this is why it is synonymous to "list construction" as I'm told
for e.g:
foldr (:) [ ] [1,2,3,4,5] [1,2,3,4,5]
In the same spirit I'm trying to construct a Foldable instance of my own type:
data Li a = Nil | Cons a (Li a) deriving (Show)
instance Foldable Li where foldr f b Nil = b foldr f b (Cons a y) = foldr f (f a b) y
So I'm trying out foldr for my type:
foldr Cons Nil (Cons 1 (Cons 2 Nil)) Cons 2 (Cons 1 Nil)
This shows my foldr implementation i'm not folding from right side, but how can I possibly do that - the data could have been an infinite stream.
A right fold on an infinite stream can terminate if the function f sometimes discards it's second argument. For example, takeWhile can be implemented this way. You are right that `foldr Cons Nil` or `foldr (:) []` will not terminate on an infinite list. On the bright side, you 've written a perfectly good left fold, even though it doesn't have quite the signature Haskell gives foldl. bergey