The foldr function does not start from the right. It associates to the
right.
This talk specifically addresses this common mistake.
http://functionaltalks.org/2013/06/19/tony-morris-explain-list-folds-to-your...
On 06/12/2015 10:21 AM, "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. It feels like I will never be able to truly write a foldr implementation with "right" folding mechanism.
Any thoughts?
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