foldl is tail recursive, so how come foldr runs faster than foldl?

Question

I wanted to test foldl vs foldr. From what I\'ve seen you should use foldl over foldr when ever you can due to tail reccursion optimization.

This makes sense. Howeve

Answer 1

Neither foldl nor foldr is tail optimized. It is only foldl'.

But in your case using ++ with foldl' is not good idea because successive evaluation of ++ will cause traversing growing accumulator again and again.