It gives a sufficient and necessary condition for when a function can be written as a (right) fold.
It's not clear whether the author means to say foldl and foldr are equivalent, or only that any foldl call which terminates can be converted into a foldr call.
The latter is true, but the former is false.
For finite lists, every foldl call can be translated into a foldr call that produces the same result (and vice versa).
Foldr still makes sense for infinite lists in a language w/ lazy evaluation, but foldl will recurse forever.
https://www.cs.nott.ac.uk/~pszgmh/fold.pdf
It gives a sufficient and necessary condition for when a function can be written as a (right) fold.
It's not clear whether the author means to say foldl and foldr are equivalent, or only that any foldl call which terminates can be converted into a foldr call.
The latter is true, but the former is false.
For finite lists, every foldl call can be translated into a foldr call that produces the same result (and vice versa).
Foldr still makes sense for infinite lists in a language w/ lazy evaluation, but foldl will recurse forever.