Why are some Prelude functions defined in terms of foldl?

Question

Generally, foldl is avoided in favor of foldl' or foldr. Quoting Real World Haskell:

Due to the thunking behavior of foldl, it is wise to avoid this function in real programs: even if it doesn't fail outright, it will be unnecessarily inefficient. Instead, import Data.List and use foldl'.

Yet some Prelude functions are defined in terms of it (e.g. (\\) and unionBy). Why is this? Is it to not introduce too much strictness to these functions?

Answer 1

The Prelude was designed before foldl' existed, and there's been pressure to maintain backwards compatibility (with regards to strictness, as you mentioned) since then.

Answer 2

In the case of (\\) and unionBy, the folded function has type

foo :: [a] -> b -> [a]

and foo xs y removes at most one element from xs, so using foldl' would not buy anything there in general, the thunks would be built on the right of the topmost (:) instead of above it then.

It would not make a difference in terms of strictness, as far as I can see, both folds would only be evaluated when the result needs to be evaluated to weak head normal form, and whenever foldl' would produce a _|_, so would foldl.

Answer 3

In both cases the accumulator is of type [a]. I can't see that forcing the list to weak-head normal form would make a huge difference, and introducing such partial strictness seems somewhat arbitrary.