foldl' and foldr' for Seq are not strict in the starting value

[meooow25]

This is unlike every other structure we have.

λ> import qualified Data.Foldable as F
λ> import qualified Data.Set as S -- or Map or IntSet or IntMap
λ> F.foldl' (\_ _ -> ()) undefined (S.singleton ())
*** Exception: Prelude.undefined
CallStack (from HasCallStack):
  ...
λ> F.foldr' (\_ _ -> ()) undefined (S.singleton ())
*** Exception: Prelude.undefined
  ...
λ> import qualified Data.Sequence as Seq
λ> F.foldl' (\_ _ -> ()) undefined (Seq.singleton ())
()
λ> F.foldr' (\_ _ -> ()) undefined (Seq.singleton ())
()

I think we should make these strict.

Perhaps it's worth mentioning that for lists, foldl' is strict in the starting value but foldr' is not (GHC #25508).

[meooow25]

@treeowl do you have opinions on this?

[treeowl]

The whole story is unfortunate. Historically, foldl' was lazy in the starting value for lists. Some other structures made it strict (I think that included HashMap, but I don't remember for sure). Then foldl' for lists became strict—by accident. Do we follow the trend? Probably yes. It's usually better that way, and there probably aren't many relying on the laziness.

[meooow25]

Interesting, I found your issue for the accidental change: GHC #12173.
It seems that no one has complained about the change since, and there is one comment in favor of it.
I expect it would be fine to do the same for Seq too then.