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foldl' and foldr' for Seq are not strict in the starting value #1072

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Tracked by #1059
meooow25 opened this issue Nov 21, 2024 · 3 comments · May be fixed by #1077
Open
Tracked by #1059

foldl' and foldr' for Seq are not strict in the starting value #1072

meooow25 opened this issue Nov 21, 2024 · 3 comments · May be fixed by #1077
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Seq strictness

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@meooow25
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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
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@treeowl do you have opinions on this?

@treeowl
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treeowl commented Nov 22, 2024

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
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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.

@meooow25 meooow25 mentioned this issue Nov 23, 2024
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@meooow25 meooow25 linked a pull request Nov 30, 2024 that will close this issue
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Make Seq foldl', foldr' strict in the initial value meooow25/containers
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@treeowl @meooow25