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Document guarantee that hash (Arg x _) == hash x #242

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josephcsible opened this issue Feb 2, 2022 · 2 comments · May be fixed by #243
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Document guarantee that hash (Arg x _) == hash x #242

josephcsible opened this issue Feb 2, 2022 · 2 comments · May be fixed by #243
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@josephcsible
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In haskell-unordered-containers/unordered-containers#349, I'm proposing to add functions that convert between HashMaps with k as keys and ones with Arg k v as keys, without needing to rehash anything. For this to be safe, it needs to be the case that hash (Arg x _) == hash x. This is indeed the case, but the documentation doesn't currently guarantee this. It only lets us infer the weaker guarantee x1 == x2 -> hash (Arg x1 y1) == hash (Arg x2 y2), which isn't sufficient for this purpose. Can we add the stronger guarantee to the documentation?
@josephcsible josephcsible mentioned this issue Feb 2, 2022
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@phadej
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phadej commented Feb 2, 2022 •
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Sure. That was a reason of hashable-1.3.0.0 major release, so we can as well document that behavior on the instance of Arg, for others to rely.

I don't see any reason to change the instance ever, and if there would be a reason, the change will be a breaking change, so we'd need to consider the motivation properly.

@josephcsible
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That was a reason of hashable-1.3.0.0 major release

I thought the reason for that was just to make the weaker guarantee hold, and the fact that the stronger one holds too was just a lucky coincidence.

But in any case, this is good news, thanks. I'll write a PR for this tonight.

josephcsible added a commit to josephcsible/hashable that referenced this issue Feb 3, 2022
@josephcsible
Document additional guarantee of Arg's Hashable instance
9c7f237 
Fixes haskell-unordered-containers#242.
@josephcsible josephcsible linked a pull request Feb 3, 2022 that will close this issue
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Document additional guarantee of Arg's Hashable instance josephcsible/hashable
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