Document additional guarantee of Arg's Hashable instance#243
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josephcsible wants to merge 1 commit intohaskell-unordered-containers:masterfrom
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Document additional guarantee of Arg's Hashable instance#243josephcsible wants to merge 1 commit intohaskell-unordered-containers:masterfrom
josephcsible wants to merge 1 commit intohaskell-unordered-containers:masterfrom
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phadej
reviewed
Feb 3, 2022
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| -- | __Note__: Prior to @hashable-1.3.0.0@ the hash computation included the second argument of 'Arg' which wasn't consistent with its 'Eq' instance. | ||
| -- Additionally, as of @hashable-1.3.0.0@, and going forward as part of the API guarantee, the hash of @'Arg' a _@ is the same value as the hash of @a@. | ||
| -- This means that it's safe to add and remove 'Arg' from elements stored in hash-indexed data structures without changing any hashes. |
Contributor
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Ok, so this is actually not true.
hashWithSalt salt (Arg x _) = hashWithSalt x by definitiuon, but hash (Arg x _) /= hash x`:
> hash (Nothing :: Maybe Int)
0
> hash (Arg (Nothing :: Maybe Int) 'x')
-5808590958014384161That is an unfortunate "oversight".
Author
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Oof, that's not good, especially since the thing I wanted to use this for does use |
Contributor
Yes |
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Fixes #242.