Declare the parameters of typed builders nominal #247
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LSLeary
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Hey. Sorry I didn't get to this earlier. Was traveling for AmeriHac. This is a minor but potentially breaking change, right? |
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@mchav No worries, but the answer to that depends somewhat on your philosophy. I consider this a bug fix; it won't break proper uses of the typed interface, but it will "break" abuses of the hole in it. |
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@LSLeary fair enough. Please bump the minor version and add a change log entry then I'll merge. |
The parameters to `Builder` and `NamedBuilder` in `Data.Csv.Incremental` are phantom, so GHC infers the same for their roles. This allows them to be `coerce`d willy nilly, defeating their purpose of using the type system to avoid building malformed CSV. Declaring the parameters `nominal` fixes this issue by ensuring that a given builder uses the same record construction instance throughout.
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@mchav Done. |
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It's also good practice to give such phantoms explicit kind signatures to avoid troubles due to accidental kind polymorphism. It doesn't look like the module is using
PolyKindsat this time, but it's the sort of extension that really should be on by default, so I've added the signatures anyway—consider it future proofing.