Make Expr's role nominal - #416
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ocharles
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Impressed I missed this! In earlier versions of Rel8 I always remembered it, but forgot it for the most important version 😄
TeofilC
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September 22, 2026 10:30
Previously Expr was inferred to have a phantom role, which meant that we could `coerce :: Expr a -> Expr b` for any `a` and `b`. We set it to nominal, which means we can only do `coerce :: a ~ b => Expr a -> Expr b`. We could have also done representational which would have additionally allowed coercing between types with the same representation, ie, if they had a Coercible instance themselves. This isn't correct since types with the same representation could have different DBType/Serializable instances, which would lead to bugs.
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Previously Expr was inferred to have a phantom role, which meant that we could
coerce :: Expr a -> Expr bfor anyaandb.We set it to nominal, which means we can only do
coerce :: a ~ b => Expr a -> Expr b.We could have also done representational which would have additionally allowed coercing between types with the same representation, ie, if they had a Coercible instance themselves. This isn't correct since types with the same representation could have different DBType/Serializable instances, which would lead to bugs.
Credit goes to @raehik for noticing this.