Use SqlBackend instances as the foundation of SqlFor instances #20
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parsonsmatt
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Apr 26, 2023
src/Database/Persist/Typed.hs
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| downcastPersistRecordBackend :: forall record backend. PersistRecordBackend record (SqlFor backend) :- PersistRecordBackend record SqlBackend | ||
| downcastPersistRecordBackend = unsafeCoerceConstraint |
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:- is the type of entailment IIRC - so we're saying "If there's a PersistRecordBackend record (SqlFor backend) instance, please coerce that into PersistRecordBackend record SqlBackend.
This currently works because SqlFor is a newtype over SqlBackend. Can we make this safer by using the Coercible constraint that exists? Coercible (SqlFor backend) SqlBackend => ... is a redundant constraint, but at least if the newtype is changed then we aren't breaking things.
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This PR solves: #19