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This should be merged after #686 GitOrigin-RevId: a372ca475a9c4a182b4819310634d4c3985225ab
114 lines
4.0 KiB
Haskell
114 lines
4.0 KiB
Haskell
module Hasura.SQL.Backend where
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import Hasura.Prelude
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import Data.GADT.Compare
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import Type.Reflection
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import Unsafe.Coerce
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-- | An enum that represents each backend we support.
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data BackendType = Postgres | MSSQL
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deriving (Show, Eq, Ord, Bounded, Enum)
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-- | A singleton-like GADT that associates a tag to each backend.
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-- It must contain one tag per backend in @BackendType@.
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data BackendTag (b :: BackendType) where
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PostgresTag :: BackendTag 'Postgres
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MSSQLTag :: BackendTag 'MSSQL
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-- | How to convert back from a tag to a runtime value.
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reify :: BackendTag b -> BackendType
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reify = \case
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PostgresTag -> Postgres
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MSSQLTag -> MSSQL
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-- We need those instances to be able to use a @BackendTag@ as a key in a
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-- dependent map. Using @BackendType@ as a data kind, makes it difficult to use
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-- @Typeable@, hence the reliance on `unsafeCoerce`.
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instance GEq BackendTag where
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geq b1 b2
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| reify b1 == reify b2 = unsafeCoerce $ Just Refl
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| otherwise = Nothing
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instance GCompare BackendTag where
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gcompare b1 b2 = case compare (reify b1) (reify b2) of
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EQ -> unsafeCoerce GEQ
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LT -> GLT
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GT -> GGT
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{- Note [Recovering Existentially Quantified Type Information]
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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See Note [Existentially Quantified Types] for a brief introduction to existential
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quantification.
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In our codebase, we have a significant amount of types indexed by 'BackendType',
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and a few existential wrappers on top of them. One such example can be found in
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Hasura.RQL.Types.Source: the 'SourceInfo b' type is existentially quantified as
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'BackendSourceInfo'.
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The idea is that we want to have a heterogenous list (or map) of 'BackendSourceInfo'
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values. However, we also want to be able to recover the information about the
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specific source.
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One way would be to have multiple constructors on 'BackendSourceInfo':
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data BackendSourceInfo where
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PostgresSourceInfo :: SourceInfo 'Postgres -> BackendSourceInfo
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MssqlSourceInfo :: SourceInfo 'MSSQL -> BackendSourceInfo
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-- etc
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However, this would mean that we need to change a significant amount of code
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when we add a new backend: we need to change all the wrappers and add a new
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constructor, as well as all the pattern matches on them.
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Instead, we use existential quantification which makes it such that we don't
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have to add constructors when adding backends. The problem then is that we
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might want to make decisions depending on the backend, but this information
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is erased.
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data BackendSourceInfo =
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forall b. Backend b => BackendSourceInfo (SourceInfo b)
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In order to circumvent this problem, we use a trick similar to the one found
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in the 'singletons' library.
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The trick involves creating a tag type, like BackendTag, which is isomorphic
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to the backend type (generally, we want as many elements in this tag type
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as different types we need to differentiate from the existential).
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Using this type, we can add the following method to the `Backend` class:
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backendTag :: BackendTag 'b
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and implement it in each backend's instance. Now we can recover the type:
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f :: BackendSourceInfo -> String
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f (BackendSourceInfo (so :: SourceInfo b)) =
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case backendTag @b of
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PostgresTag -> "Got Postgres!"
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MSSQLTag -> "Got MSSQL!"
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But that's not all! In the context of each case branch, the type of 'so'
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is actually `so :: SourceInfo 'Postgres`, and `so :: SourceInfo 'MSSQL`
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respectively, so we can call backend-specific functions!
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IMPORTANT: Please note that this function cannot be written:
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impossible :: forall b. BackendSourceInfo -> SourceInfo b
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The problem here is that 'BackendSourceInfo' contains one specific instance
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of a 'SourceInfo', but the function 'impossible' advertises that a caller
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may pick any backend.
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However, this function may be written:
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tryCastPostgres :: BackendSourceInfo -> Maybe (SourceInfo 'Postgres)
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tryCastPostgres (BackendSourceInfo (so :: SourceInfo b)) =
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case backendTag @b of
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PostgresTag -> Just so
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MSSQLTag -> Nothing -}
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