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55 lines
1.5 KiB
Haskell
55 lines
1.5 KiB
Haskell
module Hasura.SQL.Backend
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( BackendType(..)
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, BackendTag(..)
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, reify
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, backendName
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, supportedBackends
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) where
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import Hasura.Prelude
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import Data.GADT.Compare (GCompare (..), GEq (..), GOrdering (GEQ, GGT, GLT))
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import Type.Reflection (type (:~:) (Refl))
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import Unsafe.Coerce (unsafeCoerce)
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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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backendName :: BackendTag b -> Text
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backendName = \case
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PostgresTag -> "postgres"
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MSSQLTag -> "mssql"
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supportedBackends :: [Text]
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supportedBackends = tshow <$> [minBound @BackendType .. maxBound]
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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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