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Use type families instead of empty classes (#220)

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* Use type families instead of empty classes

* It's not possible to expand a closed type family.

Co-authored-by: Matt Parsons <parsonsmatt@gmail.com>

Co-authored-by: Matt Parsons <parsonsmatt@gmail.com>
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Georgi Lyubenov 2020-10-30 02:04:28 +02:00 committed by GitHub
parent b35713c09f
commit eb91208e94
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1 changed files with 11 additions and 10 deletions
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21
src/Database/Esqueleto/Experimental.hs
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@ -220,6 +220,7 @@ import qualified Control.Monad.Trans.Writer as W
#if __GLASGOW_HASKELL__ < 804 #if __GLASGOW_HASKELL__ < 804
import Data.Semigroup import Data.Semigroup
#endif #endif
import Data.Kind (Constraint)
import Data.Proxy (Proxy(..)) import Data.Proxy (Proxy(..))
import qualified Data.Text.Lazy.Builder as TLB import qualified Data.Text.Lazy.Builder as TLB
import Database.Esqueleto.Internal.Internal hiding (From, from, on) import Database.Esqueleto.Internal.Internal hiding (From, from, on)
@ -719,15 +720,15 @@ data From a where
-- | Constraint for `on`. Ensures that only types that require an `on` can be used on -- | Constraint for `on`. Ensures that only types that require an `on` can be used on
-- the left hand side. This was previously reusing the ToFrom class which was actually -- the left hand side. This was previously reusing the ToFrom class which was actually
-- a bit too lenient as it allowed to much. Expanding this class should not be needed. -- a bit too lenient as it allowed to much.
-- --
-- @since 3.4.0.0 -- @since 3.4.0.0
class ValidOnClauseValue a where type family ValidOnClauseValue a :: Constraint where
instance ValidOnClauseValue (From a) where ValidOnClauseValue (From a) = ()
instance ValidOnClauseValue (SqlQuery a) where ValidOnClauseValue (SqlQuery a) = ()
instance ValidOnClauseValue (SqlSetOperation a) where ValidOnClauseValue (SqlSetOperation a) = ()
instance ValidOnClauseValue (a -> SqlQuery b) where ValidOnClauseValue (a -> SqlQuery b) = ()
instance {-# OVERLAPPABLE #-} (TypeError ('Text "Illegal use of ON")) => ValidOnClauseValue a where ValidOnClauseValue _ = TypeError ('Text "Illegal use of ON")
-- | An @ON@ clause that describes how two tables are related. This should be -- | An @ON@ clause that describes how two tables are related. This should be
-- used as an infix operator after a 'JOIN'. For example, -- used as an infix operator after a 'JOIN'. For example,
@ -772,9 +773,9 @@ type family IsLateral a where
IsLateral (a -> SqlQuery b) = Lateral IsLateral (a -> SqlQuery b) = Lateral
IsLateral a = NotLateral IsLateral a = NotLateral
class ErrorOnLateral a where type family ErrorOnLateral a :: Constraint where
instance (TypeError ('Text "LATERAL can only be used for INNER, LEFT, and CROSS join kinds.")) => ErrorOnLateral (a -> SqlQuery b) where ErrorOnLateral (a -> SqlQuery b) = TypeError ('Text "LATERAL can only be used for INNER, LEFT, and CROSS join kinds.")
instance {-# OVERLAPPABLE #-} ErrorOnLateral a where ErrorOnLateral _ = ()
{-- Type class magic to allow the use of the `InnerJoin` family of data constructors in from --} {-- Type class magic to allow the use of the `InnerJoin` family of data constructors in from --}
class ToFrom a where class ToFrom a where