PostgreSQL – How to specify different column types according to the database type of Haskell GROUNDHOG?

instance PrimitivePersistField UUID where
 toPrimitivePersistValue _ uuid = PersistString $show uuid
 fromPrimitivePersistValue _ (PersistString a) = fromJust $UUIDmethods.fromString a

instance PersistField UUID where
 persistName _ = "UUID"
 toPersistValues ​​= primToPersistValue
 fromPersistValues ​​= primFromPersistValue
 dbType db _ = case db of
 Postgresql _ -> DbTypePrimitive (DbOther$OtherTypeDef [Left "uuid "]) False Nothing Nothing
 _ -> DbTypePrimitive (DbOther $OtherTypeDef [Left "varchar"]) False Nothing Nothing

Couldn't match expected type'proxy db'
 with actual typ e'Postgresql'
 Relevant bindings include
 db :: proxy db (bound at basicGroundhog.hs:34:10)
 dbType :: proxy db -> UUID -> DbType
 (bound at basicGroundhag.hs:34:3)
 In the pattern: Postgresql _
 In a case alternative:
 Postgresql _
 -> DbTypePrimitive
 (DbOther $ OtherTypeDef [Left "uuid"]) False Nothing Nothing
 In the expression:
 case db of {
 Postgresql _
 -> DbTypePrimitive
 (DbOther $OtherTypeDef [Left "uuid"]) False Nothing Nothing
 _ -> DbTypePrimitive
 (DbOther $OtherTypeDef [Left "varchar"]) False Nothing Nothing }

dbType db _ = case backendName db of
 "postgresql" -> DbTypePrimitive (DbOther $ OtherTypeDef [Left "uuid"]) False Nothing Nothing
 _ -> DbTypePrimitive DbString False Nothing Nothing

Using the Groundhog library in Haskell, I hope to implement a Use the column type of “uuid” when the backend is Postgresql, otherwise just use “varchar” for any other backend. Although this should be possible according to the comments in Groundhog.Core, I’m not entirely sure how to get from the proxy db Unpack the dbType in the dbType, and there is no such example in groundhog-examples, because the column types have hard-coded examples in those.

I want to get some help in case matching in Postgresql, I will Solve the remaining problems after sorting. This is where I am:

instance PrimitivePersistField UUID where
 toPrimitivePersistValue _ uuid = PersistString $show uuid
 fromPrimitivePersistValue _ (PersistString a) = fromJust $UUIDmethods.fromString a

instance PersistField UUID where
 persistName _ = "UUID"
 toPersistValues ​​= primToPersistValue
 fromPersistValues ​​= prim br /> dbType db _ = case db of
 Postgresql _ -> DbTypePrimitive (DbOther $OtherTypeDef [Left "uuid"]) False Nothing Nothing
 _ -> DbTypePrimitive (DbOther $OtherTypeDef [Left "varchar" ]) False Nothing Nothing

This problem occurred at compile time :

Couldn't match expected type'proxy db'
 with actual type'Postgresql'
 Relevant bindings include
 db :: proxy db (bound at basicGroundhog.hs:34:10)
 dbType :: proxy db -> UUID -> DbType
 (bound at basicGroundhag.hs:34:3)
 In the pattern: Postgresql _
 In a case alternative:
 Postgresql _
 -> DbTypePrimitive
 (DbOther $OtherTypeDef [Left "uuid"]) False Nothing Nothing
 In the expression: 
 case db of {
 Postgresql _
 -> DbTypePrimitive
 (DbOther $OtherTypeDef [Left "uuid"]) False Nothing Nothing
 _ -> DbTypePrimitive
 (DbOther $OtherTypeDef [Left "varchar"]) False Nothing Nothing }

You can use backendName to check the database at runtime. It is best to use DbString for The default is so that groundhog can choose a more suitable type. Varchar without a maximum length declaration is invalid in MySQL.

dbType db _ = case backendName db of
 "postgresql" -> DbTypePrimitive (DbOther $OtherTypeDef [Left "uuid"]) False Nothing Nothing
 _ -> DbTypePrimitive DbString False Nothing Nothing