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1. PersistError :: Text -> PersistException

   persistent-mtl	Database.Persist.Sql.Shim

   Generic Exception

2. data PersistException

   persistent-mtl	Database.Persist.Sql.Shim

   No documentation available.

3. class PersistField a

   persistent-mtl	Database.Persist.Sql.Shim

   This class teaches Persistent how to take a custom type and marshal it to and from a PersistValue, allowing it to be stored in a database.

   Examples

   Simple Newtype

   You can use newtype to add more type safety/readability to a basis type like ByteString. In these cases, just derive PersistField and PersistFieldSql:

   {-# LANGUAGE GeneralizedNewtypeDeriving #-}
   
   newtype HashedPassword = HashedPassword ByteString
   deriving (Eq, Show, PersistField, PersistFieldSql)
   

   Smart Constructor Newtype

   In this example, we create a PersistField instance for a newtype following the "Smart Constructor" pattern.

   {-# LANGUAGE GeneralizedNewtypeDeriving #-}
   import qualified Data.Text as T
   import qualified Data.Char as C
   
   -- | An American Social Security Number
   newtype SSN = SSN Text
   deriving (Eq, Show, PersistFieldSql)
   
   mkSSN :: Text -> Either Text SSN
   mkSSN t = if (T.length t == 9) && (T.all C.isDigit t)
   then Right $ SSN t
   else Left $ "Invalid SSN: " <> t
   
   instance PersistField SSN where
   toPersistValue (SSN t) = PersistText t
   fromPersistValue (PersistText t) = mkSSN t
   -- Handle cases where the database does not give us PersistText
   fromPersistValue x = Left $ "File.hs: When trying to deserialize an SSN: expected PersistText, received: " <> T.pack (show x)
   

   Tips:
   • This file contain dozens of PersistField instances you can look at for examples.
   • Typically custom PersistField instances will only accept a single PersistValue constructor in fromPersistValue.
   • Internal PersistField instances accept a wide variety of PersistValues to accomodate e.g. storing booleans as integers, booleans or strings.
   • If you're making a custom instance and using a SQL database, you'll also need PersistFieldSql to specify the type of the database column.

4. class PersistField a => PersistFieldSql a

   persistent-mtl	Database.Persist.Sql.Shim

   Tells Persistent what database column type should be used to store a Haskell type.

   Examples

   Simple Boolean Alternative

   data Switch = On | Off
   deriving (Show, Eq)
   
   instance PersistField Switch where
   toPersistValue s = case s of
   On -> PersistBool True
   Off -> PersistBool False
   fromPersistValue (PersistBool b) = if b then Right On else Right Off
   fromPersistValue x = Left $ "File.hs: When trying to deserialize a Switch: expected PersistBool, received: " <> T.pack (show x)
   
   instance PersistFieldSql Switch where
   sqlType _ = SqlBool
   

   Non-Standard Database Types

   If your database supports non-standard types, such as Postgres' uuid, you can use SqlOther to use them:

   import qualified Data.UUID as UUID
   instance PersistField UUID where
   toPersistValue = PersistLiteralEncoded . toASCIIBytes
   fromPersistValue (PersistLiteralEncoded uuid) =
   case fromASCIIBytes uuid of
   Nothing -> Left $ "Model/CustomTypes.hs: Failed to deserialize a UUID; received: " <> T.pack (show uuid)
   Just uuid' -> Right uuid'
   fromPersistValue x = Left $ "File.hs: When trying to deserialize a UUID: expected PersistLiteralEncoded, received: "-- >  <> T.pack (show x)
   
   instance PersistFieldSql UUID where
   sqlType _ = SqlOther "uuid"
   

   User Created Database Types

   Similarly, some databases support creating custom types, e.g. Postgres' DOMAIN and ENUM features. You can use SqlOther to specify a custom type:

   CREATE DOMAIN ssn AS text
   CHECK ( value ~ '^[0-9]{9}$');
   

   instance PersistFieldSQL SSN where
   sqlType _ = SqlOther "ssn"
   

   CREATE TYPE rainbow_color AS ENUM ('red', 'orange', 'yellow', 'green', 'blue', 'indigo', 'violet');
   

   instance PersistFieldSQL RainbowColor where
   sqlType _ = SqlOther "rainbow_color"
   

5. data PersistFilter

   persistent-mtl	Database.Persist.Sql.Shim

   No documentation available.

6. PersistForeignConstraintUnmet :: Text -> PersistException

   persistent-mtl	Database.Persist.Sql.Shim

   No documentation available.

7. PersistInt64 :: Int64 -> PersistValue

   persistent-mtl	Database.Persist.Sql.Shim

   No documentation available.

8. PersistInvalidField :: Text -> PersistException

   persistent-mtl	Database.Persist.Sql.Shim

   No documentation available.

9. PersistList :: [PersistValue] -> PersistValue

   persistent-mtl	Database.Persist.Sql.Shim

   No documentation available.

10. pattern PersistLiteral :: ByteString -> PersistValue

    persistent-mtl	Database.Persist.Sql.Shim

    This pattern synonym used to be a data constructor on PersistValue, but was changed into a catch-all pattern synonym to allow backwards compatiblity with database types. See the documentation on PersistDbSpecific for more details.

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