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1. pattern PersistDbSpecific :: ByteString -> PersistValue

   esqueleto	Database.Esqueleto

   This pattern synonym used to be a data constructor for the PersistValue type. It was changed to be a pattern so that JSON-encoded database values could be parsed into their corresponding values. You should not use this, and instead prefer to pattern match on PersistLiteral_ directly. If you use this, it will overlap a patern match on the 'PersistLiteral_, PersistLiteral, and PersistLiteralEscaped patterns. If you need to disambiguate between these constructors, pattern match on PersistLiteral_ directly.

2. PersistDouble :: Double -> PersistValue

   esqueleto	Database.Esqueleto

   No documentation available.

3. class (PersistField Key record, ToJSON Key record, FromJSON Key record, Show Key record, Read Key record, Eq Key record, Ord Key record) => PersistEntity record

   esqueleto	Database.Esqueleto

   Persistent serialized Haskell records to the database. A Database Entity (A row in SQL, a document in MongoDB, etc) corresponds to a Key plus a Haskell record. For every Haskell record type stored in the database there is a corresponding PersistEntity instance. An instance of PersistEntity contains meta-data for the record. PersistEntity also helps abstract over different record types. That way the same query interface can return a PersistEntity, with each query returning different types of Haskell records. Some advanced type system capabilities are used to make this process type-safe. Persistent users usually don't need to understand the class associated data and functions.

4. type family PersistEntityBackend record

   esqueleto	Database.Esqueleto

   Persistent allows multiple different backends (databases).

5. PersistError :: Text -> PersistException

   esqueleto	Database.Esqueleto

   Generic Exception

6. data PersistException

   esqueleto	Database.Esqueleto

   No documentation available.

7. class PersistField a

   esqueleto	Database.Esqueleto

   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.

8. class PersistField a => PersistFieldSql a

   esqueleto	Database.Esqueleto

   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"
   

9. data PersistFilter

   esqueleto	Database.Esqueleto

   No documentation available.

10. PersistForeignConstraintUnmet :: Text -> PersistException

    esqueleto	Database.Esqueleto

    No documentation available.

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