Hoogle Search

Within LTS Haskell 24.40 (ghc-9.10.3)

Note that Stackage only displays results for the latest LTS and Nightly snapshot. Learn more.

Exact lookup

1. type family PersistConfigPool c

   esqueleto	Database.Esqueleto.Legacy

   No documentation available.

2. class PersistCore backend

   esqueleto	Database.Esqueleto.Legacy

   No documentation available.

3. PersistDay :: Day -> PersistValue

   esqueleto	Database.Esqueleto.Legacy

   No documentation available.

4. pattern PersistDbSpecific :: ByteString -> PersistValue

   esqueleto	Database.Esqueleto.Legacy

   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.

5. PersistDouble :: Double -> PersistValue

   esqueleto	Database.Esqueleto.Legacy

   No documentation available.

6. 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.Legacy

   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.

7. type family PersistEntityBackend record

   esqueleto	Database.Esqueleto.Legacy

   Persistent allows multiple different backends (databases).

8. PersistError :: Text -> PersistException

   esqueleto	Database.Esqueleto.Legacy

   Generic Exception

9. data PersistException

   esqueleto	Database.Esqueleto.Legacy

   No documentation available.

10. class PersistField a

    esqueleto	Database.Esqueleto.Legacy

    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.

Page 493 of many | Previous | Next