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Within LTS Haskell 24.3 (ghc-9.10.2)
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persistent Database.Persist.Class.PersistEntity 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.
module Database.Persist.Class.
PersistField No documentation available.
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persistent Database.Persist.Class.PersistField 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.
module Database.Persist.Class.
PersistQuery No documentation available.
class (PersistCore backend, PersistStoreRead backend) =>
PersistQueryRead backendpersistent Database.Persist.Class.PersistQuery Backends supporting conditional read operations.
class (PersistQueryRead backend, PersistStoreWrite backend) =>
PersistQueryWrite backendpersistent Database.Persist.Class.PersistQuery Backends supporting conditional write operations
module Database.Persist.Class.
PersistStore No documentation available.
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persistent Database.Persist.Class.PersistStore No documentation available.
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persistent Database.Persist.Class.PersistStore A convenient alias for common type signatures
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persistent Database.Persist.Class.PersistStore No documentation available.