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Within LTS Haskell 22.13 (ghc-9.6.4)
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persistent Database.Persist.Class 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 ErrorMessage deriving (Eq, Show, PersistFieldSql) mkSSN :: ErrorMessage -> Either ErrorMessage 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.
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