Use DeriveLift to generate persistent-template's Lift instances

[RyanGlScott]

GHC 8.0 and later come with the DeriveLift extension for deriving instances of Language.Haskell.TH.Syntax.Lift. persistent-template supports GHC 8.2 and up, so it is able to make use of this. Not only does DeriveLift make for much shorter code, but it also fixes warnings that you get when compiling persistent-template with GHC 8.10 or later:

[1 of 1] Compiling Database.Persist.TH ( Database/Persist/TH.hs, interpreted )

Database/Persist/TH.hs:267:10: warning: [-Wmissing-methods]
    • No explicit implementation for
        ‘liftTyped’
    • In the instance declaration for ‘Lift SqlTypeExp’
    |
267 | instance Lift SqlTypeExp where
    |          ^^^^^^^^^^^^^^^

Database/Persist/TH.hs:278:10: warning: [-Wmissing-methods]
    • No explicit implementation for
        ‘liftTyped’
    • In the instance declaration for ‘Lift FieldsSqlTypeExp’
    |
278 | instance Lift FieldsSqlTypeExp where
    |          ^^^^^^^^^^^^^^^^^^^^^

Database/Persist/TH.hs:284:10: warning: [-Wmissing-methods]
    • No explicit implementation for
        ‘liftTyped’
    • In the instance declaration for ‘Lift FieldSqlTypeExp’
    |
284 | instance Lift FieldSqlTypeExp where
    |          ^^^^^^^^^^^^^^^^^^^^

Database/Persist/TH.hs:288:10: warning: [-Wmissing-methods]
    • No explicit implementation for
        ‘liftTyped’
    • In the instance declaration for ‘Lift EntityDefSqlTypeExp’
    |
288 | instance Lift EntityDefSqlTypeExp where
    |          ^^^^^^^^^^^^^^^^^^^^^^^^

Database/Persist/TH.hs:295:10: warning: [-Wmissing-methods]
    • No explicit implementation for
        ‘liftTyped’
    • In the instance declaration for ‘Lift ReferenceDef’
    |
295 | instance Lift ReferenceDef where
    |          ^^^^^^^^^^^^^^^^^

Database/Persist/TH.hs:302:10: warning: [-Wmissing-methods]
    • No explicit implementation for
        ‘liftTyped’
    • In the instance declaration for ‘Lift EmbedEntityDef’
    |
302 | instance Lift EmbedEntityDef where
    |          ^^^^^^^^^^^^^^^^^^^

Database/Persist/TH.hs:305:10: warning: [-Wmissing-methods]
    • No explicit implementation for
        ‘liftTyped’
    • In the instance declaration for ‘Lift EmbedFieldDef’
    |
305 | instance Lift EmbedFieldDef where
    |          ^^^^^^^^^^^^^^^^^^

Database/Persist/TH.hs:1693:10: warning: [-Wmissing-methods]
    • No explicit implementation for
        ‘liftTyped’
    • In the instance declaration for ‘Lift EntityDef’
     |
1693 | instance Lift EntityDef where
     |          ^^^^^^^^^^^^^^

Database/Persist/TH.hs:1709:10: warning: [-Wmissing-methods]
    • No explicit implementation for
        ‘liftTyped’
    • In the instance declaration for ‘Lift FieldDef’
     |
1709 | instance Lift FieldDef where
     |          ^^^^^^^^^^^^^

Database/Persist/TH.hs:1712:10: warning: [-Wmissing-methods]
    • No explicit implementation for
        ‘liftTyped’
    • In the instance declaration for ‘Lift UniqueDef’
     |
1712 | instance Lift UniqueDef where
     |          ^^^^^^^^^^^^^^

Database/Persist/TH.hs:1715:10: warning: [-Wmissing-methods]
    • No explicit implementation for
        ‘liftTyped’
    • In the instance declaration for ‘Lift CompositeDef’
     |
1715 | instance Lift CompositeDef where
     |          ^^^^^^^^^^^^^^^^^

Database/Persist/TH.hs:1718:10: warning: [-Wmissing-methods]
    • No explicit implementation for
        ‘liftTyped’
    • In the instance declaration for ‘Lift ForeignDef’
     |
1718 | instance Lift ForeignDef where
     |          ^^^^^^^^^^^^^^^

Database/Persist/TH.hs:1724:10: warning: [-Wmissing-methods]
    • No explicit implementation for
        ‘liftTyped’
    • In the instance declaration for ‘Lift FieldCascade’
     |
1724 | instance Lift FieldCascade where
     |          ^^^^^^^^^^^^^^^^^

Database/Persist/TH.hs:1730:10: warning: [-Wmissing-methods]
    • No explicit implementation for
        ‘liftTyped’
    • In the instance declaration for ‘Lift CascadeAction’
     |
1730 | instance Lift CascadeAction where
     |          ^^^^^^^^^^^^^^^^^^

Database/Persist/TH.hs:1736:10: warning: [-Wmissing-methods]
    • No explicit implementation for
        ‘liftTyped’
    • In the instance declaration for ‘Lift HaskellName’
     |
1736 | instance Lift HaskellName where
     |          ^^^^^^^^^^^^^^^^

Database/Persist/TH.hs:1738:10: warning: [-Wmissing-methods]
    • No explicit implementation for
        ‘liftTyped’
    • In the instance declaration for ‘Lift DBName’
     |
1738 | instance Lift DBName where
     |          ^^^^^^^^^^^

Database/Persist/TH.hs:1740:10: warning: [-Wmissing-methods]
    • No explicit implementation for
        ‘liftTyped’
    • In the instance declaration for ‘Lift FieldType’
     |
1740 | instance Lift FieldType where
     |          ^^^^^^^^^^^^^^

Database/Persist/TH.hs:1746:10: warning: [-Wmissing-methods]
    • No explicit implementation for
        ‘liftTyped’
    • In the instance declaration for ‘Lift PersistFilter’
     |
1746 | instance Lift PersistFilter where
     |          ^^^^^^^^^^^^^^^^^^

Database/Persist/TH.hs:1757:10: warning: [-Wmissing-methods]
    • No explicit implementation for
        ‘liftTyped’
    • In the instance declaration for ‘Lift PersistUpdate’
     |
1757 | instance Lift PersistUpdate where
     |          ^^^^^^^^^^^^^^^^^^

Database/Persist/TH.hs:1765:10: warning: [-Wmissing-methods]
    • No explicit implementation for
        ‘liftTyped’
    • In the instance declaration for ‘Lift SqlType’
     |
1765 | instance Lift SqlType where
     |          ^^^^^^^^^^^^

This is because DeriveLift fills in implementations of liftTyped, a method that was introduced to Lift in template-haskell-2.16.0.0 (bundled with GHC 8.10).

Note that not all Lift instances currently used by persistent-template can be derived, since the code that DeriveLift would produce would not be equivalent to some of the more exotic hand-written instances. These include the Lift instances for SqlTypeExp, FieldsSqlTypeExp, FieldSqlTypeExp, and EntityDefSqlTypeExp. For these instance, I simply define liftTyped implementations using CPP.

After submitting your PR:

• Update the Changelog.md file with a link to your PR
• Check that CI passes (or if it fails, for reasons unrelated to your change, like CI timeouts)

[parsonsmatt]

this is awesome, thanks! 😄