Change fieldLens to not require Entity

[pbrisbin]

Apologies if this has been discussed before, but I'd like to ask about changing the generated fieldLens from using Entity:

fieldLens :: EntityField record field -> Lens' (Entity record) field

To just be in terms of record:

fieldLens :: EntityField record field -> Lens' record field

That latter cannot be recovered through the former without unsafe (and perhaps unlawful) shenanigans:

proposedFieldLens
    :: PersistEntity record => EntityField record field -> Lens' record field
proposedFieldLens field = unsafeEntityLens . fieldLens field

unsafeEntityLens :: Lens' record (Entity record)
unsafeEntityLens = lens (Entity undefined) $ \_ e -> entityVal e

While the former can be recovered from the latter trivially and safely:

recoveredFieldLens
    :: PersistEntity record => EntityField record field -> Lens' (Entity record) field
recoveredFieldLens field = entityValLens . proposedFieldLens field

entityValLens :: Lens' (Entity record) record
entityValLens = lens entityVal $ \x y -> x { entityVal = y }

As a concrete use case, we use Graphula, and its edit function, which takes a record -> record for setting up test data. So fieldLens SomeField .~ x (at the proposed type) is a really nice way to do it. This usage is before we insert the data, so we have only the record and not an Entity record to work on.

[parsonsmatt]

The problem is with the FooId :: EntityField Foo (Key Foo) constructor value that is always generated. That is why it must be in Entity for it to be safe.

You could probably make it safe with a type family that checks that the type isn't the key of the type -

type family SafeField rec typ :: Constraint where
    SafeField rec (Key rec) = TypeError ...
    SafeField rec typ = ()

fieldLens' :: (SafeFIeld rec typ, PersistEntity rec) => EntityFIeld rec typ -> Lens' record field

This should be fine as long as you're not trying to do a non-nullable self-reference.

[pbrisbin]

Oh jeez, I didn't even think of the key field -- that explains it.