DerivingVia for Generic table codecs

[chshersh]

I've tried to use fancy tricks for table codecs with generics. You can find my commented code here:

tomland/src/Toml/Codec/Generic.hs

Lines 406 to 449 in 536a5e6

	{-
	TODO: uncomment when higher-kinded roles will be implemented
	* https://github.com/ghc-proposals/ghc-proposals/pull/233
	{- | @newtype@ for generic deriving of 'HasCodec' typeclass for custom data
	types that should we wrapped into separate table. Use it only for data types
	that are fields of another data types.
	@
	data Person = Person
	{ personName :: Text
	, personAddress :: Address
	} deriving (Generic)
	data Address = Address
	{ addressStreet :: Text
	, addressHouse :: Int
	} deriving (Generic)
	deriving HasCodec via TomlTable Address
	personCodec :: TomlCodec Person
	personCodec = genericCodec
	@
	@personCodec@ corresponds to the TOML of the following structure:
	@
	name = "foo"
	[address]
	street = \"Bar\"
	house = 42
	@
	-}
	newtype TomlTable a = TomlTable
	{ unTomlTable :: a
	}
	instance (Generic a, GenericCodec (Rep a)) => HasCodec (TomlTable a) where
	hasCodec :: Key -> TomlCodec (TomlTable a)
	hasCodec = Toml.diwrap . Toml.table (genericCodec @a)
	instance (Generic a, GenericCodec (Rep a)) => HasItemCodec (TomlTable a) where
	hasItemCodec = Right $ Toml.diwrap $ genericCodec @a
	-}

It wasn't possible to implement this feature back then. But thanks to @vrom911 nice work on simplifying the Codec type, I think we should be able to implement that feature again.

[vrom911]

Currently, I am getting

    • Couldn't match type ‘TomlTable Address’ with ‘Address’
        arising from the coercion of the method ‘hasCodec’
          from type ‘Toml.Key -> TomlCodec (TomlTable Address)’
            to type ‘Toml.Key -> TomlCodec Address’
    • When deriving the instance for (HasCodec Address)
    |
182 |       deriving HasCodec via (TomlTable Address)
    |    

With the example from docs. I don't really know how to solve this issue. Do you have any ideas, @chshersh ?

(link to the branch)

[chshersh]

@vrom911 Ah, sh*t, here we go again. The error is different, but it doesn't tell me how to solve the issue. Well, I guess not in this release...

[chshersh]

I've asked on SO:

• https://stackoverflow.com/questions/61840227/cannot-coerce-data-type-with-reader-as-a-field

[chshersh]

Should be possible to do after #311 🙂