Modify, add, or remove constructors and fields in generic types, to be used with generic implementations.
Here is a simple record type equipped with a checksum
function:
data Foo = Foo { x, y, z :: Int }
deriving (Eq, Generic, Show)
checksum :: Foo -> Checksum
Let's encode it as a JSON object with an extra "checksum"
key,
looking like this, where X
, Y
, Z
are integers:
{ "x": X
, "y": Y
, "z": Z
, "checksum": X + Y + Z
}
We use genericParseJSON
/genericToJSON
to convert between JSON values
and a generic 4-field record, and removeRField
/insertRField
to
convert between that generic 4-field record and the 3-field Foo
.
When decoding, we check the checksum and then throw it away.
instance FromJSON Foo where
parseJSON v = do
r <- genericParseJSON defaultOptions v
-- r: a generic 4-field record {x,y,z,checksum} (checksum at index 3).
let (cs, f) = (fmap fromOR . removeRField @"checksum" @3 . toOR') r
-- removeRField @"checksum" @3: split out the checksum field
-- from the three other fields. (cs, f) :: (Checksum, Foo)
if checksum f == cs then
pure f
else
fail "Checksum failed"
When encoding, we must compute the checksum to write it out. We put the
checksum in a pair (checksum f, f)
with the original record, and
insertRField
can then wrap it into a 4-field record passed into
genericToJSON
.
instance ToJSON Foo where
toJSON f =
(genericToJSON defaultOptions . fromOR' . insertRField @"checksum" @3 . fmap toOR)
(checksum f, f)
-
Surgery for data types, introductory blog post with another example.
-
The
examples/
directory in the source repo.