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README
Surgery for generic data types 
Modify, add, or remove constructors and fields in generic types, to be used with generic implementations.
Example
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.
Remove field
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"
Insert field
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)
See also
Surgery for data types, introductory blog post with another example.
The
examples/directory in the source repo.