# Surgery for generic data types [](https://hackage.haskell.org/package/generic-data-surgery) [](https://travis-ci.org/Lysxia/generic-data-surgery)
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:
```haskell
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.
```haskell
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`.
```haskell
instance ToJSON Foo where
toJSON f =
(genericToJSON defaultOptions . fromOR' . insertRField @"checksum" @3 . fmap toOR)
(checksum f, f)
```
See also the
[`examples/`](https://github.com/Lysxia/generic-data-surgery/tree/master/examples)
directory in the source repo.