# Generic data types in Haskell [](https://hackage.haskell.org/package/generic-data) [](https://travis-ci.org/Lysxia/generic-data)
Utilities for `GHC.Generics`.
## Generic deriving for standard classes
```haskell
{-# LANGUAGE DeriveGeneric #-}
-- base
import Data.Semigroup (Semigroup(..))
import GHC.Generics
-- generic-data
import Generic.Data (gmappend, Generically(..))
import Generic.Data.Orphans ()
data Foo a = Bar [a] [a] deriving Generic
instance Semigroup (Foo a) where
(<>) = gmappend
-- also with some additional extensions --
{-# LANGUAGE
DerivingStrategies,
DerivingVia #-} -- since GHC 8.6.1
data Foo a = Bar [a] [a]
deriving Generic
deriving Semigroup via (Generically (Foo a))
-- This example can be found in test/example.hs
```
Supported classes that GHC currently can't derive: `Semigroup`, `Monoid`,
`Applicative`, `Alternative`, `Eq1`, `Ord1`, `Show1`.
Other classes from base are also supported, even though GHC can already derive
them:
- `Eq`, `Ord`, `Enum`, `Bounded`, `Show` (standard);
- `Functor`, `Foldable`, `Traversable` (via extensions, `DeriveFunctor`, etc.).
(`Read` is currently not implemented.)
To derive type classes outside of the standard library, it might be worth
taking a look at [one-liner](https://hackage.haskell.org/package/one-liner).
## Type metadata
Extract type names, constructor names, number and arities of constructors, etc..
## Type surgery
generic-data offers simple operations on generic representations.
More surgeries can be found in
[generic-data-surgery](https://hackage.haskell.org/package/generic-data-surgery),
and suprisingly, in
[generic-lens](https://hackage.haskell.org/package/generic-lens) and
[one-liner](https://hackage.haskell.org/package/one-liner).
For more details, see also:
- the module `Generic.Data.Microsurgery`;
- the files `test/lens-surgery.hs` and `one-liner-surgery.hs`.
### Surgery example
Derive an instance of `Show` generically for a record type,
but as if it were not a record.
```haskell
{-# LANGUAGE DeriveGeneric #-}
import GHC.Generic (Generic)
import Generic.Data (gshowsPrec)
import Generic.Data.Microsurgery (toData, derecordify)
newtype T = T { unT :: Int } deriving Generic
-- Naively deriving Show would result in this being shown:
--
-- show (T 3) = "T {unT = 3}"
--
-- But instead, with a simple surgery, unrecordify, we can forget T was
-- declared as a record:
--
-- show (T 3) = "T 3"
instance Show T where
showsPrec n = gshowsPrec n . derecordify . toData
-- This example can be found in test/microsurgery.hs
```
---
## Related links
generic-data aims to subsume generic deriving features of the following
packages:
- [semigroups](https://hackage.haskell.org/package/semigroups): generic
`Semigroup`, `Monoid`, but with a heavy dependency footprint.
- [transformers-compat](https://hackage.haskell.org/package/transformers-compat):
generic `Eq1`, `Ord1`, `Show1`.
- [generic-deriving](https://hackage.haskell.org/package/generic-deriving):
doesn't derive the classes in base (defines clones of these classes as a toy
example); has Template Haskell code to derive `Generic`.
Here are other relevant links.
- [deriving-compat](https://hackage.haskell.org/package/deriving-compat):
deriving with Template Haskell.
- [one-liner](https://hackage.haskell.org/package/one-liner): another approach
to using `GHC.Generics` to derive instances of many type classes, including
but not restricted to the above classes (this is done in
[one-liner-instances](https://hackage.haskell.org/package/one-liner-instances)).
- [singletons](https://hackage.haskell.org/package/singletons),
[first-class-families](https://hackage.haskell.org/package/first-class-families)
(second one written by me)
libraries for dependently-typed programming in Haskell.
---
## Internal module policy
Modules under `Generic.Data.Internal` are not subject to any versioning policy.
Breaking changes may apply to them at any time.
If something in those modules seems useful, please report it or create a pull
request to export it from an external module.
---
All contributions are welcome. Open an issue or a pull request on Github!