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https://github.com/lysxia/generic-data

Generic data types in Haskell, utilities for GHC.Generics
https://github.com/lysxia/generic-data

deriving generics haskell

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Generic data types in Haskell, utilities for GHC.Generics

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# Generic data types in Haskell



[![Hackage](https://img.shields.io/hackage/v/generic-data.svg)](https://hackage.haskell.org/package/generic-data)

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Utilities for `GHC.Generics`.



## Generic deriving for standard classes



### Example: generically deriving Semigroup instances for products



Semi-automatic method using `gmappend`



```haskell

data Foo a = Bar [a] [a] deriving Generic



instance Semigroup (Foo a) where

  (<>) = gmappend

```



This library also synergizes with the `DerivingVia` extension

(introduced in GHC 8.6), thanks to the `Generically` newtype.



```haskell

data Foo a = Bar [a] [a]

  deriving Generic

  deriving Semigroup via (Generically (Foo a))

```



These examples can be found in `test/example.hs`.



---



Note for completeness, the first example uses the following extensions and

imports:



```haskell

{-# LANGUAGE DeriveGeneric #-}



-- base

import Data.Semigroup (Semigroup(..))



-- generic-data

import Generic.Data (Generic, gmappend)

import Generic.Data.Orphans ()

```



The second example makes these additions on top:



```haskell

{-# LANGUAGE

    DerivingStrategies,

    DerivingVia #-}  -- since GHC 8.6.1



-- In addition to the previous imports

import Generic.Data (Generically(..))

```



### Supported classes



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`, `Read` (derivable by the standard);

- `Functor`, `Foldable`, `Traversable` (derivable via extensions,

  `DeriveFunctor`, etc.).



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 (microsurgeries) 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 Generic.Data (Generic, gshowsPrec)

import Generic.Data.Microsurgery (toData, derecordify)



-- An example record type

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

```



Alternatively, using `DerivingVia`:



```haskell

{-# LANGUAGE DeriveGeneric, DerivingVia #-}

import Generic.Data (Generic)  -- Reexported from GHC.Generics



-- Constructors must be visible to use DerivingVia

import Generic.Data.Microsurgery (Surgery, Surgery'(..), Generically(..), Derecordify)



data V = V { v1 :: Int, v2 :: Int }

  deriving Generic

  deriving Show via (Surgery Derecordify V)



-- show (V {v1 = 3, v2 = 4}) = "V 3 4"

```



---



## 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 heavier 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` (not in generic-data).



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.

- [coercible-utils](https://hackage.haskell.org/package/coercible-utils):

  utilities for coercible types.



---



## 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!