Generic data types in Haskell
Utilities for GHC.Generics
.
Generic deriving for standard classes
Example: generically deriving Semigroup instances for products
Semi-automatic method using gmappend
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.
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:
{-# LANGUAGE DeriveGeneric #-}
-- base
import Data.Semigroup (Semigroup(..))
import GHC.Generics
-- generic-data
import Generic.Data (gmappend)
import Generic.Data.Orphans ()
The second example makes these additions on top:
{-# 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
(derivable by the standard);
Functor
, Foldable
, Traversable
(derivable 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.
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,
and suprisingly, in
generic-lens and
one-liner.
For more details, see also:
Surgery example
Derive an instance of Show
generically for a record type,
but as if it were not a record.
{-# LANGUAGE DeriveGeneric #-}
import GHC.Generic (Generic)
import Generic.Data (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
:
{-# LANGUAGE DeriveGeneric, DerivingVia #-}
import GHC.Generic (Generic)
-- 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: generic
Semigroup
, Monoid
, but with a heavier dependency footprint.
- transformers-compat:
generic
Eq1
, Ord1
, Show1
.
- 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.
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!