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.. _deriving-via:
Deriving via
------------
.. extension:: DerivingVia
:shortdesc: Enable deriving instances ``via`` types of the same runtime
representation.
Implies :extension:`DerivingStrategies`.
:implies: :extension:`DerivingStrategies`
:since: 8.6.1
This allows ``deriving`` a class instance for a type by specifying
another type of equal runtime representation (such that there exists a
``Coercible`` instance between the two: see :ref:`coercible`) that is
already an instance of the that class.
:extension:`DerivingVia` is indicated by the use of the ``via``
deriving strategy. ``via`` requires specifying another type (the ``via`` type)
to ``coerce`` through. For example, this code: ::
{-# LANGUAGE DerivingVia #-}
import Numeric
newtype Hex a = Hex a
instance (Integral a, Show a) => Show (Hex a) where
show (Hex a) = "0x" ++ showHex a ""
newtype Unicode = U Int
deriving Show
via (Hex Int)
-- >>> euroSign
-- 0x20ac
euroSign :: Unicode
euroSign = U 0x20ac
Generates the following instance ::
instance Show Unicode where
show :: Unicode -> String
show = Data.Coerce.coerce
@(Hex Int -> String)
@(Unicode -> String)
show
This extension generalizes :extension:`GeneralizedNewtypeDeriving`. To
derive ``Num Unicode`` with GND (``deriving newtype Num``) it must
reuse the ``Num Int`` instance. With ``DerivingVia``, we can explicitly
specify the representation type ``Int``: ::
newtype Unicode = U Int
deriving Num
via Int
deriving Show
via (Hex Int)
euroSign :: Unicode
euroSign = 0x20ac
Code duplication is common in instance declarations. A familiar
pattern is lifting operations over an ``Applicative`` functor.
Instead of having catch-all instances for ``f a`` which overlap
with all other such instances, like so: ::
instance (Applicative f, Semigroup a) => Semigroup (f a) ..
instance (Applicative f, Monoid a) => Monoid (f a) ..
We can instead create a newtype ``App``
(where ``App f a`` and ``f a`` are represented the same in memory)
and use :extension:`DerivingVia` to explicitly enable uses of this
pattern: ::
{-# LANGUAGE DerivingVia, DeriveFunctor, GeneralizedNewtypeDeriving #-}
import Control.Applicative
newtype App f a = App (f a) deriving newtype (Functor, Applicative)
instance (Applicative f, Semigroup a) => Semigroup (App f a) where
(<>) = liftA2 (<>)
instance (Applicative f, Monoid a) => Monoid (App f a) where
mempty = pure mempty
data Pair a = MkPair a a
deriving stock
Functor
deriving (Semigroup, Monoid)
via (App Pair a)
instance Applicative Pair where
pure a = MkPair a a
MkPair f g <*> MkPair a b = MkPair (f a) (g b)
Note that the ``via`` type does not have to be a ``newtype``.
The only restriction is that it is coercible with the
original data type. This means there can be arbitrary nesting of newtypes,
as in the following example: ::
newtype Kleisli m a b = (a -> m b)
deriving (Semigroup, Monoid)
via (a -> App m b)
Here we make use of the ``Monoid ((->) a)`` instance.
When used in combination with :extension:`StandaloneDeriving` we swap the order
for the instance we base our derivation on and the instance we define e.g.: ::
deriving via (a -> App m b) instance Monoid (Kleisli m a b)
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