DerivingVia

Implies: DerivingStrategies
Since: 8.6.1
This allows deriving
a class instance for a type by specifying another type that is already an instance of that class. This only makes sense if the methods have identical runtime representations, in the sense that coerce (see The Coercible
constraint) can convert the existing implementation into the desired implementation. The generated code will be rejected with a type error otherwise.
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 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 catchall 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 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 = Kleisli (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 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)