Language.Haskell.TH.Instances.Defaults

Documentation

defaulting :: Name -> Q Exp -> Q [Dec]

Give a default for a typeclass method that will be utilized by the instances quasiquoter. The default will be implicitly brought into scope when the module is imported, like typeclass instances.

Example:

module Data.Traversable.Defaults (module X) where
import Data.Traversable as X
import Data.Functor.Identity as X
import Data.Functor.Const as X
defaultMethod 'fmap [|\f -> runIdentity . traverse (Identity . f)|]
defaultMethod 'foldmap [|\f -> getConst . traverse (Const . f)|]

module MyData where
import Data.Traversable.Defaults

data Foo a = Foo a a
[instances| Travesable Foo where traverse f (Foo a a') = Foo <$> f a <*> f a'|]

will generate

instance Functor Foo where fmap = \f -> runIdentity . traverse (Identity . f)
instance Foldable Foo where foldMap = \f -> getConst . traverse (Const . f)
instance Travesable Foo where traverse f (Foo a a') = Foo <$> f a <*> f a'