Haskell Language Lens Classy Lenses
Example
In addition to the standard makeLenses function for generating Lenses, Control.Lens.TH also offers the makeClassy function. makeClassy has the same type and works in essentially the same way as makeLenses, with one key difference. In addition to generating the standard lenses and traversals, if the type has no arguments, it will also create a class describing all the datatypes which possess the type as a field. For example
data Foo = Foo { _fooX, _fooY :: Int }
makeClassy ''Foo
will create
class HasFoo t where
foo :: Simple Lens t Foo
instance HasFoo Foo where foo = id
fooX, fooY :: HasFoo t => Simple Lens t Int
