lens-inference

Infer lenses.

With my definition of an Inu:

newtype Breed = Breed { unBreed :: String }
  deriving Show

data Colour = White | Red | Sesame
  deriving Show

newtype Age = Age { unAge :: Int }
  deriving (Show, Num)

data Inu = Inu { _breed :: Breed, _colour :: Colour, _age :: Age }
  deriving Show
makeInferableLenses ''Inu

I use makeInferableLenses ''Inu instead of makeLenses ''Inu and get lenses I can do value inference on.

For example, I have a function to increase an Age by 1:

birthday :: Age -> Age
birthday (Age x) = Age (x + 1)

I can use lens to write a function that increments a dog's age by one:

inu_birthday :: Inu -> Inu
inu_birthday = age %~ birthday

But since a birthday applies to an Age, the only possible lens we can use is age. So instead, we infer it.

inu_birthday :: Inu -> Inu
inu_birthday = inferLens %~ birthday

Or, we can use the special modify operator %~?:

inu_birthday :: Inu -> Inu
inu_birthday = (%~?) birthday

Next we can define birds:

data Inko = Inko { _inkoAge :: Age }
  deriving Show
makeInferableLenses ''Inko

And a function that makes anything with an Age older:

older :: IsInferable a Age Identity => a -> a
older x = birthday %~? x