Optics with the Witch library

optics-witch.hs

{-# LANGUAGE TemplateHaskell #-}

import qualified Data.Text as T
import Optics as O
import Protolude hiding ((%), from)
import Witch
import Data.Coerce

-- 2 different temperature types
newtype Kelvin = Kelvin Double deriving (Eq, Show, Num)

newtype Celsius = Celsius Double deriving (Eq, Show, Num)

-- conversions between the 2 different temperature types
instance From Kelvin Celsius where
  from (Kelvin k) = Celsius (k - 273.15)

instance From Celsius Kelvin where
  from (Celsius c) = Kelvin (c + 273.15)

-- measured temperature is stored as Kelvin
data Temperature = Temperature
  { _location :: Text,
    _measured :: Kelvin
  }
  deriving (Eq, Show)

makeLenses ''Temperature

-- we can use from to implement a celsius lens
celsius :: Lens' Temperature Celsius
celsius = lens getter setter
  where
    getter = from . view measured
    setter temp c = set measured (from c) temp


-- we can also redefine set and view to use a From instance
set' :: (Is k A_Setter, From c b) => Optic k is s t a b -> c -> s -> t
set' o b s = set o (from b) s

view' :: (Is k A_Getter, From a b) => Optic' k is s a -> s -> b
view' o s = from $ view o s

-- example: we modify a measured temperature directly even if the
-- measurement is in Celsius
> let t = Temperature "here" (Kelvin 300)
> set' measured (Celsius 20) t

Temperature {_location = "here", _measured = Kelvin 293.15}
it :: Temperature