Haskell Language Learning Note (Lens) (4)

Prisms

data NewTask =  SimpleTask String |  HarderTask String Int |  CompoundTask String [NewTask]  deriving (Show)makePrisms ''NewTask

*Main> a ^? _SimpleTaskJust "Clean"*Main> b ^? _HarderTaskJust ("Clean Kitchen",15)*Main> b ^? _HarderTask._2Just 15*Main> b & _SimpleTask .~ "Clean Garage"HarderTask "Clean Kitchen" 15*Main> b & _HarderTask._2 .~ 30HarderTask "Clean Kitchen" 30

Manual calculation

_Left = prism Left $ either Right (Left . Right)prism bt seta = dimap seta (either pure (fmap bt)) . right'data Market a b s t = Market (b -> t) (s -> Either t a)instance Choice (Market a b) where  right' (Market bt seta) = Market (Right . bt) $ \cs -> case cs of    Left c -> Left (Left c)    Right s -> case seta s of      Left t -> Left (Right t)      Right a -> Right apreview l = getFirst . foldMapOf l (First . Just)foldMapOf l f = getConst . l (Const . f)

Prelude Control.Lens> preview _Left (Left 5)Just 5

Reference links

Haskell Language Learning Notes (Lens) (1)
Haskell Language Learning Notes (Lens) (2)
Haskell Language Learning Note (Lens) (3)
Taking a Close look at Lenses

Haskell Language Learning Note (Lens) (4)