Data.Lens.Light

Lenses and basic operations

newtype Lens a b

Simple lens data type

Constructors

Lens

Fields

runLens :: a -> (b -> a, b)

Instances

Category Lens

lens :: (a -> b) -> (b -> a -> a) -> Lens a b

Build a lens out of a getter and setter

iso :: (a -> b) -> (b -> a) -> Lens a b

Build a lens out of an isomorphism

getL :: Lens a b -> a -> b

Get the getter function from a lens

setL :: Lens a b -> b -> a -> a

Get the setter function from a lens

modL :: Lens a b -> (b -> b) -> a -> a

Get the modifier function from a lens

modL' :: Lens a b -> (b -> b) -> a -> a

Get the modifier function from a lens. Forces function application.

(^.) :: b -> Lens b c -> c

Infix version of getL (with the reverse order of the arguments)

Generate lenses using TH

nameMakeLens :: Name -> (String -> Maybe String) -> Q [Dec]

nameMakeLens n f where n is the name of a data type declared with data and f is a function from names of fields in that data type to the name of the corresponding accessor. If f returns Nothing, then no accessor is generated for that field.

makeLenses :: [Name] -> Q [Dec]

makeLenses n where n is the name of a data type declared with data looks through all the declared fields of the data type, and for each field beginning with an underscore generates an accessor of the same name without the underscore.

It is nameMakeLens n f where f satisfies

 f ('_' : s) = Just s
 f x = Nothing -- otherwise

For example, given the data type:

 data Score = Score { 
   _p1Score :: Int
 , _p2Score :: Int
 , rounds :: Int
 }

makeLenses will generate the following objects:

 p1Score :: Lens Score Int
 p1Score = lens _p1Score (\x s -> s { _p1Score = x })
 p2Score :: Lens Score Int
 p2Score = lens _p2Score (\x s -> s { _p2Score = x })

It is used with Template Haskell syntax like:

 $( makeLenses [''TypeName] )

And will generate accessors when TypeName was declared using data or newtype.

makeLens :: Name -> Q [Dec]

 makeLens a = makeLenses [a]

 $( makeLens ''TypeName )

MonadState operators

access :: MonadState a m => Lens a b -> m b

Get the value of a lens into state

(~=) :: MonadState a m => Lens a b -> b -> m ()

Set a value using a lens into state

(!=) :: MonadState a m => Lens a b -> b -> m ()

Set a value using a lens into state. Forces both the value and the whole state.

(%=) :: MonadState a m => Lens a b -> (b -> b) -> m ()

Infix modification of a value through a lens into state

(!%=) :: MonadState a m => Lens a b -> (b -> b) -> m ()

Infix modification of a value through a lens into state. Forces both the function application and the whole state.