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module Focus exposing (..)
{-| Sort-of lenses

# Types
@docs Setter

# Operators
@docs compose, (=>), (.=), ($=), (&)

-}

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{-| 'Setter's work by applying a function to a given field -}
type alias Setter s t a b = (a -> b) -> s -> t

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{-| Compose two setters -}
compose : Setter s t a b -> Setter a b c d -> Setter s t c d
compose f g = \ap s -> f (g ap) s


{-| Synonym for compose (TODO: Rename?) -}
(=>) : Setter s t a b -> Setter a b c d -> Setter s t c d
(=>) f g = compose f g

infixr 7 =>

{-| Set the focused item to a constant value (TODO: Rename?) -}
(.=) : Setter s t a b -> b -> s -> t
(.=) f a = \ s -> f (always a) s

infixr 6 .=

{-| Transform the focused value with a function (TODO: Rename?) -}
($=) : Setter s t a b -> (a -> b) -> s -> t
($=) f g = \s -> f g s

infixr 6 $=


{-| Reverse function application -}
(&) : a -> (a -> b) -> b
(&) x f = f x

infixr 5 &