Safe Haskell | Safe-Infered |
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*Caution*: Improper use of this module can lead to unexpected behaviour if the preconditions of the functions are not met.

A lens family is created by separating a substructure from the rest of its structure by a functor. How to create a lens family is best illustrated by the common example of a field of a record:

data MyRecord a = MyRecord { _myA :: a, _myB :: Int } -- The use of type variables a and a' allow for polymorphic updates. myA :: LensFamily (MyRecord a) (MyRecord a') a a' myA f (MyRecord a b) = (\a' -> MyRecord a' b) `fmap` (f a) -- The field _myB is monomorphic, so we can use a plain Lens type. -- However, the structure of the function is exactly the same as for LensFamily. myB :: Lens (MyRecord a) Int myB f (MyRecord a b) = (\b' -> MyRecord a b') `fmap` (f b)

By following this template you can safely build your own lenses.
To use this template, you do not need anything from this module other than the type synonyms `LensFamily`

and `Lens`

, and even they are optional.
See the `lens-family-th`

package to generate this code using Template Haskell.

*Note*: It is possible to build lenses without even depending on `lens-family`

by expanding away the type synonym.

-- A lens definition that only requires the Haskell "Prelude". myA :: Functor f => (a -> f a') -> (MyRecord a) -> f (MyRecord a') myA f (MyRecord a b) = (\a' -> MyRecord a' b) `fmap` (f a)

You can build lenses for more than just fields of records.
Any value `lens :: LensFamily a a' b b'`

is well-defined when it satisfies the two van Laarhoven lens laws:

lens Identity === Identity

lens (composeCoalgebroid f g) === composeCoalgebroid (lens f) (lens g) where composeCoalgebroid :: (Functor f, Functor g) => (b -> f c) -> (a -> g b) -> a -> (Compose g f) c composeCoalgebroid f g a = Compose $ f `fmap` g a === id

The functions `mkLens`

and `mkIsoLens`

can also be used to construct lenses.
The resulting lenses will be well-defined so long as their preconditions are satisfied.

- mkLens :: (a -> b) -> (a -> b' -> a') -> LensFamily a a' b b'
- mkIsoLens :: (a -> b) -> (b' -> a') -> LensFamily a a' b b'
- type LensFamily a a' b b' = forall f. Functor f => LensFamily f a a' b b'
- type Lens a b = LensFamily a a b b

# Documentation

:: (a -> b) | getter |

-> (a -> b' -> a') | setter |

-> LensFamily a a' b b' |

Build a lens from a `getter`

and `setter`

families.

*Caution*: In order for the generated lens family to be well-defined, you must ensure that the three lens laws hold:

getter (setter a b) === b

setter a (getter a) === a

setter (setter a b1) b2) === setter a b2

:: (a -> b) | yin |

-> (b' -> a') | yang |

-> LensFamily a a' b b' |

Build a lens from isomorphism families.

*Caution*: In order for the generated lens family to be well-defined, you must ensure that the two isomorphism laws hold:

yin . yang === id

yang . yin === id

# Types

type LensFamily a a' b b' = forall f. Functor f => LensFamily f a a' b b'Source

type Lens a b = LensFamily a a b bSource