lens-3.6: Lenses, Folds and Traversals

MaintainerEdward Kmett <ekmett@gmail.com>
Safe HaskellSafe-Inferred






Isomorphism Lenses

type Iso s t a b = forall k f. (Isomorphic k, Functor f) => k (a -> f b) (s -> f t)Source

Isomorphism families can be composed with other lenses using either (.) and id from the Prelude or from Control.Category. However, if you compose them with each other using (.) from the Prelude, they will be dumbed down to a mere Lens.

 import Control.Category
 import Prelude hiding ((.),id)
type Iso s t a b = forall k f. (Isomorphic k, Functor f) => Overloaded k f s t a b

iso :: (Isomorphic k, Functor f) => (s -> a) -> (a -> s) -> k (a -> f a) (s -> f s)Source

Build a simple isomorphism from a pair of inverse functions

 view (iso f g) ≡ f
 view (from (iso f g)) ≡ g
 set (iso f g) h ≡ g . h . f
 set (from (iso f g)) h ≡ f . h . g
iso :: (s -> a) -> (a -> s) -> Simple Iso s a

isos :: (Isomorphic k, Functor f) => (s -> a) -> (a -> s) -> (t -> b) -> (b -> t) -> k (a -> f b) (s -> f t)Source

Build an isomorphism family from two pairs of inverse functions

 view (isos sa as tb bt) ≡ sa
 view (from (isos sa as tb bt)) ≡ as
 set (isos sa as tb bt) ab ≡ bt . ab . sa
 set (from (isos ac ca bd db)) ab ≡ bd . ab . ca
isos :: (s -> a) -> (a -> s) -> (t -> b) -> (b -> t) -> Iso s t a b

Working with isomorphisms

ala :: Simple Iso s a -> ((s -> a) -> e -> a) -> e -> sSource

Based on ala from Conor McBride's work on Epigram.

>>> :m + Data.Monoid.Lens Data.Foldable
>>> ala _sum foldMap [1,2,3,4]

auf :: Simple Iso s a -> ((b -> a) -> e -> a) -> (b -> s) -> e -> sSource

Based on ala' from Conor McBride's work on Epigram.

Mnemonically, the German auf plays a similar role to à la, and the combinator is ala with an extra function argument.

under :: Isomorphism (a -> Mutator b) (s -> Mutator t) -> (s -> t) -> a -> bSource

The opposite of working over a Setter is working under an Isomorphism.

under = over . from
under :: Iso s t a b -> (s -> t) -> a -> b

mapping :: Functor f => SimpleIso s a -> SimpleIso (f s) (f a)Source

This can be used to lift any SimpleIso into an arbitrary functor.

review :: Overloaded Isomorphism (Accessor s) s t a b -> a -> sSource

This can be used to turn an Iso around and view the other way.

review = view . from

Primitive isomorphisms

from :: Isomorphic k => Isomorphism a b -> k b aSource

Invert an isomorphism.

Note to compose an isomorphism and receive an isomorphism in turn you'll need to use Category

from (from l) ≡ l

If you imported . from Control.Category, then:

from l . from r ≡ from (r . l)

via :: Isomorphic k => Isomorphism a b -> k a bSource

Convert from an Isomorphism back to any Isomorphic value.

This is useful when you need to store an isomoprhism as a data type inside a container and later reconstitute it as an overloaded function.

data Isomorphism a b Source

A concrete data type for isomorphisms.

This lets you place an isomorphism inside a container without using ImpredicativeTypes.


Isomorphism (a -> b) (b -> a) 

class Category k => Isomorphic k whereSource

Used to provide overloading of isomorphism application

This is a Category with a canonical mapping to it from the category of isomorphisms over Haskell types.


isomorphic :: (a -> b) -> (b -> a) -> k a bSource

Build this morphism out of an isomorphism

The intention is that by using isomorphic, you can supply both halves of an isomorphism, but k can be instantiated to (->), so you can freely use the resulting isomorphism as a function.

isomap :: ((a -> b) -> c -> d) -> ((b -> a) -> d -> c) -> k a b -> k c dSource

Map a morphism in the target category using an isomorphism between morphisms in Hask.

Common Isomorphisms

_const :: Iso a b (Const a c) (Const b d)Source

This isomorphism can be used to wrap or unwrap a value in Const

 x ^. _constConst x
 Const x ^. from _const ≡ x

identity :: Iso a b (Identity a) (Identity b)Source

This isomorphism can be used to wrap or unwrap a value in Identity.

 x^.identity ≡ Identity x
 Identity x ^. from identity ≡ x

simple :: Simple Iso a aSource

Composition with this isomorphism is occasionally useful when your Lens, Traversal or Iso has a constraint on an unused argument to force that argument to agree with the type of a used argument and avoid ScopedTypeVariables or other ugliness.

non :: Eq a => a -> Simple Iso (Maybe a) aSource

If v is an element of a type a, and a' is a sans the element v, then non v is an isomorphism from Maybe a' to a.

Keep in mind this is only a real isomorphism if you treat the domain as being Maybe (a sans v)

This is practically quite useful when you want to have a map where all the entries should have non-zero values.

>>> Map.fromList [("hello",1)] & at "hello" . non 0 +~ 2
fromList [("hello",3)]
>>> Map.fromList [("hello",1)] & at "hello" . non 0 -~ 1
fromList []
>>> Map.fromList [("hello",1)] ^. at "hello" . non 0
>>> Map.fromList [] ^. at "hello" . non 0

This combinator is also particularly useful when working with nested maps.

e.g. When you want to create the nested map when it is missing:

>>> Map.empty & at "hello" . non Map.empty . at "world" ?~ "!!!"
fromList [("hello",fromList [("world","!!!")])]

and when have deleting the last entry from the nested map mean that we should delete its entry from the surrounding one:

>>> fromList [("hello",fromList [("world","!!!")])] & at "hello" . non Map.empty . at "world" .~ Nothing
fromList []

enum :: Enum a => Simple Iso Int aSource

This isomorphism can be used to convert to or from an instance of Enum.

>>> LT^.from enum
>>> 97^.enum :: Char

Note: this is only an isomorphism from the numeric range actually used and it is a bit of a pleasant fiction, since there are questionable Enum instances for Double, and Float that exist solely for [1.0 .. 4.0] sugar and the instances for those and Integer don't cover all values in their range.

Storing Isomorphisms

newtype ReifiedIso s t a b Source

Useful for storing isomorphisms in containers.




reflectIso :: Iso s t a b


type SimpleIso s a = Iso s s a aSource