Control.Lens.Iso

Isomorphism Lenses

type Iso s t a b = forall p f. (Profunctor p, Functor f) => p a (f b) -> p s (f t)

Isomorphism families can be composed with another Lens using (.) and id.

Note: Composition with an Iso is index- and measure- preserving.

type Iso' s a = Iso s s a a

 type Iso' = Simple Iso

type AnIso s t a b = Exchange a b a (Mutator b) -> Exchange a b s (Mutator t)

When you see this as an argument to a function, it expects an Iso.

type AnIso' s a = AnIso s s a a

A Simple AnIso.

Isomorphism Construction

iso :: (s -> a) -> (b -> t) -> Iso s t a b

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

Consuming Isomorphisms

from :: AnIso s t a b -> Iso b a t s

Invert an isomorphism.

 from (from l) ≡ l

cloneIso :: AnIso s t a b -> Iso s t a b

Convert from AnIso back to any Iso.

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

See cloneLens or cloneTraversal for more information on why you might want to do this.

Working with isomorphisms

au :: AnIso s t a b -> ((s -> a) -> e -> b) -> e -> t

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

This version is generalized to accept any Iso, not just a newtype.

>>> au (wrapping Sum) foldMap [1,2,3,4]
10

auf :: AnIso s t a b -> ((r -> a) -> e -> b) -> (r -> s) -> e -> t

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

This version is generalized to accept any Iso, not just a newtype.

For a version you pass the name of the newtype constructor to, see alaf.

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

>>> auf (wrapping Sum) (foldMapOf both) Prelude.length ("hello","world")
10

under :: AnIso s t a b -> (t -> s) -> b -> a

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 => AnIso s t a b -> Iso (f s) (f t) (f a) (f b)

This can be used to lift any Iso into an arbitrary Functor.

Common Isomorphisms

simple :: Iso' a a

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 -> Iso' (Maybe a) a

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
1

>>> Map.fromList [] ^. at "hello" . non 0
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 []

anon :: a -> (a -> Bool) -> Iso' (Maybe a) a

anon a p generalizes non a to take any value and a predicate.

This function assumes that p a holds True and generates an isomorphism between Maybe (a | not (p a)) and a.

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

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

enum :: Enum a => Iso' Int a

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

>>> LT^.from enum
0

>>> 97^.enum :: Char
'a'

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.

curried :: Iso ((a, b) -> c) ((d, e) -> f) (a -> b -> c) (d -> e -> f)

The canonical isomorphism for currying and uncurrying a function.

 curried = iso curry uncurry

>>> (fst^.curried) 3 4
3

>>> view curried fst 3 4
3

uncurried :: Iso (a -> b -> c) (d -> e -> f) ((a, b) -> c) ((d, e) -> f)

The canonical isomorphism for uncurrying and currying a function.

 uncurried = iso uncurry curry

 uncurried = from curried

>>> ((+)^.uncurried) (1,2)
3

flipped :: Iso (a -> b -> c) (a' -> b' -> c') (b -> a -> c) (b' -> a' -> c')

The isomorphism for flipping a function.

>>> ((,)^.flipped) 1 2
(2,1)

class Bifunctor p => Swapped p where

This class provides for symmetric bifunctors.

Methods

swapped :: Iso (p a b) (p c d) (p b a) (p d c)

 swapped . swapped ≡ id
 first f . swapped = swapped . second f
 second g . swapped = swapped . first g
 bimap f g . swapped = swapped . bimap g f

>>> (1,2)^.swapped
(2,1)

Instances

Swapped Either

Swapped (,)

class Strict s a | s -> a, a -> s where

Ad hoc conversion between "strict" and "lazy" versions of a structure, such as Text or ByteString.

Methods

strict :: Iso' s a

Instances

Strict ByteString ByteString

Strict Text Text

Uncommon Isomorphisms

magma :: LensLike (Mafic a b) s t a b -> Iso s u (Magma Int t b a) (Magma j u c c)

This isomorphism can be used to inspect a Traversal to see how it associates the structure and it can also be used to bake the Traversal into a Magma so that you can traverse over it multiple times.

imagma :: Overloading (Indexed i) (->) (Molten i a b) s t a b -> Iso s t' (Magma i t b a) (Magma j t' c c)

This isomorphism can be used to inspect an IndexedTraversal to see how it associates the structure and it can also be used to bake the IndexedTraversal into a Magma so that you can traverse over it multiple times with access to the original indices.

data Magma i t b a

This provides a way to peek at the internal structure of a Traversal or IndexedTraversal

Instances

(FunctorWithIndex i (Magma i t b), FoldableWithIndex i (Magma i t b), Traversable (Magma i t b)) => TraversableWithIndex i (Magma i t b)

Foldable (Magma i t b) => FoldableWithIndex i (Magma i t b)

Functor (Magma i t b) => FunctorWithIndex i (Magma i t b)

Functor (Magma i t b)

Foldable (Magma i t b)

(Functor (Magma i t b), Foldable (Magma i t b)) => Traversable (Magma i t b)

(Show i, Show a) => Show (Magma i t b a)

Profunctors

class Profunctor p where

Formally, the class Profunctor represents a profunctor from Hask -> Hask.

Intuitively it is a bifunctor where the first argument is contravariant and the second argument is covariant.

You can define a Profunctor by either defining dimap or by defining both lmap and rmap.

If you supply dimap, you should ensure that:

dimap id id ≡ id

If you supply lmap and rmap, ensure:

 lmap id ≡ id
 rmap id ≡ id

If you supply both, you should also ensure:

dimap f g ≡ lmap f . rmap g

These ensure by parametricity:

 dimap (f . g) (h . i) ≡ dimap g h . dimap f i
 lmap (f . g) ≡ lmap g . lmap f
 rmap (f . g) ≡ rmap f . rmap g

Methods

dimap :: (a -> b) -> (c -> d) -> p b c -> p a d

Map over both arguments at the same time.

dimap f g ≡ lmap f . rmap g

lmap :: (a -> b) -> p b c -> p a c

Map the first argument contravariantly.

lmap f ≡ dimap f id

rmap :: (b -> c) -> p a b -> p a c

Map the second argument covariantly.

rmap ≡ dimap id

Instances

Profunctor (->)

Profunctor Reviewed

Monad m => Profunctor (Kleisli m)

Functor w => Profunctor (Cokleisli w)

Functor f => Profunctor (DownStar f)

Functor f => Profunctor (UpStar f)

Arrow p => Profunctor (WrappedArrow p)

Profunctor (Tagged *)

Profunctor (Indexed i)

Profunctor (Market a b)

Profunctor (Exchange a b)