Safe Haskell | None |
---|---|

Language | Haskell2010 |

## Synopsis

- module Optics.Optic
- module Optics.AffineFold
- module Optics.AffineTraversal
- module Optics.Fold
- module Optics.Getter
- module Optics.Iso
- module Optics.IxLens
- module Optics.IxGetter
- module Optics.IxAffineFold
- module Optics.IxAffineTraversal
- module Optics.IxFold
- module Optics.IxSetter
- module Optics.IxTraversal
- module Optics.Lens
- module Optics.ReversedLens
- module Optics.Prism
- module Optics.ReversedPrism
- module Optics.Review
- module Optics.Setter
- module Optics.Traversal
- module Optics.Operators
- module Optics.At
- module Optics.Cons
- module Optics.Each
- module Optics.Empty
- module Optics.Re
- module Optics.ReadOnly
- module Optics.State
- module Optics.View
- module Optics.Zoom
- module Optics.Indexed
- module Optics.TH
- module Optics.Label
- module Data.Either.Optics
- module Data.Maybe.Optics
- module Data.Tuple.Optics

# Introduction

Read on for a general introduction to the notion of optics, or if you are familiar with them already, you may wish to jump ahead to the "What is the abstract interface?" section below in Optics.

## What are optics?

An optic is a first-class, composable *notion of substructure*. As a highly
abstract concept, the idea can be approached by considering several examples
of optics and understanding their common features. What are the possible
relationships between some "outer" type `S`

and some "inner" type `A`

?

(For simplicity we will initially ignore the possibility of type-changing
update operations, which change `A`

to some other type `B`

and hence change
`S`

to some other type `T`

. These are fully supported by the library, at the
cost of some extra type parameters.)

### Optics.Iso: isomorphisms

First, `S`

and `A`

may be *isomorphic*, i.e. there exist mutually inverse
functions to convert `S -> A`

and `A -> S`

. This is a somewhat trivial
notion of substructure: `A`

is just another way to represent "all of `S`

".

An

is an isomorphism between `Iso'`

S A`S`

and `A`

, with the conversion
functions given by `view`

and `review`

. For example, given

newtype Age = Age Int

there is an isomorphism between the newtype and its representation:

`coerced`

::`Iso'`

Age`Int`

`view`

`coerced`

:: Age ->`Int`

`review`

`coerced`

::`Int`

-> Age

### Optics.Lens: generalised fields

If `S`

is a simple product type (i.e. it has a single constructor with one or
more fields), `A`

may be a single field of `S`

. More generally, `A`

may be
"part of `S`

" in the sense that `S`

is isomorphic to the pair `(A,C)`

for
some type `C`

representing the other fields. In this case, there is a
*projection* function `S -> A`

for getting the value of the field, but the
*update* function (setting the value of the field) requires the "rest of `S`

"
and so has type `A -> S -> S`

.

A

captures the structure of `Lens'`

S A`A`

being a field of `S`

, with the
projection function given by `view`

and the update function by `set`

. For
example, for the pair type `(X,Y)`

there are lenses for each component:

`_1`

::`Lens'`

(X,Y) X`_2`

::`Lens'`

(X,Y) Y`view`

`_1`

:: (X,Y) -> X`set`

`_2`

:: Y -> (X,Y) -> (X,Y)

(Note that the update function could arguably have the more precise type ```
A
-> C -> S
```

, since we do not expect the result of setting a field to depend on
the previous value of the field. However, making `C`

explicit turns out to
be awkward, so instead we impose *laws* to require that the result of setting
the field depends only on `C`

, and, more generally, that the lens behaves as
we would expect.)

### Optics.Prism: generalised constructors

If `S`

is a simple sum type (i.e. it has one or more constructors, each with
a single field), `A`

may be the type of the field for a single constructor of
`S`

. More generally, `S`

may be isomorphic to the disjoint union ```
Either D
A
```

for some type `D`

representing the other constructors. In this case,
projecting out `A`

from `S`

(pattern-matching on the constructor) may fail,
so it has type `S -> Maybe A`

. In the reverse direction we have a function
of type `A -> S`

representing the constructor itself.

A

captures the structure of `Prism'`

S A`A`

being a constructor of `S`

,
with the partial projection function given by `preview`

and the constructor
function given by `review`

. For example, for the type

there
is a prism for each constructor:`Either`

X Y

`_Left`

::`Prism'`

(`Either`

X Y) X`_Right`

::`Prism'`

(`Either`

X Y) Y`preview`

`_Left`

::`Either`

X Y ->`Maybe`

X`review`

`_Right`

:: Y ->`Either`

X Y

### Optics.Traversal: multiple substructures

Alternatively, `S`

may "contain" the substructure `A`

a variable number of
times. In this case, the projection function extracts the (possibly zero or
many) elements so has type `S -> [A]`

, while the update function may take
different values for different elements so has type `(A -> A) -> S -> S`

(though in fact more general formulations are possible).

A

captures the structure of `Traversal'`

S A`A`

being contained in `S`

perhaps multiple times, with the list of values given by `toListOf`

and the
update function given by `over`

. For example, for the type `Maybe X`

there
is a traversal that may return zero or one element:

`traversed`

::`Traversal'`

(`Maybe`

X) X`toListOf`

`traversed`

::`Maybe`

X -> [X]`over`

`traversed`

:: (X -> X) ->`Maybe`

X ->`Maybe`

X

(In fact, traversals of at most one element are known as *affine* traversals,
see Optics.AffineTraversal.)

### In general

So far we have seen four different kinds of optic or "notions of substructure", and many more are possible. Observe the important properties they have in common:

- There are subtyping relationships between different optic kinds. Any
isomorphism is trivially a lens and a prism (with no other fields or
constructors, respectively). Any lens is a traversal (where the list of
elements is always a singleton list), and any prism is also a traversal
(where there will be zero or one element depending on whether the
constructor matches). This was implicit in the fact that we used that we
used the same operators in multiple cases:
`view`

gives the projection function of both an isomorphism and a lens, but cannot be applied to a traversal. - Optics can be composed. If
`S`

is isomorphic to`U`

and`U`

is isomorphic to`A`

then`S`

is isomorphic to`A`

, and similarly for other optic kinds. - Composition and subtyping interact: a lens and a prism can be composed, by
first thinking of them as traverals using the subtyping relationship. That
is, if
`S`

has a field`U`

, and`U`

has a constructor`A`

, then`S`

contains zero or one`A`

s that we can pick out with a traversal (but in general there is neither a lens from`S`

to`A`

nor a prism). - Each optic kind can be described by certain operations it enables. For example lenses support projection and update, while prisms support partial projection and construction.
- Optics are subject to laws, which are necessary for the operations to make sense.

The point of the `optics`

library is to capture this common pattern.

## What is the abstract interface?

A key principle behind this library is the belief that optics are useful as an abstract concept, and that the purpose of types is to capture abstract concepts and make them useful. The programmer using optics should be able to think in terms of the abstract interface, rather than the details of the implementation, and implementation choices should (as far as possible) not dictate the interface.

Each optic kind is identified by a "tag type" (such as `A_Lens`

), which is an
empty data type. The type of the actual optics (such as `Lens`

) is obtained
by applying the `Optic`

newtype wrapper to the tag type.

type`Lens`

s t a b =`Optic`

`A_Lens`

`NoIx`

s t a b type`Lens'`

s a =`Optic'`

`A_Lens`

`NoIx`

s a

`NoIx`

as the second parameter to `Optic`

indicates that the optic is not
indexed. See the "Indexed optics" section below in Optics for
further discussion of indexed optics.

The details of the internal implementation of `Optic`

are hidden behind an
abstraction boundary, so that the library can be used without needing to
think about the particular implementation choices.

### Specification of optics interfaces

Each different kind of optic is documented in a separate module describing
its abstract interface, in a standard format with at least *formation*,
*introduction*, *elimination*, and *well-formedness* sections. See "Optic
kinds" below in Optics for a list of these modules.

The

**formation**sections contain type definitions. For example Optics.Lens defines:-- Type synonym for a type-modifying lens. type

`Lens`

s t a b =`Optic`

`A_Lens`

`NoIx`

s t a bThe

**introduction**sections describe the canonical way to construct each particular optic. Continuing with a`Lens`

example:-- Build a lens from a getter and a setter.

`lens`

:: (s -> a) -> (s -> b -> t) ::`Lens`

s t a bCorrespondingly, the

**elimination**sections show how you can destruct the optic into the pieces from which it was constructed.-- A

`Lens`

is a`Setter`

and a`Getter`

, therefore you can specialise types to obtain`view`

::`Lens`

s t a b -> s -> a`set`

::`Lens`

s t a b -> b -> s -> tThe

**computation**rules tie introduction and elimination forms together. These rules are automatically fulfilled by the library (for well-formed optics).`view`

(`lens`

f g) s ≡ f s`set`

(`lens`

f g) a s ≡ g s aThe

**well-formedness**sections describe the laws that each optic should obey. As far as possible, all optics provided by the library are well-formed, but in some cases this depends on invariants that cannot be expressed in types. Ill-formed optics*might*behave differently from what the computation rules specify.For example, a

`Lens`

should obey three laws, known as*GetPut*,*PutGet*and*PutPut*. See the Optics.Lens module for their definitions. The user of the`lens`

introduction form must ensure that these laws are satisfied.- Some optic kinds have
**additional introduction forms**,**additional elimination forms**or**combinators**sections, which give alternative ways to create and use optics of that kind. In principle these are expressible in terms of the canonical introduction and elimination rules. - The
**subtyping**section gives the "tag type" (such as`A_Lens`

), which in particular is accompanied by`Is`

instances that define the subtyping relationship discussed in the following section.

### Subtyping

There is a subtyping relationship between optics, implemented using
typeclasses. The `Is`

typeclass captures the property that one optic kind
can be used as another, and the `castOptic`

function can be used to
explicitly cast between optic kinds. `Is`

forms a partial order, represented
in the graph below. For example, a lens can be used as a traversal, so there
are arrows from `Lens`

to `Traversal`

(via `AffineTraversal`

) and there is an
instance of

.`Is`

`A_Lens`

`A_Traversal`

Introduction forms (constructors) return a concrete optic kind, while elimination forms (destructors) are generally polymorphic in the optic kind they accept. This means that it is not normally necessary to explicitly cast between optic kinds. For example, we have

`view`

::`Is`

k`A_Getter`

=>`Optic'`

k is s a -> s -> a

so `view`

can be used with isomorphisms or lenses, as these can be converted
to a `Getter`

.

If an explicit cast is needed, you can use `castOptic`

. This arises when you
use optics of different kinds in a context that requires them to have the
same type. For example `[`

gives a type error (since
`folded`

, `traversed`

]`A_Traversal`

is not `A_Fold`

) but `[`

works.`folded`

, `castOptic`

`traversed`

]

The optic kind module (e.g. Optics.Lens) does not list all ways to
construct or use particular the optic kind. For example, since a `Lens`

is
also a `Traversal`

, a `Fold`

etc, so you can use `traverseOf`

, `preview`

and
many other combinators with lenses.

#### Subtype hierarchy

This graph gives an overview of the optic kinds and their subtype relationships:

In addition to the optic kinds included in the diagram, there are also
indexed variants such as `IxLens`

, `IxGetter`

, `IxAffineTraversal`

,
`IxTraversal`

, `IxAffineFold`

, `IxFold`

and `IxSetter`

. These are explained
in more detail in the "Indexed optics" section below in Optics.

### Composition

Since *optics are not functions*, they cannot be composed with the (`.`

)
operator. Instead there is a separate composition operator (`%`

). The
composition operator returns the common supertype of its arguments, or
generates a type error if the composition does not make sense.

The optic kind resulting from a composition is the least upper bound (join)
of the optic kinds being composed, if it exists. The `Join`

type family
computes the least upper bound given two optic kind tags. For example the
`Join`

of a `Lens`

and a `Prism`

is an `AffineTraversal`

.

`>>>`

Join A_Lens A_Prism :: * = An_AffineTraversal`:kind! Join A_Lens A_Prism`

The join does not exist for some pairs of optic kinds, which means that they
cannot be composed. For example there is no optic kind above both `Setter`

and `Fold`

:

`>>>`

Join A_Setter A_Fold :: * = (TypeError ...)`:kind! Join A_Setter A_Fold`

`>>>`

... ...A_Setter cannot be composed with A_Fold ...`:t mapped % folded`

## Comparison with `lens`

The `lens`

package is the best known Haskell library for optics, and
established many of the foundations on which the `optics`

package builds (not
least in quite a bit of code having been directly ported). It defines optics
based on the *van Laarhoven* representation, where each optic kind is
introduced as a *transparent* type synonym for a complex polymorphic type,
for example:

`type Lens s t a b = forall f. ``Functor`

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

In contrast, `optics`

tries to preserve an abstraction boundary between the
interface of optics and their implementation. Optic kinds are expressed
directly in the types, as `Optic`

is an *opaque* newtype:

type`Lens`

s t a b =`Optic`

`A_Lens`

`NoIx`

s t a b

The choice of representation of `Optic`

is then an implementation detail, not
essential for understanding the library. (In fact, `optics`

uses the
*profunctor* representation rather than the *van Laarhoven* representation;
this affects the optic kinds and operations that can be conveniently
supported, but not the essence of the design.)

Our design choice to use *opaque* rather than *transparent* abstractions
leads to various consequences, both positive and negative, which are explored
in the following subsections.

## Advantages of the opaque design

Since the interface is deliberately chosen rather than to some extent
determined by the implementation, we are free to choose a more restricted
interface where doing so leads to conceptual simplicity. For example, in
`lens`

, the `view`

function can be used with a `Fold`

provided the result
type has a `Monoid`

instance, and the multiple targets of the `Fold`

will be
combined monoidally. This behaviour can be confusing, so in `optics`

a
`Fold`

cannot be silently used as a `Getter`

, and we prefer to have `view`

work on `Getter`

s and define a separate `foldOf`

operator for use on
`Fold`

s. (But the `gview`

function is available for users who may prefer
otherwise.)

In general, opaque abstractions lead to better results from type inference (the optic kind is preserved in the inferred type):

`>>>`

traversed % to not :: Traversable t => Optic A_Fold '[] (t Bool) (t Bool) Bool Bool`:t traversed % to not`

Error messages are domain-specific:

`>>>`

... ...A_Getter cannot be used as A_Setter ...`set (to fst)`

Composing incompatible optics yields a sensible error:

`>>>`

... ...A_Setter cannot be composed with A_Getter ...`sets map % to not`

Since `Optic`

is a rank-1 type, it is easy to store optics in a
datastructure:

`>>>`

[folded, backwards_ folded] :: Foldable f => [Fold (f a) a]`:t [folded, backwards_ folded]`

It is possible to define aliases for optics without the monomorphism restriction spoiling the fun:

`>>>`

('x',('c','y'))`let { myoptic = _1; p = ('x','y') } in (view myoptic p, set myoptic 'c' p)`

Finally, having an abstract interface gives more freedom of choice in the internal implementation. If there is a compelling reason to switch to an alternative representation, one can in principle do so without changing the interface.

## Disadvantages of the opaque design

Since `Optic`

is a newtype, other libraries that wish to define optics must
depend upon its definition. In contrast, with a transparent representation,
and since the van Laarhoven representations of lenses and traversals depend
only on definitions from `base`

, it is possible for libraries to define them
without any extra library dependencies (although this does not hold for more
advanced optic kinds such as prisms or indexed optics). To address this, the
present library is split into a package `optics-core`

, which has a minimal
dependency footprint intended for use in libraries, and the
"batteries-included" `optics`

package for use in applications.

It is something of an amazing fact that the composition operator for
transparent optics is just function composition. Moreover, since Haskell
uses (`.`

) for function composition, `lens`

is able to support a pseudo-OOP
syntax. In contrast, `optics`

must use a different composition operator
(`%`

). `Optic`

does not quite form a `Category`

, thanks to
type-changing optics.

Rather than emerging naturally from the definitions, opportunities for
polymorphism have to be identified in advance and explicitly introduced using
type classes. Similarly, the set of optic kinds and the subtyping
relationships between them must be fixed in advance, and cannot be added to
in downstream libraries. Thus in a sense the opaque approach is more
restrictive than the transparent one. There are cases in `lens`

where the
types work out nicely and permit abstraction-breaking-but-convenient
shortcuts, such as applying a `Traversal`

as a `traverse`

-like function,
whereas `optics`

requires a call to `traverseOf`

.

## More specific differences

The sections above set out the major conceptual differences from the `lens`

package, and their advantages and disadvantages. Some more specific design
differences, which may be useful for comparison or porting code between the
libraries. This list is no doubt incomplete.

- The composition operator is (
`%`

) rather than (`.`

) and is defined as`infixl 9`

instead of`infixr 9`

. - Fewer operators are provided, and some of them are not exported from the main Optics module. Import Optics.State.Operators if you want them.
- The
`view`

function and corresponding (`^.`

) operator work only for`Getter`

s and have a more restricted type. The equivalent for`Fold`

s is`foldOf`

, and you can use`preview`

for`AffineFold`

s. Alternatively you can use`gview`

which is more compatible with`view`

from`lens`

, but it uses a type class to choose between`view`

,`preview`

and`foldOf`

. - Indexed optics are rather different, as described in the "Indexed optics" section below in Optics. All ordinary optics are "index-preserving", so there is no separate notion of an index-preserving optic.
`Each`

provides indexed traversals.`firstOf`

from`lens`

is replaced by`headOf`

.`concatOf`

from`lens`

is omitted in favour of the more general`foldOf`

.`set'`

is a strict version of`set`

, not`set`

for type-preserving optics.- Numbered lenses for accessing fields of tuples positionally are provided
only up to
`_9`

, rather than`_19`

. - There are four variants of
`backwards`

for (indexed)`Traversal`

s and`Fold`

s:`backwards`

,`backwards_`

,`ibackwards`

and`ibackwards_`

. - There is no
`Traversal1`

and`Fold1`

. - There are affine variants of (indexed) traversals and folds
(
`AffineTraversal`

,`AffineFold`

,`IxAffineTraversal`

and`IxAffineFold`

). An affine optic targets at most one value. Composing a`Lens`

with a`Prism`

produces an`AffineTraversal`

, so for example

is well-typed.`matching`

(`_1`

`%`

`_Left`

) - Functions
`ifiltered`

and`indices`

are defined as optic combinators due to restrictions of internal representation. - We can't use
`traverse`

as an optic directly. Instead there is a`Traversal`

called`traversed`

. Similarly`traverseOf`

must be used to apply a`Traversal`

, rather than simply using it as a function. - The
`re`

combinator produces a different optic kind depending on the kind of the input`Iso`

, for example`Review`

reverses to`Getter`

while a reversed`Iso`

is still an`Iso`

. Thus there is no separate`from`

combinator for reversing`Iso`

s.

## Other resources

- Through a Glass, Abstractly: Lenses and the Power of Abstraction a talk on the principles behind this library with accompanying slides by Adam Gundry (but note that the design details of
`optics`

have changed substantially since this talk was given) - Profunctors and Data Accessors a talk on basics of profunctors and how they relate to data accessors such as lenses, prisms and traversals by Andrzej Rybczak
- Profunctor Optics: Modular Data Accessors a paper by Matthew Pickering, Jeremy Gibbons and Nicolas Wu
- Glassery and Indexed Profunctor optics, blog posts by Oleg Grenrus
- The lens package by Edward Kmett and contributors

# Using the library

To get started, you can just add `optics`

as a dependency to your `.cabal`

file, and then:

import Optics

If you are writing a library for which it is important to keep the dependency
footprint minimal, you may wish to depend upon `optics-core`

instead (and
perhaps `optics-extra`

or `optics-th`

), and then:

import Optics.Core

module Optics.Optic

## Optic kinds

module Optics.AffineFold

module Optics.AffineTraversal

module Optics.Fold

module Optics.Getter

module Optics.Iso

module Optics.IxLens

module Optics.IxGetter

module Optics.IxAffineFold

module Optics.IxAffineTraversal

module Optics.IxFold

module Optics.IxSetter

module Optics.IxTraversal

module Optics.Lens

module Optics.ReversedLens

module Optics.Prism

module Optics.ReversedPrism

module Optics.Review

module Optics.Setter

module Optics.Traversal

## Optic operators

module Optics.Operators

# Optics utilities

## At

An `AffineTraversal`

to traverse a key in a map or an element of a
sequence:

`>>>`

Just 'b'`preview (ix 1) ['a','b','c']`

a `Lens`

to get, set or delete a key in a map:

`>>>`

fromList [(0,'b')]`set (at 0) (Just 'b') (Map.fromList [(0, 'a')])`

and a `Lens`

to insert or remove an element of a set:

`>>>`

fromList [1,2,4]`IntSet.fromList [1,2,3,4] & contains 3 .~ False`

module Optics.At

## Cons

`Prism`

s to match on the left or right side of a list, vector or other
sequential structure:

`>>>`

Just ('a',"bc")`preview _Cons "abc"`

`>>>`

Just ("ab",'c')`preview _Snoc "abc"`

module Optics.Cons

## Each

An `IxTraversal`

for each element of a (potentially monomorphic) container.

`>>>`

(10,20,30)`over each (*10) (1,2,3)`

module Optics.Each

## Empty

A `Prism`

for a container type that may be empty.

`>>>`

True`isn't _Empty [1,2,3]`

module Optics.Empty

## Re

Some optics can be reversed with `re`

. This is mainly useful to invert
`Iso`

s:

`>>>`

`let _Identity = iso runIdentity Identity`

`>>>`

Identity 'x'`view (_1 % re _Identity) ('x', "yz")`

Yet we can use a `Lens`

as a `Review`

too:

`>>>`

'x'`review (re _1) ('x', "yz")`

In the following diagram, red arrows illustrate how `re`

transforms optics.
The `ReversedLens`

and `ReversedPrism`

optic kinds are backwards versions
of `Lens`

and `Prism`

respectively, and are present so that

does not change the optic kind.`re`

. `re`

module Optics.Re

## ReadOnly

Defines `getting`

, which turns a read-write optic into its read-only
counterpart.

module Optics.ReadOnly

`Setter`

utilities for working in `MonadState`

.

module Optics.State

## View

A generalized view function `gview`

, which returns a single result (like
`view`

) if the optic is a `Getter`

, a `Maybe`

result (like `preview`

) if
the optic is an `AffineFold`

, or a monoidal summary of results (like
`foldOf`

) if the optic is a `Fold`

. In addition, it works for any
`MonadReader`

, not just `(->)`

.

`>>>`

'x'`gview _1 ('x','y')`

`>>>`

Just 'x'`gview _Left (Left 'x')`

`>>>`

"ab"`gview folded ["a", "b"]`

`>>>`

'x'`runReaderT (gview _1) ('x','y') :: IO Char`

This module is experimental. Using the more type-restricted variants is encouraged where possible.

module Optics.View

## Zoom

A class to `zoom`

in, changing the `State`

supplied
by many different monad transformers, potentially quite deep in a monad
transformer stack.

`>>>`

('c','b')`flip execState ('a','b') $ zoom _1 $ equality .= 'c'`

module Optics.Zoom

# Indexed optics

The `optics`

library also provides indexed optics, which provide
an additional *index* value in mappings:

`over`

::`Setter`

s t a b -> (a -> b) -> s -> t`iover`

::`IxSetter`

i s t a b -> (i -> a -> b) -> s -> t

Note that there aren't any laws about indices. Especially in compositions the same index may occur multiple times.

The machinery builds on indexed variants of `Functor`

, `Foldable`

, and `Traversable`

classes:
`FunctorWithIndex`

, `FoldableWithIndex`

and `TraversableWithIndex`

respectively.
There are instances for types in the boot libraries.

class (`FoldableWithIndex`

i t,`Traversable`

t) =>`TraversableWithIndex`

i t | t -> i where`itraverse`

::`Applicative`

f => (i -> a -> f b) -> t a -> f (t b)

Indexed optics *can* be used as regular ones, i.e. indexed optics
gracefully downgrade to regular ones.

`>>>`

"foo"`toListOf ifolded "foo"`

`>>>`

[(0,'f'),(1,'o'),(2,'o')]`itoListOf ifolded "foo"`

But there is also a combinator `noIx`

to explicitly erase indices:

`>>>`

(ifolded % simple) :: FoldableWithIndex i f => Optic A_Fold '[i] (f b) (f b) b b`:t (ifolded % simple)`

`>>>`

noIx (ifolded % simple) :: FoldableWithIndex i f => Optic A_Fold NoIx (f b) (f b) b b`:t noIx (ifolded % simple)`

λ> :t noIx (ifolded % ifolded) noIx (ifolded % ifolded) :: (FoldableWithIndex i1 f1, FoldableWithIndex i2 f2) => Optic A_Fold NoIx (f1 (f2 b)) (f1 (f2 b)) b b

As the example above illustrates, regular and indexed optics have the same
tag in the first parameter of `Optic`

, in this case `A_Fold`

. Regular optics
simply don't have any indices. The provided type aliases `IxLens`

,
`IxGetter`

, `IxAffineTraversal`

, `IxAffineFold`

, `IxTraversal`

, `IxFold`

and
`IxSetter`

are variants with a single index. In general, the second parameter
of the `Optic`

newtype is a type-level list of indices, which will typically
be `NoIx`

(the empty index list) or `(`

(a singleton list).`WithIx`

i)

When two optics are composed with (`%`

), the index lists are concatenated.
Thus composing an unindexed optic with an indexed optic preserves the
indices, or composing two indexed optics retains both indices:

λ> :t (ifolded % ifolded) (ifolded % ifolded) :: (FoldableWithIndex i1 f1, FoldableWithIndex i2 f2) => Optic A_Fold '[i1, i2] (f1 (f2 b)) (f1 (f2 b)) b b

In order to use such an optic, it is necessary to flatten the indices into a
single index using `icompose`

or a similar function:

λ> :t icompose (,) (ifolded % ifolded) icompose (,) (ifolded % ifolded) :: (FoldableWithIndex i1 f1, FoldableWithIndex i2 f2) => Optic A_Fold (WithIx (i1, i2)) (f1 (f2 b)) (f1 (f2 b)) b b

For example:

`>>>`

[((0,0),'a'),((0,1),'b'),((1,0),'c'),((1,1),'d')]`itoListOf (icompose (,) (ifolded % ifolded)) [['a','b'], ['c', 'd']]`

Alternatively, you can use one of the (`<%`

) or (`%>`

) operators to compose
indexed optics and pick the index to retain, or the (`<%>`

) operator to
retain a pair of indices:

`>>>`

[(0,'a'),(0,'b'),(1,'c'),(1,'d')]`itoListOf (ifolded <% ifolded) [['a','b'], ['c', 'd']]`

`>>>`

[(0,'a'),(1,'b'),(0,'c'),(1,'d')]`itoListOf (ifolded %> ifolded) [['a','b'], ['c', 'd']]`

`>>>`

[((0,0),'a'),((0,1),'b'),((1,0),'c'),((1,1),'d')]`itoListOf (ifolded <%> ifolded) [['a','b'], ['c', 'd']]`

In the diagram below, the optics hierachy is amended with these (singly) indexed variants (in blue).
Orange arrows mean
"can be used as one, assuming it's composed with any optic below the
orange arrow first". For example. `_1`

is not an indexed fold, but

is, because it's an indexed traversal, so it's
also an indexed fold.`itraversed`

% `_1`

`>>>`

`let fst' = _1 :: Lens (a, c) (b, c) a b`

`>>>`

fst' % itraversed :: TraversableWithIndex i f => Optic A_Traversal '[i] (f a, c) (f b, c) a b`:t fst' % itraversed`

module Optics.Indexed

# Generation of optics

## ...with Template Haskell

module Optics.TH

## ...with `OverloadedLabels`

module Optics.Label

# Optics for concrete base types

module Data.Either.Optics

module Data.Maybe.Optics

module Data.Tuple.Optics