This chapter is about functional references. By "references", we mean they point at parts of values, allowing us to access and modify them. By "functional", we mean they do so in a way that provides the flexibility and composability we came to expect from functions. We will study functional references as implemented by the powerful lens library. lens is named after lenses, a particularly well known kind of functional reference. Beyond being very interesting from a conceptual point of view, lenses and other functional references allow for several convenient and increasingly common idioms, put into use by a number of useful libraries.

## A taste of lenses

As a warm-up, we will demonstrate the simplest use case for lenses: as a nicer alternative to the vanilla Haskell records. There will be little in the way of explanations in this section; we will fill in the gaps through the remainder of the chapter.

Consider the following types, which are not unlike something you might find in a 2D drawing library:

```-- A point in the plane.
data Point = Point
{ positionX :: Double
, positionY :: Double
} deriving (Show)

-- A line segment from one point to another.
data Segment = Segment
{ segmentStart :: Point
, segmentEnd :: Point
} deriving (Show)

-- Helpers to create points and segments.
makePoint :: (Double, Double) -> Point
makePoint (x, y) = Point x y

makeSegment :: (Double, Double) -> (Double, Double) -> Segment
makeSegment start end = Segment (makePoint start) (makePoint end)
```

Record syntax gives us functions for accessing the fields. With them, getting the coordinates of the points that define a segment is easy enough:

```GHCi> let testSeg = makeSegment (0, 1) (2, 4)
GHCi> positionY . segmentEnd \$ testSeg
GHCi> 4.0
```

```GHCi> testSeg { segmentEnd = makePoint (2, 3) }
Segment {segmentStart = Point {positionX = 0.0, positionY = 1.0}
, segmentEnd = Point {positionX = 2.0, positionY = 3.0}}
```

... and get downright ugly when we need to reach a nested field. Here is what it takes to double the value of the y coordinate of the end point:

```GHCi> :set +m -- Enabling multi-line input in GHCi.
GHCi> let end = segmentEnd testSeg
GHCi| in testSeg { segmentEnd = end { positionY = 2 * positionY end } }
Segment {segmentStart = Point {positionX = 0.0, positionY = 1.0}
, segmentEnd = Point {positionX = 2.0, positionY = 8.0}}
```

Lenses allow us to avoid such nastiness, so let's start over with them:

```-- Some of the examples in this chapter require a few GHC extensions:
-- TemplateHaskell is needed for makeLenses; RankNTypes is needed for
-- a few type signatures later on.

import Control.Lens

data Point = Point
{ _positionX :: Double
, _positionY :: Double
} deriving (Show)
makeLenses ''Point

data Segment = Segment
{ _segmentStart :: Point
, _segmentEnd :: Point
} deriving (Show)
makeLenses ''Segment

makePoint :: (Double, Double) -> Point
makePoint (x, y) = Point x y

makeSegment :: (Double, Double) -> (Double, Double) -> Segment
makeSegment start end = Segment (makePoint start) (makePoint end)
```

The only real change here is the use of `makeLenses`, which automatically generates lenses for the fields of `Point` and `Segment` (the extra underscores are required by the naming conventions of `makeLenses`). As we will see, writing lenses definitions by hand is not difficult at all; however, it can be tedious if there are lots of fields to make lenses for, and thus automatic generation is very convenient.

Thanks to `makeLenses`, we now have a lens for each field. Their names match that of the fields, except with the leading underscore removed:

```GHCi> :info positionY
positionY :: Lens' Point Double
-- Defined at WikibookLenses.hs:9:1
GHCi> :info segmentEnd
segmentEnd :: Lens' Segment Point
-- Defined at WikibookLenses.hs:15:1
```

The type `positionY :: Lens' Point Double` tells us that `positionY` is a reference to a `Double` within a `Point`. To work with such references, we use the combinators provided by the lens library. One of them is `view`, which gives us the value pointed at by a lens, just like a record accessor:

```GHCi> let testSeg = makeSegment (0, 1) (2, 4)
GHCi> view segmentEnd testSeg
Point {_positionX = 2.0, _positionY = 4.0}
```

Another one is `set`, which overwrites the value pointed at:

```GHCi> set segmentEnd (makePoint (2, 3)) testSeg
Segment {_segmentStart = Point {_positionX = 0.0, _positionY = 1.0}
, _segmentEnd = Point {_positionX = 2.0, _positionY = 3.0}}
```

One of the great things about lenses is that they are easy to compose:

```GHCi> view (segmentEnd . positionY) testSeg
GHCi> 4.0
```

Note that when writing composed lenses, such as `segmentEnd . positionY`, the order is from large to small. In this case, the lens that focuses on a point of the segment comes before the one that focuses on a coordinate of that point. While that might look a little surprising in contrast to how record accessors work (compare with the equivalent lens-less example at the beginning of this section), the `(.)` used here is just the function composition operator we know and love.

Composition of lenses provide a way out of the nested record update quagmire. Here is a translation of the coordinate-doubling example using `over`, through which we can apply a function to the value pointed at by a lens:

```GHCi> over (segmentEnd . positionY) (2 *) testSeg
Segment {_segmentStart = Point {_positionX = 0.0, _positionY = 1.0}
, _segmentEnd = Point {_positionX = 2.0, _positionY = 8.0}}
```

These initial examples might look a bit magical at first. What makes it possible to use one and the same lens to get, set and modify a value? How come composing lenses with `(.)` just works? Is it really so easy to write lenses without the help of `makeLenses`? We will answer such questions by going behind the curtains to find what lenses are made of.

## The scenic route to lenses

There are many ways to make sense of lenses. We will follow a sinuous yet gentle path, one which avoids conceptual leaps. Along the way, we will introduce a few different kinds of functional references. Following lens terminology, from now on we will use the word "optics" to refer collectively to the various species of functional references. As we will see, the optics in lens are interrelated, forming a hierarchy. It is this hierarchy which we are now going to explore.

### Traversals

We will begin not with lenses, but with a closely related optic: traversals. The Traversable chapter discussed how `traverse` makes it possible to walk across a structure while producing an overall effect:

```traverse
:: (Applicative f, Traversable t) => (a -> f b) -> t a -> f (t b)
```

With `traverse`, you can use any `Applicative` you like to produce the effect. In particular, we have seen how `fmap` can be obtained from `traverse` simply by picking `Identity` as the applicative functor, and that the same goes for `foldMap` and `Const m`, using `Monoid m => Applicative (Const m)`:

```fmap f = runIdentity . traverse (Identity . f)
foldMap f = getConst . traverse (Const . f)
```

lens takes this idea and lets it blossom.

Manipulating values within a `Traversable` structure, as `traverse` allows us to, is an example of targeting parts of a whole. As flexible as it is, however, `traverse` only handles a rather limited range of targets. For one, we might want to walk across structures that are not `Traversable` functors. Here is an entirely reasonable function that does so with our `Point` type:

```pointCoordinates
:: Applicative f => (Double -> f Double) -> Point -> f Point
pointCoordinates g (Point x y) = Point <\$> g x <*> g y
```

`pointCoordinates` is a traversal of `Point`. It looks a lot like a typical implementation of `traverse`, and can be used in pretty much the same way. Here is an adaptation of the `rejectWithNegatives` example from the Traversable chapter:

```GHCi> let deleteIfNegative x =  if x < 0 then Nothing else Just x
GHCi> pointCoordinates deleteIfNegative (makePoint (1, 2))
Just (Point {_positionX = 1.0, _positionY = 2.0})
GHCi> pointCoordinates deleteIfNegative (makePoint (-1, 2))
Nothing
```

This generalised notion of a traversal that `pointCoordinates` exemplifies is captured by one of the core types of lens: `Traversal`.

```type Traversal s t a b =
forall f. Applicative f => (a -> f b) -> s -> f t
```

Note

The `forall f.` on the right side of the `type` declaration means that any `Applicative` can be used to replace `f`. That makes it unnecessary to mention `f` on the left side, or to specify which `f` to pick when using a `Traversal`.

With the `Traversal` synonym, the type of `pointCoordinates` can be expressed as:

```Traversal Point Point Double Double
```

Let's have a closer look at what became of each type variable in `Traversal s t a b`:

• `s` becomes `Point`: `pointCoordinates` is a traversal of a `Point`.
• `t` becomes `Point`: `pointCoordinates` produces a `Point` (in some `Applicative` context).
• `a` becomes `Double`: `pointCoordinates` targets `Double` values in a `Point` (the X and Y coordinates of the points).
• `b` becomes `Double`: the targeted `Double` values become `Double` values (possibly different than the original ones).

In the case of `pointCoordinates`, `s` is the same as `t`, and `a` is the same as `b`. `pointCoordinates` does not change the type of the traversed structure, or that of the targets in it, but that need not be the case. One example is good old `traverse`, whose type can be expressed as:

```Traversable t => Traversal (t a) (t b) a b
```

`traverse` is able to change the types of the targeted values in the `Traversable` structure and, by extension, the type of the structure itself.

The Control.Lens.Traversal module includes generalisations of Data.Traversable functions and various other tools for working with traversals.

Exercises
1. Write `extremityCoordinates`, a traversal that goes through all coordinates of the points that define a `Segment` in the order suggested by the `data` declaration. (Hint: use the `pointCoordinates` traversal.)

### Setters

Next in our programme comes the generalisation of the links between `Traversable`, `Functor` and `Foldable`. We shall begin with `Functor`.

To recover `fmap` from `traverse`, we picked `Identity` as the applicative functor. That choice allowed us to modify the targeted values without producing any extra effects. We can reach similar results by picking the definition of a `Traversal`...

```forall f. Applicative f => (a -> f b) -> s -> f t
```

... and specialising `f` to `Identity`:

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

In lens parlance, that is how you get a `Setter`. For technical reasons, the definition of `Setter` in Control.Lens.Setter is a little different...

```type Setter s t a b =
forall f. Settable f => (a -> f b) -> s -> f t
```

... but if you dig into the documentation you will find that a `Settable` functor is either `Identity` or something very much like it, so the difference need not concern us.

When we take `Traversal` and restrict the choice of `f` we actually make the type more general. Given that a `Traversal` works with any `Applicative` functor, it will also work with `Identity`, and therefore any `Traversal` is a `Setter` and can be used as one. The reverse, however, is not true: not all setters are traversals.

`over` is the essential combinator for setters. It works a lot like `fmap`, except that you pass a setter as its first argument in order to specify which parts of the structure you want to target:

```GHCi> over pointCoordinates negate (makePoint (1, 2))
Point {_positionX = -1.0, _positionY = -2.0}
```

In fact, there is a `Setter` called `mapped` that allows us to recover `fmap`:

```GHCi> over mapped negate [1..4]
[-1,-2,-3,-4]
GHCi> over mapped negate (Just 3)
Just (-3)
```

Another very important combinator is `set`, which replaces all targeted values with a constant. `set setter x = over setter (const x)`, analogously to how `(x <\$) = fmap (const x)`:

```GHCi> set pointCoordinates 7 (makePoint (1, 2))
Point {_positionX = 7.0, _positionY = 7.0}
```
Exercises
1. Use `over` to implement...
`scaleSegment :: Double -> Segment -> Segment`
... so that `scaleSegment n` multiplies all coordinates of a segment by `x`. (Hint: use your answer to the previous exercise.)
2. Implement `mapped`. For this exercise, you can specialise the `Settable` functor to `Identity`. (Hint: you will need Data.Functor.Identity.)

### Folds

Having generalised the `fmap`-as-traversal trick, it is time to do the same with the `foldMap`-as-traversal one. We will use `Const` to go from...

```forall f. Applicative f => (a -> f b) -> s -> f t
```

... to:

```forall r. Monoid r => (a -> Const r a) -> s -> Const r s
```

Since the second parameter of `Const` is irrelevant, we replace `b` with `a` and `t` with `s` to make our life easier.

Just like we have seen for `Setter` and `Identity`, Control.Lens.Fold uses something slightly more general than `Monoid r => Const r`:

```type Fold s a =
forall f. (Contravariant f, Applicative f) => (a -> f a) -> s -> f s
```

Note

`Contravariant` is a type class for contravariant functors. The key `Contravariant` method is `contramap`...

```contramap :: Contravariant f => (a -> b) -> f b -> f a
```

... which looks a lot like `fmap`, except that it, so to say, turns the function arrow around on mapping. Types parametrised over function arguments are typical examples of `Contravariant`. For instance, Data.Functor.Contravariant defines a `Predicate` type for boolean tests on values of type `a`:

```newtype Predicate a = Predicate { getPredicate :: a -> Bool }
```
```GHCi> :m +Data.Functor.Contravariant
GHCi> let largerThanFour = Predicate (> 4)
GHCi> getPredicate largerThanFour 6
True
```

`Predicate` is a `Contravariant`, and so you can use `contramap` to modify a `Predicate` so that the values are adjusted in some way before being submitted to the test:

```GHCi> getPredicate (contramap length largerThanFour) "orange"
True
```

`Contravariant` has laws which are analogous to the `Functor` ones:

```contramap id = id
contramap (g . f) = contramap f . contramap g
```

`Monoid r => Const r` is both a `Contravariant` and an `Applicative`. Thanks to the functor and contravariant laws, anything that is both a `Contravariant` and a `Functor` is, just like `Const r`, a vacuous functor, with both `fmap` and `contramap` doing nothing. The additional `Applicative` constraint corresponds to the `Monoid r`; it allows us to actually perform the fold by combining the `Const`-like contexts created from the targets.

Every `Traversal` can be used as a `Fold`, given that a `Traversal` must work with any `Applicative`, including those that are also `Contravariant`. The situation parallels exactly what we have seen for `Traversal` and `Setter`.

`Control.Lens.Fold` offers analogues to everything in Data.Foldable. Two commonly seen combinators from that module are `toListOf`, which produces a list of the `Fold` targets...

```GHCi> -- Using the solution to the exercise in the traversals subsection.
GHCi> toListOf extremityCoordinates (makeSegment (0, 1) (2, 3))
[0.0,1.0,2.0,3.0]
```

... and `preview`, which extracts the first target of a `Fold` using the `First` monoid from Data.Monoid.

```GHCi> preview traverse [1..10]
Just 1
```

### Getters

So far we have moved from `Traversal` to more general optics (`Setter` and `Fold`) by restricting the functors available for traversing. We can also go in the opposite direction, that is, making more specific optics by broadening the range of functors they have to deal with. For instance, if we take `Fold`...

```type Fold s a =
forall f. (Contravariant f, Applicative f) => (a -> f a) -> s -> f s
```

... and relax the `Applicative` constraint to merely `Functor`, we obtain `Getter`:

```type Getter s a =
forall f. (Contravariant f, Functor f) => (a -> f a) -> s -> f s
```

As `f` still has to be both `Contravariant` and `Functor`, it remains being a `Const`-like vacuous functor. Without the `Applicative` constraint, however, we can't combine results from multiple targets. The upshot is that a `Getter` always has exactly one target, unlike a `Fold` (or, for that matter, a `Setter`, or a `Traversal`) which can have any number of targets, including zero.

The essence of `Getter` can be brought to light by specialising `f` to the obvious choice, `Const r`:

```someGetter :: (a -> Const r a) -> s -> Const r s
```

Since a `Const r whatever` value can be losslessly converted to a `r` value and back, the type above is equivalent to:

```someGetter' :: (a -> r) -> s -> r

someGetter' k x = getConst (someGetter (Const . k) x)
someGetter g x = Const (someGetter' (getConst . g) x)
```

An `(a -> r) -> s -> r` function, however, is just an `s -> a` function in disguise (the camouflage being continuation passing style):

```someGetter'' :: s -> a

someGetter'' x = someGetter' id x
someGetter' k x = k (someGetter'' x)
```

Thus we conclude that a `Getter s a` is equivalent to a `s -> a` function. From this point of view, it is only natural that it takes exactly one target to exactly one result. It is not surprising either that two basic combinators from Control.Lens.Getter are `to`, which makes a `Getter` out of an arbitrary function, and `view`, which converts a `Getter` back to an arbitrary function.

```GHCi> -- The same as fst (4, 1)
GHCi> view (to fst) (4, 1)
4
```

Note

Given what we have just said about `Getter` being less general than `Fold`, it may come as a surprise that `view` can work `Fold`s and `Traversal`s as well as with `Getter`s:

```GHCi> :m +Data.Monoid
GHCi> view traverse (fmap Sum [1..10])
Sum {getSum = 55}
GHCi> -- both traverses the components of a pair.
GHCi> view both ([1,2],[3,4,5])
[1,2,3,4,5]
```

That is possible thanks to one of the many subtleties of the type signatures of lens. The first argument of `view` is not exactly a `Getter`, but a `Getting`:

```type Getting r s a = (a -> Const r a) -> s -> Const r s

view :: MonadReader s m => Getting a s a -> m a
```

`Getting` specialises the functor parameter to `Const r`, the obvious choice for `Getter`, but leaves it open whether there will be an `Applicative` instance for it (i.e. whether `r` will be a `Monoid`). Using `view` as an example, as long as `a` is a `Monoid` `Getting a s a` can be used as a `Fold`, and so `Fold`s can be used with `view` as long as the fold targets are monoidal.

Many combinators in both `Control.Lens.Getter` and `Control.Lens.Fold` are defined in terms of `Getting` rather than `Getter` or `Fold`. One advantage of using `Getting` is that the resulting type signatures tell us more about the folds that might be performed. For instance, consider `hasn't` from `Control.Lens.Fold`:

```hasn't :: Getting All s a -> s -> Bool
```

It is a generalised test for emptiness:

```GHCi> hasn't traverse [1..4]
False
GHCi> hasn't traverse Nothing
True
```

`Fold s a -> s -> Bool` would work just as well as a signature for `hasn't`. However, the `Getting All` in the actual signature is quite informative, in that it strongly suggests what `hasn't` does: it converts all `a` targets in `s` to the `All` monoid (more precisely, to `All False`), folds them and extracts a `Bool` from the overall `All` result.

### Lenses at last

If we go back to `Traversal`...

```type Traversal s t a b =
forall f. Applicative f => (a -> f b) -> s -> f t
```

... and relax the `Applicative` constraint to `Functor`, just as we did when going from `Fold` to `Getter`...

```type Lens s t a b =
forall f. Functor f => (a -> f b) -> s -> f t
```

... we finally reach the `Lens` type.

What changes when moving from `Traversal` to `Lens`? As before, relaxing the `Applicative` constraint costs us the ability to traverse multiple targets. Unlike a `Traversal`, a `Lens` always focuses on a single target. As usual in such cases, there is a bright side to the restriction: with a `Lens`, we can be sure that exactly one target will be found, while with a `Traversal` we might end up with many, or none at all.

The absence of the `Applicative` constraint and the uniqueness of targets point towards another key fact about lenses: they can be used as getters. `Contravariant` plus `Functor` is a strictly more specific constraint than just `Functor`, and so `Getter` is strictly more general than `Lens`. As every `Lens` is also a `Traversal` and therefore a `Setter`, we conclude that lenses can be used as both getters and setters. That explains why lenses can replace record labels.

Note

On close reading, our claim that every `Lens` can be used as a `Getter` might seem rash. Placing the types side by side...

```type Lens s t a b =
forall f. Functor f => (a -> f b) -> s -> f t

type Getter s a =
forall f. (Contravariant f, Functor f) => (a -> f a) -> s -> f s
```

... shows that going from `Lens s t a b` to `Getter s a` involves making `s` equal to `t` and `a` equal to `b`. How can we be sure that is possible for any lens? An analogous issue might be raised about the relationship between `Traversal` and `Fold`. For the moment, this question will be left suspended; we will return to it in the section about optic laws.

Here is a quick demonstration of the flexibility of lenses using `_1`, a lens that focuses on the first component of a tuple:

```GHCi> _1 (\x -> [0..x]) (4, 1) -- Traversal
[(0,1),(1,1),(2,1),(3,1),(4,1)]
GHCi> set _1 7 (4, 1) -- Setter
(7,1)
GHCi> over _1 length ("orange", 1) -- Setter, changing the types
(6,1)
GHCi> toListOf _1 (4, 1) -- Fold

GHCi> view _1 (4, 1) -- Getter
4
```
Exercises
1. Implement the lenses for the fields of `Point` and `Segment`, that is, the ones we generated with `makeLenses` early on. (Hint: Follow the types. Once you write the signatures down you will notice that beyond `fmap` and the record labels there is not much else you can use to write them.)
2. Implement the `lens` function, which takes a getter function `s -> a` and a setter function `s -> b -> t` and produces a `Lens s t a b`. (Hint: Your implementation will be able to minimise the repetitiveness in the solution of the previous exercise.)

## Composition

The optics we have seen so far fit the shape...

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

... in which:

• `f` is a `Functor` of some sort;
• `s` is the type of the whole, that is, the full structure the optic works with;
• `t` is the type of what the whole becomes through the optic;
• `a` is the type of the parts, that is, the targets within `s` that the optic focuses on; and
• `b` is the type of what the parts becomes through the optic.

One key thing those optics have in common is that they are all functions. More specifically, they are mapping functions that turn a function acting on a part (`a -> f b`) into a function acting on the whole (`s -> f t`). Being functions, they can be composed in the usual manner. Let's have a second look at the lens composition example from the introduction:

```GHCi> let testSeg = makeSegment (0, 1) (2, 4)
GHCi> view (segmentEnd . positionY) testSeg
GHCi> 4.0
```

An optic modifies the function it receives as argument to make it act on a larger structure. Given that `(.)` composes functions from right to left, we find that, when reading code from left to right, the components of an optic assembled with `(.)` focus on progressively smaller parts of the original structure. The conventions used by the lens type synonyms match this large-to-small order, with `s` and `t` coming before `a` and `b`. The table below illustrates how we can look at what an optic does either a mapping (from small to large) or as a focusing (from large to small), using `segmentEnd . positionY` as an example:

 Lens `segmentEnd` `positionY` `segmentEnd . positionY` Bare type ```Functor f => (Point -> f Point) -> (Segment -> f Segment)``` ```Functor f => (Double -> f Double) -> (Point -> f Point)``` ```Functor f => (Double -> f Double) -> (Segment -> f Segment)``` "Mapping" interpretation From a function on `Point` to a function on `Segment`. From a function on `Double` to a function on `Point`. From a function on `Double` to a function on `Segment`. Type with `Lens` `Lens Segment Segment Point Point` `Lens Point Point Double Double` `Lens Segment Segment Double Double` Type with `Lens'` `Lens' Segment Point` `Lens' Point Double` `Lens' Segment Double` "Focusing" interpretation Focuses on a `Point` within a `Segment` Focuses on a `Double` within a `Point` Focuses on a `Double` within a `Segment`

Note

The `Lens'` synonym is just convenient shorthand for lenses that do not change types (that is, lenses with `s` equal to `t` and `a` equal to `b`).

```type Lens' s a = Lens s s a a
```

There are analogous `Traversal'` and `Setter'` synonyms as well.

The types behind synonyms such as `Lens` and `Traversal` only differ in which functors they allow in place of `f`. As a consequence, optics of different kinds can be freely mixed, as long as there is a type which all of them fit. Here are some examples:

```GHCi> -- A Traversal on a Lens is a Traversal.
GHCi> (_2 . traverse) (\x -> [-x, x]) ("foo", [1,2])
[("foo",[-1,-2]),("foo",[-1,2]),("foo",[1,-2]),("foo",[1,2])]
GHCi> -- A Getter on a Lens is a Getter.
GHCi> view (positionX . to negate) (makePoint (2,4))
-2.0
GHCi> -- A Getter on a Traversal is a Fold.
GHCi> toListOf (both . to negate) (2,-3)
[-2,3]
GHCi> -- A Getter on a Setter does not exist (there is no unifying optic).
GHCi> set (mapped . to length) 3 ["orange", "apple"]

<interactive>:49:15:
No instance for (Contravariant Identity) arising from a use of ‘to’
In the second argument of ‘(.)’, namely ‘to length’
In the first argument of ‘set’, namely ‘(mapped . to length)’
In the expression: set (mapped . to length) 3 ["orange", "apple"]
```

## Operators

Several lens combinators have infix operator synonyms, or at least operators nearly equivalent to them. Here are the correspondences for some of the combinators we have already seen:

Prefix Infix
`view _1 (1,2)` `(1,2) ^. _1`
`set _1 7 (1,2)` `(_1 .~ 7) (1,2)`
`over _1 (2 *) (1,2)` `(_1 %~ (2 *)) (1,2)`
`toListOf traverse [1..4]` `[1..4] ^.. traverse`
`preview traverse []` `[] ^? traverse`

lens operators that extract values (e.g. `(^.)`, `(^..)` and `(^?)`) are flipped with respect to the corresponding prefix combinators, so that they take the structure from which the result is extracted as the first argument. That improves readability of code using them, as writing the full structure before the optics targeting parts of it mirrors how composed optics are written in large-to-small order. With the help of the `(&)` operator, which is defined simply as `flip (\$)`, the structure can also be written first when using modifying operators (e.g. `(.~)` and `(%~)`). `(&)` is particularly convenient when there are many fields to modify:

```sextupleTest = (0,1,0,1,0,1)
& _1 .~ 7
& _2 %~ (5 *)
& _3 .~ (-1)
& _4 .~ "orange"
& _5 %~ (2 +)
& _6 %~ (3 *)
```
```GHCi> sextupleTest
(7,5,-1,"orange",2,3)
```

## A swiss army knife

Thus far we have covered enough of lens to introduce lenses and show that they aren't arcane magic. That, however, is only the tip of the iceberg. lens is a large library providing a rich assortment of tools, which in turn realise a colourful palette of concepts. The odds are that if you think of anything in the core libraries there will be a combinator somewhere in lens that works with it. It is no exaggeration to say that a book exploring every corner of lens might be made as long as this one you are reading. Unfortunately, we cannot undertake such an endeavour right here. What we can do is briefly discussing a few other general-purpose lens tools you are bound to encounter in the wild at some point.

### `State` manipulation

There are quite a few combinators for working with state functors peppered over the `lens` modules. For instance:

• `use` from `Control.Lens.Getter` is an analogue of `gets` from `Control.Monad.State` that takes a getter instead of a plain function.
• `Control.Lens.Setter` includes suggestive-looking operators that modify parts of a state targeted a setter (e.g. `.=` is analogous to `set`, `%=` to `over` and `(+= x)` to `over (+x)`).
• Control.Lens.Zoom offers the remarkably handy `zoom` combinator, which uses a traversal (or a lens) to zoom into a part of a state. It does so by lifiting a stateful computation into one that works with a larger state, of which the original state is a part.

Such combinators can be used to write highly intention-revealing code that transparently manipulates deep parts of a state:

```import Control.Monad.State

stateExample :: State Segment ()
stateExample = do
segmentStart .= makePoint (0,0)
zoom segmentEnd \$ do
positionX += 1
positionY *= 2
pointCoordinates %= negate
```
```GHCi> execState stateExample (makeSegment (1,2) (5,3))
Segment {_segmentStart = Point {_positionX = 0.0, _positionY = 0.0}
, _segmentEnd = Point {_positionX = -6.0, _positionY = -6.0}}
```

### Isos

In our series of `Point` and `Segment` examples, we have been using the `makePoint` function as a convenient way to make a `Point` out of `(Double, Double)` pair.

```makePoint :: (Double, Double) -> Point
makePoint (x, y) = Point x y
```

The X and Y coordinates of the resulting `Point` correspond exactly to the two components of the original pair. That being so, we can define an `unmakePoint` function...

```unmakePoint :: Point -> (Double, Double)
unmakePoint (Point x y) = (x,y)
```

... so that `makePoint` and `unmakePoint` are a pair of inverses, that is, they undo each other:

```unmakePoint . makePoint = id
makePoint . unmakePoint = id
```

In other words, `makePoint` and `unmakePoint` provide a way to losslessly convert a pair to a point and vice-versa. Using jargon, we can say that `makePoint` and `unmakePoint` form an isomorphism.

`unmakePoint` might be made into a `Lens' Point (Double, Double)`. Symmetrically. `makePoint` would give rise to a `Lens' (Double, Double) Point`, and the two lenses would be a pair of inverses. Lenses with inverses have a type synonym of their own, `Iso`, as well as some extra tools defined in Control.Lens.Iso.

An `Iso` can be built from a pair of inverses through the `iso` function:

```iso :: (s -> a) -> (b -> t) -> Iso s t a b
```
```pointPair :: Iso' Point (Double, Double)
pointPair = iso unmakePoint makePoint
```

`Iso`s are `Lens`es, and so the familiar lens combinators work as usual:

```GHCi> import Data.Tuple (swap)
GHCi> let testPoint = makePoint (2,3)
GHCi> view pointPair testPoint -- Equivalent to unmakePoint
(2.0,3.0)
GHCi> view (pointPair . _2) testPoint
3.0
GHCi> over pointPair swap testPoint
Point {_positionX = 3.0, _positionY = 2.0}
```

Additionally, `Iso`s can be inverted using `from`:

```GHCi> :info from pointPair
from :: AnIso s t a b -> Iso b a t s
-- Defined in ‘Control.Lens.Iso’
pointPair :: Iso' Point (Double, Double)
-- Defined at WikibookLenses.hs:77:1
GHCi> view (from pointPair) (2,3) -- Equivalent to makePoint
Point {_positionX = 2.0, _positionY = 3.0}
GHCi> view (from pointPair . positionY) (2,3)
3.0
```

Another interesting combinator is `under`. As the name suggests, it is just like `over`, except that it uses the inverted `Iso` that `from` would give us. We will demonstrate it by using the `enum` isomorphism to play with the `Int` representation of `Char`s without using `chr` and `ord` from `Data.Char` explicitly:

```GHCi> :info enum
enum :: Enum a => Iso' Int a 	-- Defined in ‘Control.Lens.Iso’
GHCi> under enum (+7) 'a'
'h'
```

`newtype`s and other single-constructor types give rise to isomorphisms. Control.Lens.Wrapped exploits that fact to provide `Iso`-based tools which, for instance, make it unnecessary to remember record label names for unwrapping `newtype`s...

```GHCi> let testConst = Const "foo"
GHCi> -- getConst testConst
GHCi> op Const testConst
"foo"
GHCi> let testIdent = Identity "bar"
GHCi> -- runIdentity testIdent
GHCi> op Identity testIdent
"bar"
```

... and that make `newtype` wrapping for instance selection less messy:

```GHCi> :m +Data.Monoid
GHCi> -- getSum (foldMap Sum [1..10])
GHCi> ala Sum foldMap [1..10]
55
GHCi> -- getProduct (foldMap Product [1..10])
GHCi> ala Product foldMap [1..10]
3628800
```

### Prisms

With `Iso`, we have reached for the first time a rank below `Lens` in the hierarchy of optics: every `Iso` is a `Lens`, but not every `Lens` is an `Iso`. By going back to `Traversal`, we can observe how the optics get progressively less precise in what they point to:

• An `Iso` is an optic that has exactly one target and is invertible.
• A `Lens` also has exactly one target but is not invertible.
• A `Traversal` can have any number of targets and is not invertible.

Along the way, we first dropped invertibility and then the uniqueness of targets. If we follow a different path by dropping uniqueness before invertibility, we find a second kind of optic between isomorphisms and traversals: prisms. A `Prism` is an invertible optic that need not have exactly one target. As invertibility is incompatible with multiple targets, we can be more precise: a `Prism` can reach either no targets or exactly one target.

Aiming at a single target with the possibility of failure sounds a lot like pattern matching, and prisms are indeed able to capture that. If tuples and records provide natural examples of lenses, `Maybe`, `Either` and other types with multiple constructors play the same role for prisms.

Every `Prism` is a `Traversal`, and so the usual combinators for traversals, setters and folds all work with prisms:

```GHCi> set _Just 5 (Just "orange")
Just 5
GHCi> set _Just 5 Nothing
Nothing
GHCi> over _Right (2 *) (Right 5)
Right 10
GHCi> over _Right (2 *) (Left 5)
Left 5
GHCi> toListOf _Left (Left 5)

```

A `Prism` is not a `Getter`, though: the target might not be there. For that reason, we use `preview` rather than `view` to retrieve the target:

```GHCi> preview _Right (Right 5)
Just 5
GHCi> preview _Right (Left 5)
Nothing
```

For inverting a `Prism`, we use `re` and `review` from Control.Lens.Review. `re` is analogous to `from`, though it gives merely a `Getter`. `review` is equivalent to `view` with the inverted prism.

```GHCi> view (re _Right) 3
Right 3
GHCi> review _Right 3
Right 3
```

Just like there is more to lenses than reaching record fields, prisms are not limited to matching constructors. For instance, Control.Lens.Prism defines `only`, which encodes equality tests as a `Prism`:

```GHCi> :info only
only :: Eq a => a -> Prism' a ()
-- Defined in ‘Control.Lens.Prism’
GHCi> preview (only 4) (2 + 2)
Just ()
GHCi> preview (only 5) (2 + 2)
Nothing
```

The `prism` and `prism'` functions allow us to build our own prisms. Here is an example using `stripPrefix` from `Data.List`:

```GHCi> :info prism
prism :: (b -> t) -> (s -> Either t a) -> Prism s t a b
-- Defined in ‘Control.Lens.Prism’
GHCi> :info prism'
prism' :: (b -> s) -> (s -> Maybe a) -> Prism s s a b
-- Defined in ‘Control.Lens.Prism’
GHCi> import Data.List (stripPrefix)
GHCi> :t stripPrefix
stripPrefix :: Eq a => [a] -> [a] -> Maybe [a]
```
```prefixed :: Eq a => [a] -> Prism' [a] [a]
prefixed prefix = prism' (prefix ++) (stripPrefix prefix)
```
```GHCi> preview (prefixed "tele") "telescope"
Just "scope"
GHCi> preview (prefixed "tele") "orange"
Nothing
GHCi> review (prefixed "tele") "graph"
"telegraph"
```

`prefixed` is available from lens, in the Data.List.Lens module.

Exercises
1. `Control.Lens.Prism` defines an `outside` function, which has the following (simplified) type:

```outside :: Prism s t a b
-> Lens (t -> r) (s -> r) (b -> r) (a -> r)```
1. Explain what `outside` does without mentioning its implementation. (Hint: The documentation says that with it we can "use a `Prism` as a kind of first-class pattern". Your answer should expand on that, explaining how we can use it in such a way.)
2. Use `outside` to implement `maybe` and `either` from the Prelude:

`maybe :: b -> (a -> b) -> Maybe a -> b`

`either :: (a -> c) -> (b -> c) -> Either a b -> c`

## Laws

There are laws specifying how sensible optics should behave. We will now survey those that apply to the optics that we covered here.

Starting from the top of the taxonomy, `Fold` does not have laws, just like the `Foldable` class. `Getter` does not have laws either, which is not surprising, given that any function can be made into a `Getter` via `to`.

`Setter`, however, does have laws. `over` is a generalisation of `fmap`, and is therefore subject to the functor laws:

```over s id = id
over s g . over s f = over s (g . f)
```

As `set s x = over s (const x)`, a consequence of the second functor law is that:

```set s y . set s x = set s y
```

That is, setting twice is the same as setting once.

`Traversal` laws, similarly, are generalisations of the `Traversable` laws:

```t pure = pure
fmap (t g) . t f = getCompose . t (Compose . fmap g . f)
```

The consequences discussed in the Traversable chapter follow as well: a traversal visits all of its targets exactly once, and must either preserve the surrounding structure or destroy it wholly.

Every `Lens` is a `Traversal` and a `Setter`, and so the laws above also hold for lenses. In addition, every `Lens` is also a `Getter`. Given that a lens is both a getter and a setter, it should get the same target that it sets. This common sense requirement is expressed by the following laws:

```view l (set l x) = x
set l (view l z) z = z
```

Together with the "setting twice" law of setters presented above, those laws are commonly referred to as the lens laws.

Analogous laws hold for `Prism`s, with `preview` instead of `view` and `review` instead of `set`:

```preview p (review p x) = Just x
review p <\$> preview p z = Just z -- If preview p z isn't Nothing.
```

`Iso`s are both lenses and prisms, so all of the laws above hold for them. The prism laws, however, can be simplified, given that for isomorphisms `preview i = Just . view i` (that is, `preview` never fails):

```view i (review i x) = x
review i (view i z) = z
```

When we look at optic types such as `Setter s t a b` and `Lens s t a b` we see four independent type variables. However, if we take the various optic laws into account we find out that not all choices of `s`, `t`, `a` and `b` are reasonable. For instance, consider the "setting twice" law of setters:

```set s y . set s x = set s y
```

For "setting twice is the same than setting once" to make sense, it must be possible to set twice using the same setter. As a consequence, the law can only hold for a `Setter s t a b` if `t` can somehow be specialised so that it becomes equal to `s` (otherwise the type of the whole would change on every `set`, leading to a type mismatch).

From considerations about the types involved in the laws such as the one above, it follows that the four type parameters in law-abiding `Setter`s, `Traversal`s, `Prism`s and `Lens`es are not fully independent from each other. We won't examine the interdependency in detail, but merely point out some of its consequences. Firstly, `a` and `b` are cut from the same cloth, in that even if an optic can change types there must be a way of specialising `a` and `b` to make them equal; furthermore, the same holds for `s` and `t`. Secondly, if `a` and `b` are equal then `s` and `t` must be equal as well.

In practice, those restrictions mean that valid optics that can change types usually have `s` and `t` parametrised in terms of `a` and `b`. Type-changing updates in this fashion are often referred to as polymorphic updates. For the sake of illustration, here are a few arbitrary examples taken from lens:

```-- To avoid distracting details,
-- we specialised the types of argument and _1.
mapped :: Functor f => Setter (f a) (f b) a b
contramapped :: Contravariant f => Setter (f b) (f a) a b
argument :: Setter (b -> r) (a -> r) a b
traverse :: Traversable t => Traversal (t a) (t b) a b
both :: Bitraversable r => Traversal (r a a) (r b b) a b
_1 :: Lens (a, c) (b, c) a b
_Just :: Prism (Maybe a) (Maybe b) a b
```

At this point, we can return to the question left open when we presented the `Lens` type. Given that `Lens` and `Traversal` allow type changing while `Getter` and `Fold` do not, it would be indeed rash to say that every `Lens` is a `Getter`, or that every `Traversal` is a `Fold`. However, the interdependence of the type variables mean that every lawful `Lens` can be used as a `Getter`, and every lawful `Traversal` can be used as a `Fold`, as lawful lenses and traversals can always be used in non type-changing ways.

## No strings attached

As we have seen, we can use lens to define optics through functions such as `lens` and auto-generation tools such as `makeLenses`. Strictly speaking, though, these are merely convenience helpers. Given that `Lens`, `Traversal` and so forth are just type synonyms, their definitions are not needed when writing optics − for instance, we can always write `Functor f => (a -> f b) -> (s -> f t)` instead of `Lens s t a b`. That means we can define optics compatible with lens without using lens at all! In fact, any `Lens`, `Traversal`, `Setter` or `Getting` can be defined with no dependencies other than the base package.

The ability to define optics without depending on the lens library provides considerable flexibility in how they can be leveraged. While there are libraries that do depend on lens, library authors are often wary of acquiring a dependency on large packages with several dependencies such as lens, especially when writing small, general-purpose libraries. Such concerns can be sidestepped by defining the optics without using the type synonyms or the helper tools in lens. Furthermore, the types being only synonyms makes it possible to have multiple optic frameworks (i.e. lens and similar libraries) that can be used interchangeably.