Haskell/Lens
本章中我們將討論函數式引用. "引用"指的是能夠對值的一部分進行訪問和修改的能力; "函數式"指我們不會因此失去 Haskell 中函數所具有的靈活性和可複合性. 我們將討論強大的 lens
庫所實現的函數式引用. lens
庫的名字來源於 lenses(透鏡), 我們將會介紹的一種臭名昭著的函數式引用. 除了作為一些非常有趣的概念外, lens 和其它函數式引用為我們帶來了一些方便而且越來越普遍的語言用法, 而且被許多實用的庫所採用.
初嘗 lens
編輯作為熱身, 我們將演示 lens 最簡單的使用案例: 替代傳統的 Haskell 的具有命名的數據類型 (record syntax). 我們先不給出詳細的解釋; 隨着本章的進展, 我們會逐漸覆蓋所需的知識的.
我們來看看下面兩個數據類型, 或許我們會在一些2D繪圖庫中看見它們:
 平面上一点.
data Point = Point
{ positionX :: Double
, positionY :: Double
} deriving (Show)
 两点间的一条线段.
data Segment = Segment
{ segmentStart :: Point
, segmentEnd :: Point
} deriving (Show)
 构造线和点的函数.
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 自動為我們定義了訪問這兩個數據類型中的域的函數. 有了這些函數, 從一條線段中讀取它的兩個端點並不困難:
GHCi> let testSeg = makeSegment (0, 1) (2, 4)
GHCi> positionY . segmentEnd $ testSeg
GHCi> 4.0
但修改就略嫌麻煩了...
GHCi> testSeg { segmentEnd = makePoint (2, 3) }
Segment {segmentStart = Point {positionX = 0.0, positionY = 1.0}
, segmentEnd = Point {positionX = 2.0, positionY = 3.0}}
...而當我們需要修改嵌套在深處的域時, 代碼變得非常醜陋. 例如, 修改線段終點的 y 軸坐標:
GHCi> :set +m  开启 GHCi 的多行模式.
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}}
lens 允許我們繞開這些討厭的嵌套, 觀察下面的代碼:
 本章中的一些例子需要开启一些 GHC 扩展:
 makeLenses 需要 TemplateHaskell 扩展;
 之后的一些类型声明需要 RankNTypes 扩展.
{# LANGUAGE TemplateHaskell, RankNTypes #}
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)
這裡唯一的改變就是 makeLenses
, 其自動生成了 Point
和 Segment
的 lens (域名前的下劃線是 makeLenses
的特殊要求). 我們將會看到, 手寫 lens 定義並不複雜; 然而, 如果有許多域都需要使用 lense, 這個過程就會變得很枯燥, 因此我們使用方便的自動生成.
使用了 makeLenses
後, 每一個域都有各自的 lens 了. 這些 lens 的名字和域名一一對應, 區別之處在於頭部的下劃線被刪除了:
GHCi> :info positionY
positionY :: Lens' Point Double
 定义于 WikibookLenses.hs:9:1
GHCi> :info segmentEnd
segmentEnd :: Lens' Segment Point
 定义于 WikibookLenses.hs:15:1
類型簽名 positionY :: Lens' Point Double
告訴我們, positionY
是一個 Point
中 Double
的引用. 我們使用 lens
庫提供的組合函數來操作這些引用. 其中一個函數是 view
, 其返回一個 lens 所指向的值, 如同 record 所自動生成的那些訪問函數一樣:
GHCi> let testSeg = makeSegment (0, 1) (2, 4)
GHCi> view segmentEnd testSeg
Point {_positionX = 2.0, _positionY = 4.0}
另一個是 set
, 它能夠修改其指向的值:
GHCi> set segmentEnd (makePoint (2, 3)) testSeg
Segment {_segmentStart = Point {_positionX = 0.0, _positionY = 1.0}
, _segmentEnd = Point {_positionX = 2.0, _positionY = 3.0}}
lens 的一大優點是它們能夠互相組合:
GHCi> view (segmentEnd . positionY) testSeg
4.0
注意到, 在組合 lens, 例如 segmentEnd . positionY
時, 順序是由總到分. 本例中, 指向線段的終點的 lens 寫在指向點的坐標的 lens 前面. 或許這和 record 所提供的函數的工作方式不盡相同 (和本節開頭不使用 lens 的等價寫法比較), 但這裡的 (.)
確實是我們所熟悉的函數組合.
lens 的組合為修改嵌套 record 內部的值提供了一個解決方案. 我們將之前給出的將坐標翻倍的例子改寫成使用 lens 和 over
函數的寫法, 後者將一個函數應用到被 lens 指向的值上 (並返回整個 record 修改後的值):
GHCi> over (segmentEnd . positionY) (2 *) testSeg
Segment {_segmentStart = Point {_positionX = 0.0, _positionY = 1.0}
, _segmentEnd = Point {_positionX = 2.0, _positionY = 8.0}}
這些例子或許看起來有些不可思議. 為什麼用同一個 lens 我們不僅能訪問, 還能夠修改一個值呢? 為什麼 lens 能夠用 (.)
組合呢? 不使用 makeLenses
而是改為手寫 lens 真的並不困難嗎? 為了回答這些問題, 我們將介紹 lens 的工作原理.
lens 前方800m
編輯我們能從許多角度解讀 lens. 我們將遵循一條蜿蜒曲折而平緩的道路, 避免跳躍過大. 一路上, 我們將介紹好幾個種類的函數式引用. 我們將使用 lens 的命名趣味, 使用"光學元件(optic)"^{[1]}來統稱函數式引用. 正如我們將看到的, lens
中的 optic 互相關聯, 形成了有序的上下關係. 我們即將介紹這種關係.
traversal
編輯我們選擇不從 lens, 而是從一個緊密相關的 optic  traversal
 入手. 我們知道, traverse
能夠遍歷一個結構並產生一個最終結果.
traverse
:: (Applicative f, Traversable t) => (a > f b) > t a > f (t b)
有了 traverse
, 你能夠使用任何你想要的 Applicative
來產生這個最終結果. 特別的, 我們知道 fmap
能夠用 traverse
定義: 只要選擇 Identity
作為其中的 Applicative
就好了. foldMap
和 Const m
也存在類似的關係:
fmap f = runIdentity . traverse (Identity . f)
foldMap f = getConst . traverse (Const . f)
lens
是在這個基礎上一次漂亮的延伸.
操作 Traversable
結構內部的數據, 也就是 traverse
的功能, 恰恰就是一個操作整體數據內部特定部分的例子. 然而 traverse
的靈活性僅僅允許我們處理有限範圍內的類型. 例如, 我們或許會想要操作非 Traversable
的值. 比如說, 我們或許會想要這樣一個處理 Point
值的函數:
pointCoordinates
:: Applicative f => (Double > f Double) > Point > f Point
pointCoordinates g (Point x y) = Point <$> g x <*> g y
pointCoordinates
是對 Point
類型值的一種 traversal (遍歷). 它和 traverse
具有相似的實現和使用方法. 這是來自之前章節^{[2]}的 rejectWithNegatives
的使用樣例:
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
這種 pointCoordinates
的例子中出現的一般意義上的遍歷被 lens
庫的核心類型之一  Traversal
所表示:
type Traversal s t a b =
forall f. Applicative f => (a > f b) > s > f t
註解

有了 Traversal
類型別名的定義, pointCoordinates
的類型能夠被表示為:
Traversal Point Point Double Double
讓我們看看 Traversal s t a b
中每個類型變量的值:
s
=Point
:pointCoordinates
是一個Point
上的遍歷.t
=Point
:pointCoordinates
將產生一個Point
(某種Applicative
的意義上).a
=Double
:pointCoordinates
指向一個Point
內的Double
值 (點的 X 和 Y 坐標).b
=Double
: 指向的Double
將被修改為一個Double
(有時不一定相同).
在 pointCoordinates
的例子中, s
和 t
相同, a
也和 b 相同. pointCoordinates
並不改變被遍歷結構和它的"內部目標"的類型, 但這並不對所有 lens 成立. 例如我們熟悉的 traverse
, 其類型可以被表示為:
Traversable t => Traversal (t a) (t b) a b
traverse
能夠改變 Traversable
結構內部值的類型, 因此也能夠改變整個結構的類型.
Control.Lens.Traversal 模塊中包含了 Data.Traversable 模塊中函數的推廣, 以及一些額外的操作 traversal 的函數.
練習 


設置器
編輯接下來我們的程序中將推廣 Traversable
, Functor
和 Foldable
之間的聯繫. 我們將從 Functor
開始.
為了從 traverse
中恢復 fmap
, 我們選擇 Identity
作為相應的應用函子. 這使得我們能夠修改目標值而不產生別的影響. 我們可以通過選擇一個 Traversal
的定義實現相似的功能...
forall f. Applicative f => (a > f b) > s > f t
... 並設定 f
為 Identity
:
(a > Identity b) > s > Identity t
用 lens
相關的說法, 這樣做使你得到了一個 Setter
. 由於一些專門的原因, 在 Template:Haskell lib 中 Setter
的定義有點不同...
type Setter s t a b =
forall f. Settable f => (a > f b) > s > f t
...但如果你從文檔中深入發掘你會發現一個 Settable
函子不過就是一個 Identity
或者差不多的東西, 因此不必在意這裡面的差異.
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}
練習 


Folds
編輯Having generalised the fmap
astraversal trick, it is time to do the same with the foldMap
astraversal 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
, Template:Haskell lib 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
註解
contramap :: Contravariant f => (a > b) > f b > f a
... which looks a lot like newtype Predicate a = Predicate { getPredicate :: a > Bool }
GHCi> :m +Data.Functor.Contravariant
GHCi> let largerThanFour = Predicate (> 4)
GHCi> getPredicate largerThanFour 6
True
GHCi> getPredicate (contramap length largerThanFour) "orange"
True
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 Template:Haskell lib. 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 Template:Haskell lib.
GHCi> preview traverse [1..10]
Just 1
Getters
編輯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 Template:Haskell lib 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
註解
Given what we have just said about 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 type Getting r s a = (a > Const r a) > s > Const r s
view :: MonadReader s m => Getting a s a > m a
Many combinators in both 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

Lenses at last
編輯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.
註解
On close reading, our claim that every 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 
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
[4]
GHCi> view _1 (4, 1)  Getter
4
練習 


Composition
編輯The optics we have seen so far fit the shape...
(a > f b) > (s > f t)
... in which:
f
is aFunctor
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 withins
that the optic focuses on; andb
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 largetosmall 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

註解
The type Lens' s a = Lens s s a a
There are analogous 
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
編輯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 largetosmall 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
編輯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 generalpurpose lens
tools you are bound to encounter in the wild at some point.
State
manipulation
編輯
There are quite a few combinators for working with state functors peppered over the lens
modules. For instance:
use
fromControl.Lens.Getter
is an analogue ofgets
fromControl.Monad.State
that takes a getter instead of a plain function.Control.Lens.Setter
includes suggestivelooking operators that modify parts of a state targeted a setter (e.g..=
is analogous toset
,%=
toover
and(+= x)
toover (+x)
). Template:Haskell lib 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 intentionrevealing 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
編輯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 viceversa. 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 Template:Haskell lib.
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 singleconstructor types give rise to isomorphisms. Template:Haskell lib 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
編輯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
Traversable
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)
[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 Template:Haskell lib. 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, Template:Haskell lib 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 Template:Haskell lib module.
練習 


Laws
編輯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
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
Polymorphic updates
編輯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 lawabiding Setter
s, Traversal
s, Prism
s 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
. Typechanging 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 typechanging ways.
No strings attached
編輯As we have seen, we can use lens
to define optics through functions such as lens
and autogeneration 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
and 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, generalpurpose 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 make it possible to have multiple optic frameworks (i.e. lens
and similar libraries) that can be used interchangeably.
Further reading
編輯 Several paragraphs above, we said that
lens
easily provides enough material for a full book. The closest thing to that we currently have is Artyom Kazak's "lens over tea" series of blog posts. It explores the implementation of functional references inlens
and the concepts behind it in far more depth than what we are able to do here. Highly recommended reading.  Useful information can be reached through
lens
' GitHub wiki, and of courselens
' API documentation is well worth exploring. lens
is a large and complex library. If you want to study its implementation but would rather begin with something simpler, a good place to start are minimalisticlens
compatible libraries such asmicrolens
andlenssimple
. Studying (and using!) opticpowered libraries is a good way to get the hang of how functional references are used. Some arbitrary examples:
diagrams
, a vector graphics library that useslens
extensively to deal with properties of graphic elements.wreq
, a web client library with alens
based interface.xmllens
, which provides optics for manipulating XML.formattable
, a library for date, time and number formattting. Template:Haskell lib is an example of a module that provideslens
compatible lenses without depending on thelens
package.
Lens 
習題解答 
Advanced Haskell 
Haskell 
Haskell基礎
>> 初級Haskell
>> Haskell進階
>> Monads
