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

Language | Haskell2010 |

This `lens`

tutorial targets Haskell beginners and assumes only basic
familiarity with Haskell. By the end of this tutorial you should:

- understand what problems the
`lens`

library solves, - know when it is appropriate to use the
`lens`

library, - be proficient in the most common
`lens`

idioms, - understand the drawbacks of using lenses, and:
- know where to look if you wish to learn more advanced tricks.

If you would like to follow along with these examples, just import this module:

$ ghci >>> import Control.Lens.Tutorial

- data Atom = Atom {}
- element :: Lens' Atom String
- point :: Lens' Atom Point
- data Point = Point {}
- x :: Lens' Point Double
- y :: Lens' Point Double
- data Molecule = Molecule {}
- atoms :: Iso' Molecule [Atom]
- data Pair a = Pair a a
- traverse :: Traversable t => forall a f b. Applicative f => (a -> f b) -> t a -> f (t b)

# Motivation

The simplest problem that the `lens`

library solves is updating deeply
nested records. Suppose you had the following nested Haskell data types:

data Atom = Atom { _element :: String, _point :: Point } data Point = Point { _x :: Double, _y :: Double }

If you wanted to increase the `x`

coordinate of an `Atom`

by one unit, you
would have to write something like this in Haskell:

shiftAtomX :: Atom -> Atom shiftAtomX (Atom e (Point x y)) = Atom e (Point (x + 1) y)

This unpacking and repacking of data types grows increasingly difficult the more fields you add to each data type or the more deeply nested your data structures become.

The `lens`

library solves this problem by letting you instead write:

-- atom.hs {-# LANGUAGE TemplateHaskell #-} import Control.Lens hiding (element) data Atom = Atom { _element :: String, _point :: Point } deriving (Show) data Point = Point { _x :: Double, _y :: Double } deriving (Show) makeLenses ''Atom makeLenses ''Point shiftAtomX :: Atom -> Atom shiftAtomX = over (point . x) (+ 1)

Let's convince ourselves that this works:

`>>>`

`let atom = Atom { _element = "C", _point = Point { _x = 1.0, _y = 2.0 } }`

`>>>`

Atom {_element = "C", _point = Point {_x = 2.0, _y = 2.0}}`shiftAtomX atom`

The above solution does not change no matter how many fields we add to
`Atom`

or `Point`

.

Now suppose that we added yet another data structure:

data Molecule = Molecule { _atoms :: [Atom] } deriving (Show)

We could shift an entire `Molecule`

by writing:

makeLenses ''Molecule shiftMoleculeX :: Molecule -> Molecule shiftMoleculeX = over (atoms . traverse . point . x) (+ 1)

Again, this works the way we expect:

`>>>`

`let atom1 = Atom { _element = "C", _point = Point { _x = 1.0, _y = 2.0 } }`

`>>>`

`let atom2 = Atom { _element = "O", _point = Point { _x = 3.0, _y = 4.0 } }`

`>>>`

`let molecule = Molecule { _atoms = [atom1, atom2] }`

`>>>`

Molecule {_atoms = [Atom {_element = "C", _point = Point {_x = 2.0, _y = 2.0}},Atom {_element = "O", _point = Point {_x = 4.0, _y = 4.0}}]}`shiftMoleculeX molecule -- Output formatted for clarity`

... or formatted for clarity:

Molecule { _atoms = [ Atom { _element = "C", _point = Point { _x = 2.0, _y = 2.0 } } , Atom { _element = "O", _point = Point { _x = 4.0, _y = 4.0 } } ] }

Many people stumble across lenses while trying to solve this common problem of working with data structures with a large number of fields or deeply nested values. These sorts of situations arise commonly in:

- games with complex and deeply nested state
- scientific data formats
- sensor or instrument output
- web APIs
- XML and JSON
- enterprise code where data structures can have tens, hundreds, or even thousands of fields (true story!)

# Lenses

You might have some basic questions like:

*Question:* What is a lens?

*Answer:* A lens is a first class getter and setter

We already saw how to use lenses to update values using `over`

, but we can
also use lenses to retrieve values using `view`

:

`>>>`

`let atom = Atom { _element = "C", _point = Point { _x = 1.0, _y = 2.0 } }`

`>>>`

1.0`view (point . x) atom`

In other words, lenses package both "get" and "set" functionality into a single value (the lens). You could pretend that a lens is a record with two fields:

data Lens a b = Lens { view :: a -> b , over :: (b -> b) -> (a -> a) }

That's not how lenses are actually implemented, but it's a useful starting intuition.

*Question:* What is the type of a lens?

*Answer:* We used two lenses in the above `Atom`

example, with these types:

point :: Lens' Atom Point x :: Lens' Point Double

The `point`

lens contains all the information we need to get or set the
`_point`

field of the `Atom`

type (which is a `Point`

). Similarly, the `x`

lens contains all the information we need to get or set the `_x`

field of
the `Point`

data type (which is a `Double`

).

The convention for the `Lens'`

type parameters is:

-- +-- Bigger type -- | -- v Lens' bigger smaller -- ^ -- | -- +-- Smaller type within the bigger type

The actual definition of `Lens'`

is:

type Lens' a b = forall f . Functor f => (b -> f b) -> (a -> f a)

You might wonder how you can fit both getter and setter functionality in
a single value like this. The trick is that we get to pick what `Functor`

we specialize `f`

to and depending on which `Functor`

we pick we get
different features.

For example, if you pick `(f = `

:`Identity`

)

type ASetter' a b = (b -> Identity b) -> (a -> Identity a) -- ... equivalent to: (b -> b) -> (a -> a)

... you can build an `over`

-like function.

Similarly, if you pick `(f = `

:`Const`

b)

type Getting b a b = (b -> Const b b) -> (a -> Const b a) -- ... equivalent to: (b -> b ) -> (a -> b ) -- ... equivalent to: (a -> b )

... you can build a `view`

-like function.

Those are not the only two `Functor`

s we can pick. In fact, we can do a
lot more with lenses than just get and set values, but those are the two
most commonly used features.

*Question:* How do I create lenses?

*Answer:* You can either auto-generate them using Template Haskell or
create them by hand

In our `Atom`

example, we auto-generated the lenses using Template Haskell,
like this:

makeLenses ''Atom makeLenses ''Point

This created four lenses of the following types:

element :: Lens' Atom String point :: Lens' Atom Point x :: Lens' Point Double y :: Lens' Point Double

`makeLenses`

creates one lens per field prefixed with an underscore. The
lens has the same name as the field without the underscore.

However, sometimes Template Haskell is not an option, so we can also use
the `lens`

utility function to build lenses. This utility has type:

lens :: (a -> b) -> (b -> a -> a) -> Lens' a b

The first argument is a "getter" (a way to extract a `'b'`

from an
`'a'`

). The second argument is a "setter" (given a `b`

, update an
`a`

). The result is a `Lens'`

built from the getter and setter. You would
use `lens`

like this:

point :: Lens' Atom Point point = lens _point (\newPoint atom -> atom { _point = newPoint })

You can even define lenses without incurring a dependency on the `lens`

library. Remember that lenses are just higher-order functions over
`Functor`

s, so we could instead write:

-- point :: Lens' Atom Point point :: Functor f => (Point -> f Point) -> Atom -> f Atom point k atom = fmap (\newPoint -> atom { _point = newPoint }) (k (_point atom))

This means that you can provide lenses for your library's types without
depending on the `lens`

library. All you need is the `fmap`

function,
which is provided by the Haskell Prelude.

*Question:* How do I combine lenses?

*Answer:* You compose them, using function composition (Yes, really!)

You can think of the function composition operator as having this type:

(.) :: Lens' a b -> Lens' b c -> Lens' a c

We can compose lenses using function composition because `Lens'`

is a
type synonym for a higher-order function:

type Lens' a b = forall f . Functor f => (b -> f b) -> (a -> f a)

So under the hood we are composing two higher-order functions to get back a new higher-order function:

(.) :: Functor f => ((b -> f b) -> (a -> f a)) -> ((c -> f c) -> (b -> f b)) -> ((c -> f c) -> (a -> f a))

In our original `Atom`

example, we composed the `point`

and `x`

lenses to
create a new composite lens:

point :: Lens' Atom Point x :: Lens' Point Double point . x :: Lens' Atom Double

This composite lens lets us get or set the `x`

coordinate of an `Atom`

.
We can use `over`

and `view`

on the composite `Lens'`

and they will behave
exactly the way we expect:

view (point . x) :: Atom -> Double over (point . x) :: (Double -> Double) -> (Atom -> Atom)

*Question:* How do I consume lenses?

*Answer:* Using `view`

, `set`

or `over`

Here are their types:

view :: Lens' a b -> a -> b over :: Lens' a b -> (b -> b) -> a -> a set :: Lens' a b -> b -> a -> a set lens b = over lens (\_ -> b)

`view`

and `over`

are the two fundamental functions on lenses. `set`

is
just a special case of `over`

.

`view`

and `over`

are fundamental because they distribute over lens
composition:

view (lens1 . lens2) = (view lens2) . (view lens1) view id = id

over (lens1 . lens2) = (over lens1) . (over lens2) over id = id

*Question:* What else do I need to know?

*Answer:* That's pretty much it!

For 90% of use cases, you just:

- Create lenses (using
`makeLens`

,`lens`

or plain-old`fmap`

) - Compose them (using (
`.`

)) - Consume them (using
`view`

,`set`

, and`over`

)

You could actually stop reading here if you are in a hurry since this covers the overwhelmingly common use case for the library. On the other hand, keep reading if you would like to learn additional tricks and features.

# Accessor notation

You might be used to object-oriented languages where you could retrieve a nested field using:

atom.point.x

You can do almost the exact same thing using the `lens`

library, except
that the first dot will have a `^`

right before the dot:

`>>>`

`let atom = Atom { _element = "C", _point = Point { _x = 1.0, _y = 2.0 } }`

`>>>`

1.0`atom^.point.x`

You can better understand why this works, by adding whitespace and explicit parentheses:

atom ^. (point . x)

This trick uses (`^.`

), which is an infix operator equivalent to `view`

:

(^.) :: a -> Lens' a b -> b x ^. l = view l x

... and you just keep adding dots after that for each lens you compose. This gives the appearance of object-oriented accessors if you omit the whitespace around the operators.

# First-class

Lenses are "first class" values, meaning that you can manipulate them using ordinary functional programming techniques. You can take them as inputs, return them as outputs, or stick them in data structures. Anything goes!

For example, suppose we don't want to define separate shift functions for
`Atom`

s and `Molecule`

s:

shiftAtomX :: Atom -> Atom shiftAtomX = over (point . x) (+ 1)

shiftMoleculeX :: Molecule -> Molecule shiftMoleculeX = over (atoms . traverse . point . x) (+ 1)

We can instead unify them into a single function by parametrizing the shift function on the lens:

shift lens = over lens (+ 1)

This lets us write:

shift (point . x) :: Atom -> Atom shift (atoms . traverse . point . x) :: Molecule -> Molecule

Even better, we can define synonyms for our composite lenses:

atomX :: Lens' Atom Double atomX = point . x -- We'll learn what `Traversal` means shortly moleculeX :: Traversal' Molecule Double moleculeX = atoms . traverse . point . x

Now we can write code almost identical to the original code:

shift atomX :: Atom -> Atom shift moleculeX :: Molecule -> Molecule

... but we also get several other utilities for free:

set atomX :: Double -> Atom -> Atom set moleculeX :: Double -> Molecule -> Molecule view atomX :: Atom -> Double -- We can't use `view` for `Traversal'`s. Read on to find out why toListOf moleculeX :: Molecule -> [Double]

That's much more reusable, but you might wonder what this `Traversal'`

and
`toListOf`

business is all about.

# Traversals

*Question:* What is a traversal?

*Answer:* A first class getter and setter for an arbitrary number of values

A traversal lets you get all the values it points to as a list and it also lets you update or set all the values it points to. Think of a traversal as a record with two fields:

data Traversal' a b = Traversal' { toListOf :: a -> [b] , over :: (b -> b) -> (a -> a) }

That's not how traversals are actually implemented, but it's a useful starting intuition.

We can still use `over`

and `set`

(a special case of `over`

) with a
traversal, but we use `toListOf`

instead of `view`

.

*Question:* What is the type of a traversal?

*Answer:* We used one traversal in the above `Molecule`

example:

moleculeX :: Traversal' Molecule Double

This `Traversal'`

lets us get or set an arbitrary number of x coordinates,
each of which is a `Double`

. There could be less than one x coordinate
(i.e. 0 coordinates) or more than one x coordinate. Contrast this with a
`Lens'`

which can only get or set exactly one value.

Like `Lens'`

, `Traversal'`

is a type synonym for a higher-order function:

type Traversal' a b = forall f . Applicative f => (b -> f b) -> (a -> f a) type Lens' a b = forall f . Functor f => (b -> f b) -> (a -> f a)

Notice that the only difference between a `Lens'`

and a `Traversal'`

is the
type class constraint. A `Lens'`

has a `Functor`

constraint and
`Traversal'`

has an `Applicative`

constraint. This means that any `Lens'`

is automatically also a valid `Traversal'`

(since `Functor`

is a superclass
of `Applicative`

).

Since every `Lens'`

is a `Traversal'`

, all of our example lenses also
double as traversals:

atoms :: Traversal' Molecule [Atom] element :: Traversal' Atom String point :: Traversal' Atom Point x :: Traversal' Point Double y :: Traversal' Point Double

We actually used yet another `Traversal'`

, which was `traverse`

(from
Data.Traversable):

traverse :: Traversable t => Traversal' (t a) a

This works because the `Traversal'`

type synonym expands out to:

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

... which is exactly the traditional type signature of `traverse`

.

In our `Molecule`

example, we were using the special case where `t = []`

:

traverse :: Traversal' [a] a

In Haskell, you can derive `Functor`

, `Foldable`

and
`Traversable`

for many data types using the `DeriveFoldable`

and
`DeriveTraversable`

extensions. This means that you can autogenerate a
valid `traverse`

for these data types:

{-# LANGUAGE DeriveFoldable #-} {-# LANGUAGE DeriveFunctor #-} {-# LANGUAGE DeriveTraversable #-} import Control.Lens import Data.Foldable data Pair a = Pair a a deriving (Functor, Foldable, Traversable)

We could then use `traverse`

to navigate from `Pair`

to its two children:

traverse :: Traversal' (Pair a) a over traverse :: (a -> a) -> (Pair a -> Pair a) over traverse (+ 1) (Pair 3 4) = Pair 4 5

*Question:* How do I create traversals?

*Answer:* There are three main ways to create primitive traversals:

`traverse`

is a`Traversal'`

that you get for any type that implements`Traversable`

- Every
`Lens'`

will also type-check as a`Traversal'`

- You can use Template Haskell to generate
`Traversal'`

s using`makePrisms`

since every`Prism'`

is also a`Traversal'`

(not covered in this tutorial)

*Question:* How do I combine traversals?

*Answer:* You compose them, using function composition

You can think of the function composition operator as having this type:

(.) :: Traversal' a b -> Traversal' b c -> Traversal' a c

We can compose traversals using function composition because a
`Traversal'`

is a type synonym for a higher-order function:

type Traversal' a b = forall f . Applicative f => (b -> f b) -> (a -> f a)

So under the hood we are composing two functions to get back a new function:

(.) :: Applicative f => ((b -> f b) -> (a -> f a)) -> ((c -> f c) -> (b -> f b)) -> ((c -> f c) -> (a -> f a))

In our original `Molecule`

example, we composed four `Traversal'`

s
together to create a new `Traversal'`

:

-- Remember that `atoms`, `point`, and `x` are also `Traversal'`s atoms :: Traversal' Molecule [Atom] traverse :: Traversal' [Atom] Atom point :: Traversal' Atom Point x :: Traversal' Point Double -- Now compose them atoms :: Traversal' Molecule [Atom] atoms . traverse :: Traversal' Molecule Atom atoms . traverse . point :: Traversal' Molecule Point atoms . traverse . point . x :: Traversal' Molecule Double

This composite traversal lets us get or set the `x`

coordinates of a
`Molecule`

.

over (atoms . traverse . point . x) :: (Double -> Double) -> (Molecule -> Molecule) toListOf (atoms . traverse . point . x) :: Molecule -> [Double]

*Question:* How do I consume traversals?

*Answer:* Using `toListOf`

, `set`

or `over`

Here are their types:

toListOf :: Traversal' a b -> a -> [b] over :: Traversal' a b -> (b -> b) -> a -> a set :: Traversal' a b -> b -> a -> a set traversal b = over traversal (\_ -> b)

Note that `toListOf`

distributes over traversal composition:

toListOf (traversal1 . traversal2) = (toListOf traversal1) >=> (toListOf traversal2) toListOf id = return

If you prefer object-oriented syntax you can also use (`^..`

), which is an
infix operator equivalent to `toListOf`

:

`>>>`

[3,4]`Pair 3 4 ^.. traverse`

# Types

You might wonder why you can use `over`

on both a `Lens'`

and a
`Traversal'`

but you can only use `view`

on a `Lens'`

. We can see why by
studying the (simplified) type and implementation of `over`

:

over :: ((b -> Identity b) -> (a -> Identity a)) -> (b -> b) -> a -> a over setter f x = runIdentity (setter (\y -> Identity (f y)) x)

To follow the implementation, just step slowly through the types. Here
are the types of the arguments to `over`

:

setter :: (b -> Identity b) -> (a -> Identity a) f :: b -> b x :: a

... and here are the types of the sub-expressions on the right-hand side:

\y -> Identity (f y) :: b -> Identity b setter (\y -> Identity (f y)) :: a -> Identity a setter (\y -> Identity (f y)) x :: Identity a runIdentity (setter (\y -> Identity (f y)) x) :: a

We can replace `setter`

with `point`

and replace `x`

with `atom`

to see
that this generates the correct code for updating an atom's point:

over point f atom -- Definition of `over` = runIdentity (point (\y -> Identity (f y)) atom) -- Definition of `point` = runIdentity (fmap (\newPoint -> atom { _point = newPoint }) (Identity (f (_point atom))) -- fmap g (Identity y) = Identity (g y) = runIdentity (Identity (atom { _point = f (_point atom) })) -- runIdentity (Identity z) = z = atom { _point = f (_point atom) }

... which is exactly what we would have written by hand without lenses.

The reason `over`

works for both `Lens'`

es and `Traversal'`

s is because
`Identity`

implements both `Functor`

and `Applicative`

:

instance Functor Identity where ... instance Applicative Identity where ...

So both the `Lens'`

type and `Traversal'`

type synonyms:

type Traversal' a b = forall f . Applicative f => (b -> f b) -> (a -> f a) type Lens' a b = forall f . Functor f => (b -> f b) -> (a -> f a)

... can both be specialized to use `Identity`

in place of `f`

:

(b -> Identity b) -> (a -> Identity a)

... making them valid arguments to `over`

.

Now let's study the (simplified) type and implementation of `view`

:

view :: ((b -> Const b b) -> (a -> Const b a)) -> a -> b view getter x = getConst (getter Const x)

Again, we can walk slowly through the types of the arguments:

getter :: (b -> Const b b) -> (a -> Const b a) x :: a

... and the types of the sub-expressions on the right-hand side:

getter Const :: a -> Const b a getter Const x :: Const b a getConst (getter Const x) :: b

Let's see how this plays out for the `point`

lens:

view point atom -- Definition of `view` = getConst (point Const atom) -- Definition of `point` = getConst (fmap (\newPoint -> atom { _point = newPoint }) (Const (_point atom))) -- fmap g (Const y) = Const y = getConst (Const (_point atom)) -- getConst (Const z) = z = _point atom

... which is exactly what we would have written by hand without lenses.

`view`

accepts `Lens'`

es because `Const`

implements `Functor`

:

instance Functor (Const b)

... so the `Lens'`

type synonym:

type Lens' a b = forall f . Functor f => (b -> f b) -> (a -> f a)

... can be specialized to use `(`

in place of `Const`

b)`f`

:

(b -> Const b b) -> (a -> Const b a)

... making it a valid argument to `view`

.

Interestingly, `Const`

implements also `Applicative`

, but with a
constraint:

instance Monoid b => Applicative (Const b)

This implies that we *can* use `view`

on a `Traversal'`

, but only if the
value that we extract is a `Monoid`

. Let's try this out:

`>>>`

`let atom1 = Atom { _element = "C", _point = Point { _x = 1.0, _y = 2.0 } }`

`>>>`

`let atom2 = Atom { _element = "O", _point = Point { _x = 3.0, _y = 4.0 } }`

`>>>`

`let molecule = Molecule { _atoms = [atom1, atom2] }`

`>>>`

"CO"`view (atoms . traverse . element) molecule`

This works because our traversal's result is a `String`

:

atoms . traverse . element :: Traversal' Molecule String

... and `String`

implements the `Monoid`

interface. When you
try to extract multiple strings using `view`

they get flattened together
into a single `String`

using `mappend`

.

If you try to extract the element from an empty molecule:

`>>>`

""`view (atoms . traverse . element) (Molecule { _atoms = [] })`

You get the empty string (i.e. `mempty`

).

This is why the result of a `Traversal'`

needs to be a `Monoid`

when using `view`

. If the `Traversal'`

points to more than one value you
need some way to combine them into a single value (using
`mappend`

) and if the `Traversal'`

points to less than one
value you need a default value to return (using `mempty`

).

If you try to `view`

a `Traversal'`

that doesn't point to a
`Monoid`

, you will get the following type error:

>>> view (atoms . traverse . point . x) molecule No instance for (Data.Monoid.Monoid Double) arising from a use of `traverse' In the first argument of `(.)', namely `traverse' In the second argument of `(.)', namely `traverse . point . x' In the first argument of `view', namely `(atoms . traverse . point . x)'

The compiler complains that `Double`

does not implement the
`Monoid`

type class, so there is no sensible way to merge all
the x coordinates that our `Traversal'`

points to. For these cases you
should use `toListOf`

instead.

# Drawbacks

Lenses come with trade-offs, so you should use them wisely.

For example, lenses do not produce the best error messages. Unless you
understand how `Traversal'`

s work you will probably not understand the
above error message.

Also, lenses increase the learning curve for new Haskell programmers, so you should consider avoiding them in tutorial code targeting novice Haskell programmers.

Lenses also add a level of boilerplate to all data types to auto-generate lenses and increase compile times. So for small projects the overhead of adding lenses may dwarf the benefits.

`lens`

is also a library with a large dependency tree, focused on being
"batteries included" and covering a large cross-section of the Haskell
ecosystem. Browsing the Hackage listing you will find support modules
ranging from System.FilePath.Lens to Control.Parallel.Strategies.Lens,
and many more. If you need a more light-weight alternative you can use
the `lens-simple`

or `microlens`

library, each of which provides a
restricted subset of the `lens`

library with a much smaller dependency tree.

The ideal use case for the `lens`

library is a medium-to-large project with
rich and deeply nested types. In these large projects the benefits of using
lenses outweigh the costs.

# Conclusion

This tutorial covers an extremely small subset of this library. If you would like to learn more, you can begin by skimming the example code in the following modules:

- Control.Lens.Getter
- Control.Lens.Setter
- Control.Lens.Traversal
- Control.Lens.Tuple
- Control.Lens.Lens
- Control.Lens.Review
- Control.Lens.Prism
- Control.Lens.Iso

The documentation for these modules includes several examples to get you started and help you build an intuition for more advanced tricks that were not covered in this tutorial.

You can also study several long-form examples here:

https://github.com/ekmett/lens/tree/master/examples

If you prefer light-weight `lens`

-compatible libraries, then check out
`lens-simple`

or `micro-lens`

:

If you would like a broader survey of lens features, then you can check out these tutorials:

- A little lens starter tutorial - Introduces Prisms, Isos and JSON functionality
- Program imperatively using Haskell lenses - Illustrates lens support for stateful code

# Exports

These are the same types and lenses used throughout the tutorial, exported for your convenience.

traverse :: Traversable t => forall a f b. Applicative f => (a -> f b) -> t a -> f (t b)

Map each element of a structure to an action, evaluate these actions
from left to right, and collect the results. For a version that ignores
the results see `traverse_`

.