Edward Kmett ekmett at gmail.com
Thu Jun 27 05:11:02 CEST 2013

```Let me take a couple of minutes to summarize how the lens approach tackles
the composition problem today without requiring confusing changes in the
lexical structure of the language.

I'll digress a few times to showcase how this actually lets us make more
powerful tools than are available in standard OOP programming frameworks as
I go.

The API for lens was loosely inspired once upon a time by Erik Meijer's old
'the power is in the dot' paper, but the bits and pieces have nicely become
more orthogonal.

Lens unifies the notion of (.) from Haskell with the notion of (.) as a
field accessor by choosing an interesting form for the domain and codomain
of the functions it composes.

I did a far more coherent introduction at New York Haskell
sitting through if you have more time.

In particular in that talk I spend a lot of time talking about all of the
other lens-like constructions you can work with. More resources including
several blog posts, announcements, a tutorial, etc. are available on
http://lens.github.com/

A lens that knows how to get a part p out of a whole w looks like

type Lens' w p = forall f. Functor f => (p -> f p) -> w -> f w

In the talk I linked above, I show how this is equivalent to a
getter/setter pair.

Interestingly because the function is already CPSd, this composition is the
'reverse' composition you expect.

You can check that:

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

The key here is that a lens is a function from a domain of (p -> f p)   to
a codomain of (w -> f w) and therefore they compose with (.) from the
Prelude.

We can compose lenses that know how to access parts of a structure in a
manner analogous to writing a Traversable instance.

Lets consider the lens that accesses the second half of a tuple:

_2 f (a,b) = (,) a <\$> f b

We can write a combinator that use these lenses to read and write their
respective parts:

import Control.Applicative

infixl 8 ^.

s ^. l = getConst (l Const s)

With that combinator in hand:

("hello","world")^._2 = "world"

(1,(3,4))^._2._2 = 4 -- notice the use of (.) not (^.) when chaining these.

Again this is already in the order an "OOP programmer" expects when you go
compose them!

_1 f (a,b) = (,b) <\$> f a

(1,(3,4))^._2._1 = 3

The fixity of (^.) was chosen carefully so that the above parses as

(1,(3,4))^.(_2._1)

If you just write the definitions for the lenses I gave above and let type
inference give you their types they turn out to be more general than the
signature for Lens'  above.

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

With that type you could choose to write the signatures above as:

_1 :: Lens (a,c) (b,c) a b
_2 :: Lens (c,a) (c,b) a b

(^.) :: s -> ((a -> Const a b) -> s -> Const a t) -> a

But we don't need the rank-2 aliases for anything other than clarity.
In particular the code above can be written and typechecked entirely

We can also generate a 'getter' from a normal haskell function such
that it can be composed with lenses and other getters:

to :: (s -> a) -> (a -> Const r b) -> s -> Const r t

to sa acr = Const . getConst . acr . sa

x^.to f = getConst (to f Const s) = getConst ((Const . getConst .
Const . f) s) = f s

Then the examples where folks have asked to be able to just compose in

(1,"hello")^._2.to length = 5

We can also write back through a lens:

They take on the more general pattern that actually allows type
changing assignment.

modify :: ((a -> Identity b) -> s -> Identity t) -> (a -> b) -> s -> t

modify l ab = runIdentity . l (Identity . ab)

set l b = modify l (const b)

These can be written entirely using 'base' rather than with Identity
from transformers by replacing Identity with (->) ()

With that in hand we can state the 'Setter' laws:

modify l id = id

modify l f . modify l g = modify l (f . g)

These are just the Functor laws!

and we can of course make a 'Setter' for any Functor that you could
pass to modify:

mapped :: Functor f => (a -> Identity b) -> f a -> Identity (f b)

mapped aib = Identity . fmap (runIdentity . aib)

then you can verify that

modify mapped ab = runIdentity . Identity . fmap (Identity .
runIdentity ab) = fmap ab

modify (mapped.mapped) = fmap.fmap

'mapped' isn't a full lens. You can't read from 'mapped' with (^.).
Try it. Similarly 'to' gives you merely a 'Getter', not something
suitable to modify. You can't 'modify the output of 'to', the types
won't let you. (The lens type signatures are somewhat more complicated
here because they want the errors to be in instance resolution rather

But we can still use modify on any lens, because Identity is a
perfectly cromulent Functor.

modify _2 (+2) (1,2) = (1,4)

modify _2 length (1,"hello") = (1,5) -- notice the change of type!

modify (_2._1) (+1) (1,(2,3)) = (1,(3,3))

modify (_2.mapped) (+1) (1,[2,3,4]) = (1,[3,4,5])

We can also define something very lens-like that has multiple targets.
In fact we already know the canonical example of this, 'traverse' from
Data.Traversable. So we'll call them traversals.

We can use modify on any 'traversal' such as traverse:

modify traverse (+1) [1,2,3] = [2,3,4]

This permits us to modify multiple targets with a lens in a coherent,
possibly type changing manner.

We can make new traversals that don't exactly match the types in
Data.Traversable as well:

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

both :: Traversal (a,a) (b,b) a b

both f (a,b) = (,) <\$> f a <*> f b

modify both (+1) (1,2) = (3,4)

The laws for a traversal are a generalization of the Traversable laws.

Compositions of traversals form valid traversals.

Lens goes farther and provides generalizations of Foldables as
'Folds', read-only getters, etc. just by changing the constraints on
'f' in the (a -> f b) -> s -> f t form.

The key observation here is that we don't need to make up magic syntax
rules for (.) just to get reverse application. We already have it!

The only thing we needed was a slightly different (.)-like operator to
start the chain ((^.) above.).

This is nice because it allows us to talk about compositions of lenses
as first class objects we can pass around.

Moreover they compose naturally with traversals, and the idioms we
already know how to use with traverse apply. In fact if you squint you
can recognize the code for modify and (^.) from the code for
foldMapDefault and fmapDefault in Data.Traversable, except we just
pass in the notion of 'traverse' as the extra lens-like argument.

Every Lens is a valid Traversal.

modify (both._1) (+1) ((1,2),(3,4)) = ((2,2),(4,4))

If you have a lens foo and a lens bar then baz = foo.bar is also a lens.

We can make lenses that can access fairly complex structures. e.g. we
can make lenses that let us both read and write whether or not
something is in a Set:

contains :: Ord k => k -> Lens' (Set k) Bool

contains k f s = (\b -> if b then Set.insert k s else Set.delete k s) <\$> f
(Set.member k s)

singleton 4 ^. contains 4 = True

singleton 4 ^. contains 5 = False

set (contains 5) True (singleton 4) = fromList [4,5]

This sort of trick has been often used to idiomatically allow for sets of
flags to be thrown in data types as a field.

data Flags = Foo | ...
data Bar a = Bar { barA :: a,  barFlags :: Set Flags }

flags f (Bar a flgs) = Bar a <\$> f flgs

foo = flags.contains Foo

We can similarly access the membership of a map as a lens.

alterF :: Ord k => Int -> (Maybe a -> f (Maybe a)) -> Map k a -> f (Map k a)

This can be viewed as:

alterF :: Ord k => Int -> Lens' (Map k a) (Maybe a)

or the lens that accesses a field out of a record type:

data Foo = Foo { _fooX, _fooY :: Int }

fooY f (Foo x y) = Foo x <\$> f y

The latter usecase is the only one that we've been considering in the
record debate, but having a solution that extends to cover all of these
strikes me as valuable.

Defining these lenses do not take us outside of Haskell 98. They do not
require anything that isn't currently provided by base.

Just a couple more notes:

I tried to keep the above more or less self-contained. It doesn't use very
'idiomatic' lens code. Normally most of the lens users would use code like:

(1,2) & _2 .~ "hello" = (1,"hello")
where
x & f = f x
l .~ a = modify l (const a) -- with appropriate fixities, etc.

Also of concern to me is that it is already common practice among uses of
lens to elide spaces around (.) when composing lenses, so such a syntactic
change is going to break a lot of code or at least break a lot of habits.

The relevance to the discussion at hand I think is that (^.) is a rather
simple combinator that can be defined in the language today. It is one that
has been defined in multiple libraries (lens, lens-family, etc.) It doesn't
require weird changes to the syntax of the language and notably once you
'start' accessing into a structure with it, the subsequent dots are just
Prelude dots and the result is more powerful in that it generalizes in more
directions.

This approach already has hundreds of users (we have 90+ users in
#haskell-lens 24 hours a day on freenode, packdeps shows ~80 reverse
doesn't break any existing code.

Simon, the 'makeLenses' 'makeClassy' and 'makeFields' template-haskell
functions for lens try to tackle the SORF/DORF-like aspects. These are what
Greg Weber was referring to in that earlier email. Kickstarting that
discussion probably belongs in another email as this one is far to long, as
there a lot of points in the design space there that can be explored.

-Edward

On Wed, Jun 26, 2013 at 4:39 PM, Simon Peyton-Jones
<simonpj at microsoft.com>wrote:

> |  record projections.  I would prefer to have dot notation for a
> |  general, very tightly-binding reverse application, and the type of the
> record
> |  selector for a field f changed to "forall r t. r { f :: t } => r -> t"
> |  instead of "SomeRecordType -> t".  Such a general reverse application
> dot would
> |  allow things like "string.toUpper" and for me personally, it would
> |  make a Haskell OO library that I'm working on more elegant...
>
> Actually I *hadn't* considered that.   I'm sure it's been suggested before
> (there has been so much discussion), but I had not really thought about it
> in the context of our very modest proposal.
>
> We're proposing, in effect, that ".f" is a postfix function with type
> "forall r t. r { f :: t } => r -> t".   You propose to decompose that idea
> further, into (a) reverse function application and (b) a first class
> function f.
>
> It is kind of weird that
>         f . g  means    \x. f (g x)
> but     f.g    means    g f
>
> but perhaps it is not *more* weird than our proposal.
>
> Your proposal also allows things like
>
>         data T = MkT { f :: Int }
>
>         foo :: [T] -> [Int]
>         foo = map f xs
>
> because the field selector 'f' has the very general type you give, but the
> type signature would be enough to fix it.  Or, if foo lacks a type
> signature, I suppose we'd infer
>
>         foo :: (r { f::a }) => [r] -> [a]
>
> which is also fine.
>
> It also allows you to use record field names in prefix position, just as
> now, which is a good thing.
>
> In fact, your observation allows us to regard our proposal as consisting
> of two entirely orthogonal parts
>   * Generalise the type of record field selectors
>   * Introduce period as reverse function application
>
> Both have merit.
>
> Simon
>
> |  -----Original Message-----
> |  bounces at haskell.org] On Behalf Of Dominique Devriese
> |  Sent: 26 June 2013 13:16
> |  Subject: Re: Overloaded record fields
> |
> |  I think it's a good idea to push forward on the records design because
> |  it seems futile to hope for an ideal consensus proposal.
> |
> |  The only thing I dislike though is that dot notation is special-cased to
> |  record projections.  I would prefer to have dot notation for a
> |  general, very tightly-binding reverse application, and the type of the
> record
> |  selector for a field f changed to "forall r t. r { f :: t } => r -> t"
> |  "SomeRecordType -> t".  Such a general reverse application dot would
> |  allow things like "string.toUpper" and for me personally, it would
> |  make a Haskell OO library that I'm working on more elegant...
> |
> |  But I guess you've considered such a design and decided against it,
> |  perhaps because of the stronger backward compatibility implications of
> |  changing the selectors' types?
> |
> |  Dominique
> |
> |  > Hi everyone,
> |  >
> |  > I am implementing an overloaded record fields extension for GHC as a
> |  > GSoC project. Thanks to all those who gave their feedback on the
> |  > original proposal! I've started to document the plan on the GHC wiki:
> |  >
> |  >
> |  >
> |  > If you have any comments on the proposed changes, or anything is
> unclear
> |  > about the design, I'd like to hear from you.
> |  >
> |  > Thanks,
> |  >
> |  >
> |  > _______________________________________________
> |  > Glasgow-haskell-users mailing list
> |
> |  _______________________________________________