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lens over tea #2: composition, laws, getters/actions/setters

2 obligatory topics of every lens tutorial seem to be “wow, look how lenses compose, it's just like OOP” and “wow, look how lenses make working with records easy”. Until these 2 are covered, then, this tutorial can't be considered a proper lens tutorial and so should probably be called “a text which for whatever reason mentions the word “lens” often”.

Template Haskell is unwieldy, so let's start with composition.

Lens composition

First of all, why had somebody thought in the first place that the words “lens” and “composition” should occur in a sentence together? (It's an important question – without at least trying to address it, lens composition would just look like something pulled out of thin air.)

I don't know. This Luke Palmer's post from 2007 suggests that at least some people were motivated by OOP:

And then given the Accessor abstraction you could do stuff like define a := operator for readability and whatnot. I was proud that you could write accessors that accessed things other than the top level of the data structure.

But something still wasn't right. I wanted to be able to do something like a.b.c in OO languages, where a, b, and c were accessors. So here is my new Accessor abstraction [...]

A bit of history:

Old lenses

When lenses were still looking like this:

data OldLens s a = OldLens
  { get    :: s -> a
  , modify :: (a -> a) -> s -> s }

the idea of composing them probably felt more natural than it does now with type Lens s t a b. I mean, it's an easy enough step from

If you have a function from a to b, and from b to c, then you can get a function from a to c.


If you can access b in a, and c in b, then you can access c in a.

Let's write a composition function for OldLens, then.

(@.) :: OldLens b c -> OldLens a b -> OldLens a c
(@.) _c _b = OldLens get' modify'

get' is simple. To get c from a, first get b from a, and then c from b.

    get' a = let b = get _b a
             in  get _c b

modify' is pretty simple as well. To modify c in a, first write a function which modifies c in b, and then use it to modify b in a.

    modify' f a = let modifyB :: b -> b
                      modifyB = modify _c f
                  in  modify _b modifyB a

(I added the type signature for extra clarity.)

If you try to compile it, tho, you'll get a weird-looking error message saying Couldn't match type ‘b’ with ‘b1’. This is because in modifyB :: b -> b, b could've been just as well replaced by a or x or pony – there's no relation to the b in (@.)'s type signature. So, we'll have to enable the ScopedTypeVariables extension, which does exactly what we want – it allows type variables to be brought into scope so that they can be used later. Finally, just enabling the extension doesn't do anything by itself, only allows variables to be brought into scope – you still have to do it yourself by specifying forall b in the signature:

{-# LANGUAGE ScopedTypeVariables #-}


-- “forall a b c” and not “forall b” because it's all-or-none for “forall”s.
(@.) :: forall a b c . OldLens b c -> OldLens a b -> OldLens a c
(@.) _c _b = OldLens get' modify'
    get' a = let b = get _b a
             in  get _c b
    modify' f a = let modifyB :: b -> b
                      modifyB = modify _c f
                  in  modify _b modifyB a

If you understand everything, let's shorten things a bit.

get' a = let b = get _b a
         in  get _c b

-- Substitute “b”:

get' a = get _c (get _b a)

-- Get rid of “a”:

get' = get _c . get _b

Same with modify':

modify' f a = let modifyB :: b -> b
                  modifyB = modify _c f
              in  modify _b modifyB a

-- Substitute “modifyB”:

modify' f a = modify _b (modify _c f) a

-- Get rid of “a”:

modify' f = modify _b (modify _c f)

-- Get rid of “f”:

modify' = modify _b . modify _c

Final definition of (@.):

(@.) :: OldLens b c -> OldLens a b -> OldLens a c
(@.) _c _b = OldLens
  { get    = get    _c . get    _b
  , modify = modify _b . modify _c }

And here's an example of how to use lens composition to work with the first element of a pair in a nested list (row x, column y) [darn, so that's probably why tutorial writers start with records – to be able to use more interesting lenses in examples]. Assume that ^. (reminding: that's flip view), *~ (like set which multiplies instead of setting), _1, _2, ix were already defined for OldLens – there's nothing interesting or surprising about their definitions anyway.

> let field = [[(x, y) | x <- [0..3]] | y <- [0..3]]

> field
[ [ (0,0), (1,0), (2,0), (3,0) ]
, [ (0,1), (1,1), (2,1), (3,1) ]
, [ (0,2), (1,2), (2,2), (3,2) ]
, [ (0,3), (1,3), (2,3), (3,3) ] ]

> -- Getting.

> field ^. (ix 3 @. ix 1)  -- 1st row, 3rd column.
(3, 1)

> field ^. (_1 @. ix 3 @. ix 1)

> -- Modifying.

> field & ((_1 @. ix 3 @. ix 1) *~ 100)
[ [ (0,0), (1,0), (2,0), (3  ,0) ]
, [ (0,1), (1,1), (2,1), (300,1) ]
, [ (0,2), (1,2), (2,2), (3  ,2) ]
, [ (0,3), (1,3), (2,3), (3  ,3) ] ]


If you have been programming in Haskell long enough, you might've noticed that there's quite a number of things that can be composed.


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

Monad functions (this one isn't used often, probably for reasons like “we have do-notation”, but I don't really know):

(<=<) :: Monad m => (b -> m c) -> (a -> m b) -> a -> m c

Conduits (in case you haven't heard about these before, they are an abstraction for streams – a Conduit i m o consumes some items of type i and produces items of type o. They are very useful for doing IO efficiently):

(=$=) :: Monad m => Conduit a m b -> ConduitM b c m r -> ConduitM a c m r

Pipes (same as conduits, but don't ask me which one is better):

(<-<) :: Monad m => Pipe b c m r -> Pipe a b m r -> Pipe a c m r

Can this pattern be generalised? Meet Category:

class Category cat where
  -- An “identity element” – doesn't change anything when composed.
  id :: cat a a
  -- Composition – if you replace “cat” with e.g. “~>”, its type becomes
  -- “(b ~> c) -> (a ~> b) -> (a ~> c)”.
  (.) :: cat b c -> cat a b -> cat a c

Now, I haven't actually ever used this instead of usual . and id, and I suspect that you won't either. At least until people become more accepting of categories (and maybe then we all will be able to stop inventing yet another way to draw an arrow with ASCII symbols, for heavens' sake). However, since other lens libraries (such as fclabels or data-lens) have Category instances for their lenses, we should at least know what all this means and how to use it and whether it's the best thing since sliced bread and so on and so forth.

Here's the trivial instance of Category for OldLens:

instance Category OldLens where
  id = OldLens id id
  (.) = (@.)

With this instance at hand we can achieve our Ultimate Goal, namely, composing lenses without annoying operators such as @.. Observe (um, just in case, reminding you again that it won't work in GHCi unless you've bothered to rewrite all the lens functions):

> field ^. _1 . ix 3 . ix 1

> field & _1 . ix 3 . ix 1 *~ 100
[ [ (0,0), (1,0), (2,0), (3  ,0) ]
, [ (0,1), (1,1), (2,1), (300,1) ]
, [ (0,2), (1,2), (2,2), (3  ,2) ]
, [ (0,3), (1,3), (2,3), (3  ,3) ] ]

You might notice that it's hard to read these examples – they mess with the concept of “flow” in your head. When you write f . g . h $ x, you see the flow – “here's x, it goes to h, then g does something, then f”. It doesn't even matter whether it's left-to-right or right-to-left, as long as there's continuity. For instance, (at least for me) this is easier to read:

> view (_1 . ix 3 . ix 1) field

> 100 ~* _1 . ix 3 . ix 1 $ field
[ [ (0,0), (1,0), (2,0), (3  ,0) ]
, [ (0,1), (1,1), (2,1), (300,1) ]
, [ (0,2), (1,2), (2,2), (3  ,2) ]
, [ (0,3), (1,3), (2,3), (3  ,3) ] ]

(~* doesn't exist in lens library – I made it up. If it did exist, it would've been flip (*~).)

Okay, enough of those “old lenses”. Can we get composition for our lenses?

New lenses

Before making a Category instance, let's work out how to actually do composition. I'm going to reuse the @. name for our operator:

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

Here goes the process:

“Hey, why darn?” – because it's almost, almost function composition, but it's flipped function composition, so we'd still have to wrap it in a newtype to make a Category instance— you know, screw Category, I'm satisfied with the outcome anyway – after all, we can compose our lenses with a mere . from Prelude and they all can't. Ha.

But still... why?

“Backward composition”

Let's rewrite our examples with OldLens to lens (by just reversing the order of composed lenses):

> field ^. ix 1 . ix 3 . _1

> field & ix 1 . ix 3 . _1 *~ 100
[ [ (0,0), (1,0), (2,0), (3  ,0) ]
, [ (0,1), (1,1), (2,1), (300,1) ]
, [ (0,2), (1,2), (2,2), (3  ,2) ]
, [ (0,3), (1,3), (2,3), (3  ,3) ] ]

Now the flow is right – in the first example field “goes” to ix 1, then to ix 3, then to _1 and we get the result. In the second example * 100 “passes” thru a chain of lenses which turn it into a more and more complex modifier, until it finally gets applied to field.

In case of view, I clearly prefer the “normal” order. view (_1 . ix 3 . ix 1) field reads just like fst . (!! 3) . (!! 1) $ field, which is what I'm used to when I'm working with getters. However, when I'm modifying, the “backward” order is better – field & ix 1 . ix 3 . _1 *~ 100 (or, alternatively, (ix 1 . ix 3 . _1 *~ 100) field) reads like mapElem 1 (mapElem 3 (mapFst (* 100))) field, which, again, reads like what I'm used to when I'm working with setters.

So, what's the answer to “why”? It's simple: a Lens' s a is a setter first and foremost, and a getter only by a clever (and not very obvious) Const hack. If the order of type parameters was changed appropriately – if it was Lens' a s instead of Lens' s a. would've been a “proper” composition operator for lenses.

Okay, but can we fix it if we really want to?

Again: it's only a “bug” if you view lenses as getters. When you're using them as setters, all other libraries are buggy. That said... yeah, we can, and we don't even need newtypes or anything.

The trick is slightly reminiscent of the one we used with difference lists – if x is a lens, then we'll define x' to be a function which “appends” x to the end of a “lens chain”:

_1' l = l . _1
_2' l = l . _2
ix' i l = l . ix i

or, in pointfree style:

_1' = (. _1)
_2' = (. _2)
ix' i = (. ix i)

Here's how it lets us reverse our “backward” lens chain:

view (ix 1 . ix 3 . _1)

-- Add “id” to the chain (we can do it since it's the “constant lens”).

view (id . ix 1 . ix 3 . _1)

-- Move “_1” to the beginning.

view (_1' (id . ix 1 . ix 3))

-- Move “ix 3” to the beginning.

view (_1' (ix' 3 (id . ix 1)))

-- Move “ix 1” to the beginning.

view (_1' (ix' 3 (ix' 1 (id))))

-- Use “.” and “$” instead of brackets.

view (_1' . ix' 3 . ix' 1 $ id)

-- Move “$ id” to “view”.

(view . ($ id)) (_1' . ix' 3 . ix' 1)

-- And call it «view'».

view' = view . ($ id)

view' (_1' . ix' 3 . ix' 1)

In the same way we can change lens, over, and every other lens or function using lenses. By the way, what's the type of these lenses?

_1  :: Functor f => (a -> f a) -> (a, x) -> f (a, x)
_1' :: Functor f => (((a, x) -> f (a, x)) -> c) -> (a -> f a) -> c

Not very illuminating. I'll add a couple of type synonyms to make it easier (warning: it won't compile due to type unification errors):

type Modifier = Functor f => a -> f a
type TupleModifier = Functor f => (a, x) -> f (a, x)

_1  :: Modifier -> TupleModifier

_1' :: (TupleModifier -> c) -> (Modifier -> c)

Or in English:

_1 takes a simple modifier and makes a modifier which acts on tuples from it.

_1' takes a function which needs a tuple modifier to produce some c, and then it takes a simple modifier and produces a c.

Is it clear how/why they compose? [Just in case it's not obvious already: when I say “is it clear”/“do you understand”, it almost always means that either I don't understand, or I did 5min ago but not anymore.] Here's another step-by-step explanation:

-- These are the types for ordinary lenses.

_1   :: Modifer -> TupleModifier
ix 3 :: Modifier -> ListModifier

-- These are the types for new lenses.

_1'   :: (TupleModifier -> c) -> (Modifier -> c)
ix' 3 :: (ListModifier  -> c) -> (Modifier -> c)

-- In order for «_1' . ix' 3» to work, the output type of «ix' 3» has to
-- match the input type of «_1». So, here's the more specialised type:

ix' 3 :: (ListTupleModifier -> c) -> (TupleModifier -> c)

-- So, the composition has this type:

_1' . ix' 3 :: (ListTupleModifier -> c) -> (Modifier -> c)

Various stuff

There are a few things left to mention:


A bit about laws

Traversals and lenses have some associated laws, pretty much like monads or functors. I'll describe these laws in a moment.

Disclaimer: I won't preach about Sticking To The Laws in this section (yada yada “or else lens police will come after you!”), but it doesn't mean that I think that you can go and do whatever you want and make all kind of weird improper lenses and put them on Hackage and it would be morally acceptable. On the other hand, I don't -think that you shouldn't ever use improper lenses- either. And there's place for head and fromJust in this world, too.

Traversal laws

(Why start with traversals? Because every lens is a traversal, but not every traversal is a lens, which means that lenses have some additional laws -traversals don't have-, which means that starting with traversals is easier.)


If t is some Traversal, then:

t pure ≡ pure

And since nobody can forbid me to quote the definition of Traversal as many times as I please:

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

Now, what this law implies is that you can't change the shape of the structure. Omitting every second element of a list isn't okay; duplicating elements isn't okay either; even things which technically don't change the shape are wrong – for instance, a traversal of a list can't reverse it (but it can process the list in reversed order or even random order).

An interesting question is whether this law implies that you can't leave some elements unprocessed. For polymorphic traversals, it does – but only because you can't leave an element unprocessed without omitting it entirely (how can you leave an Int unprocessed and yet include it when you're converting [Int] into, say, Maybe [String]?). However, for simple traversals which don't claim to be able to perform polymorphic updates – i.e. Traversal's – it doesn't mean any such thing. A Traversal' which alternates between applying the given function and applying pure is a valid traversal (even const pure is a valid traversal), and in fact there are several such traversals in lens.

ignored is exactly const pure – just an empty traversal:

> 1 ^.. ignored

> "hi" & ignored +~ 2

taking takes an existing traversal and makes a traversal which visits only the first n elements:

> [1..10] & taking 5 each %~ negate

dropping does... well, guess what:

> [1..10] & dropping 5 each %~ negate

(Don't try to implement them yet, it's hard.)

elementsOf generalises taking and dropping to arbitrary index predicates:

> [[1],[3,5],[7,9,11],[13,15,17,19]] & elementsOf (each.each) even .~ 0

(The lists only have odd numbers on purpose – otherwise someone might think elementsOf applies even to elements and not to indexes.)

Finally, this law also means that it should be possible to restrict every t :: Traversal s t a b to t :: Traversal s s a a – otherwise t pure won't even typecheck. In general, there are restrictions on what s, t, a and b can be for lenses and traversals – see Edward's Mirrored Lenses post for details.


This one looks kinda scary at first. If t is some Traversal, then:

fmap (t f) . t g ≡ getCompose . t (Compose . fmap f . g)

For the sake of simplicity, I'll make some assumptions. Let's say t is a list traversal:

t :: forall f . Applicative f => (a -> f b) -> [a] -> f [b]

f and g are just some functions:

g :: a -> G b
f :: b -> F c

t f and t g would be the same functions, “lifted” to work on lists:

t g :: [a] -> G [b]
t f :: [b] -> F [c]

The left part is therefore:

fmap (t f) . t g :: [a] -> G (F [c])

The right part is built as follows:

fmap f . g :: a -> G (F c)

We already used Compose in the previous part to combine 2 functors, but I haven't defined it, and so I'll do it here:

newtype Compose f g a = Compose {getCompose :: f (g a)}

-- And some easy-to-write instances:
(Functor     f, Functor     g) => Functor     (Compose f g) where ...
(Applicative f, Applicative g) => Applicative (Compose f g) where ... 
(Foldable    f, Foldable    g) => Foldable    (Compose f g) where ... 
(Traversable f, Traversable g) => Traversable (Compose f g) where ... 
(Alternative f, Applicative g) => Alternative (Compose f g) where ... 

Armed with this knowledge, we can infer the type of Compose . fmap f . g:

Compose . fmap f . g :: a -> Compose G F c

When using t with this, Compose G F would be our Applicative:

t (Compose . fmap f . g) :: [a] -> Compose G F [c]

Finally, getCompose brings us G (F [c]).

As an example of a traversal ruled out by this law, let's take


Why did I think several months ago that going to sleep instead of finishing this sentence was a good idea.


Okay, I remember vaguely that I had asked a question on Stackoverflow about it... Aha! Here it is.

I'll retell it here for you; all credit goes to— do I link to Twitter, or SO profile, or Github? and do I use their real name or just nick? decisions, decisions— whatever, bennofs.

We take a data type:

data Present = Wrapped Present | Toy

Now we write a traversal to unwrap the present:

unwrapped :: Traversal' Present Present
unwrapped f (Wrapped x) = Wrapped <$> f x
unwrapped f Toy         = f Toy             -- Instead of “pure Toy”.

Instead of not doing anything when there's nothing to unwrap, it just pretends the toy was wrapped and does something to it. Too darn smart for its own good.

The intuitive law this traversal doesn't follow is “you get what you put in” – specifically, (whatever & t .~ x) ^.. t should give you a list consisting of xs back. Which it won't, as you'll see now.

Let's take Toy:

> Toy

And set the “present inside” (which is just Toy itself, because there is no wrapping at all) to be Wrapped Toy:

> Toy & unwrapped .~ Wrapped Toy
Wrapped Toy

And now unwrap it:

> (Toy & unwrapped .~ Wrapped Toy) ^.. unwrapped

We should've gotten [Wrapped Toy] back, but got just [Toy]. Ouch.

Now, what does it all have to do with the second traversal law? Well, let's rewrite the example using definitions of .~ and ^..:

t .~ b = runIdentity . t (\_ -> Identity b)

v ^.. t = getConst $ t (\x -> Const [x]) v

Which brings us to:

Toy & runIdentity . unwrapped (\_ -> Identity (Wrapped Toy))
    & getConst . unwrapped (\x -> Const [x])

I'll also get rid of runIdentity and getConst to make lines shorter, and change the order to the one I'm more used to. And rename some pieces:

> let set' = \_ -> Identity (Wrapped Toy)

> let get' = \x -> Const [x]

> fmap (unwrapped get') . unwrapped set' $ Toy
Identity (Const [Toy] Toy)

I had to use fmap because we didn't unwrap Identity after doing unwrapped set'. Also, there's no Show instance for Const, so I just made the output up.

Anyway, it's exactly the left part of the law: fmap (t f) . t g. What about the right part? Let's try to write it without looking at it. (I looked accidentally before the idea to write it without looking came to my head, but I have bad memory when it comes to things I saw a minute ago, so it's alright.) The point of the right part is that we somehow apply the traversal only once. Okay, what have we got?

get' :: t -> Const [t] b
set' :: t -> Identity Present

Or, specialised:

get' :: Present -> Const [Present] b
set' :: Present -> Identity Present

And we need to give unwrapped something of the type Present -> f Present. Hm.

We could just compose get' and set':

get' . set' :: Present -> Const [Identity Present] b

It would give us:

unwrapped (get' . set') Toy
  :: Const [Identity Present] Present

But the left part had a different type:

fmap (unwrapped get') . unwrapped set' $ Toy
  :: Identity (Const [Present] Present)

Can't we get the same type?

We can, but it needs some shenanigans. First, instead of just applying get' let's fmap it:

fmap get' . set' :: Present -> Identity (Const [Present] b)

Now if we give this to unwrapped, it will only see Identity as the functor part, which is not what we want. We'd like to have both Identity and Const [Present] handled together.

That's exactly what Compose does – it lets us join functors!

Compose :: f (g a) -> (Compose f g) a

So, the whole thing looks like this:

Compose . fmap get' . set'
  :: Present -> Compose Identity (Const [Present]) b

And here's what would happen after unwrapped:

unwrapped (Compose . fmap get' . set') Toy
  :: Compose Identity (Const [Present]) Present
> unwrapped (Compose . fmap get' . set') Toy
Compose (Identity (Const [Wrapped Toy]))

Now it's only left to strip Compose off:

> getCompose $ unwrapped (Compose . fmap get' . set') Toy
Identity (Const [Wrapped Toy])

Finally, the same shape as the left part of the equation – but not the same output! It happened because in the left part get' operated on the thing unwrapped gave to it, and in the right part get' operated directly on the result of set'. Here's exactly what I said right now but in list form because lists are cool:

No duplicates

Another consequence of the composition law is that you can't visit the same element twice – at least, it says so in the lens documentation:

Another testament to the strength of these laws is that the caveat expressed in section 5.5 of the “Essence of the Iterator Pattern” about exotic Traversable instances that traverse the same entry multiple times was actually already ruled out by the second law in that same paper!

So, something like

traverseTwice :: Traversal [a] [b] a b
traverseTwice f = T.traverse (\x -> f x *> f x)

is disallowed.

This remark was added in version 2.4, and previously “no visiting the same entry twice” was a separate law. The commit introducing the remark is kinda concise:

removed the third traversal law. roconnor proved it can't be infringed

I asked Edward to send me the proof, but...

06:56 <puregreen> edwardk: have you still got that roconnor's proof that
the (former) 3rd traversal law follows from the second law? could you put it on
lpaste or somewhere?
07:00 <edwardk> puregreen: not handy

So, if you really need it, bug Edward or Russell.

Months ago I left myself a cryptic remark here:

real ix, real at, all'

I don't know what it means. I only remember vaguely that ix— [looks at the previous part] ah, right, I defined ix as a lens, and it really should be a traversal. Well, then I guess “rewriting ix” shall be left as homework or something. Or just don't bother.

I don't even know what all' could be doi— [curiosity wins, looks at the previous part again] turns out all' just traverses all elements which are equal to some value. And it's not a proper traversal because it violates the composition law – 2 all's in a row might not be looking at the same elements (for instance, if you traverse all 0s and change them to 2s, then the next time you'll look at all' 0 you won't see anything).

Still could be pretty useful, y'know.

Lens laws

There are 3 lens laws. I am going to copy them from lens documentation. I'm so lazy. I hate long sentences. I've read the Harry the Hufflepuff fanfic recently and I think I got infected with laziness or something. Or maybe it's sleep deprivation. Whatever. Let's go.

  1. You get back what you put in:

    view l (set l v s) ≡ v
  2. Putting back what you got doesn't change anything:

    set l (view l s) s ≡ s
  3. Setting twice is the same as setting once:

    set l v' (set l v s) ≡ set l v' s

The third law seems to follow from the second traversal law. I don't see why it doesn't. Tell me if it doesn't, so that I could be properly embarrassed.

The second law is not even applicable to traversals, and the first law is almost not applicable.

Study the 3 laws and be wise in your ways.



You can regard view as something which makes a simple function from a lens. It's useful if you want to employ an occasional lens when writing “ordinary” code:

-- This is a lens which lets you access a deeply concealed value in a record.
-- I don't want to actually write it merely for the sake of an example, but
-- just believe me that it's *very* deep inside and everything is complicated
-- and so you really, really like the idea of not having to write *both* a
-- getter and a setter for it.
deepValue :: Lens' AmazinglyComplicatedStructure Int
deepValue = ...

main = do
  putStrLn "Looking for it..."
  ws <- filter ((== 42) . view deepValue) <$> getWorlds
  forM_ ws $ \world -> do
    new <- randomRIO (1, 100)
    modifyWorld world (set deepValue new)

However, when the code is fully (or mostly) lens-based, we would want to inject functions into the flow of lenses, rather than the other way round. For instance, lenses are more convenient than ordinary functions when it comes to various maps:

import qualified Data.Map as M

bindings :: M.Map String String
bindings = M.fromList
  [ ("cancel" , "C-g")
  , ("search" , "C-s")
  , ("refill" , "M-q")
  , ("save"   , "C-x C-s")
  , ("occur"  , "C-s o")

You want to get a keybinding for “save”:

> bindings ^?! ix "save"
"C-x C-s"

By the way, ^?! is an unsafe version of ^? – it doesn't wrap its result in Maybe, but it also throws an exception when there aren't any elements:

> bindings ^? ix "bark"

> bindings ^?! ix "bark"
"*** Exception: (^?!): empty Fold

Okay, now you want to take the first key group of the binding:

> takeWhile (not . isSpace) $ bindings ^?! ix "save"

Unfortunately, the flow is broken – first goes bindings, then ix, then takeWhile, and the result is slightly messy. You can fix the situation with &:

> bindings ^?! ix "save" & takeWhile (not . isSpace)

But now you lose the ability to easily get back to safety:

> bindings ^? ix "save" & takeWhile (not . isSpace)

    Couldn't match type ‘[Char]’ with ‘Maybe [Char]’
    Expected type: Maybe [Char] -> [Char]
      Actual type: [Char] -> [Char]
    In the second argument of ‘(&)’, namely
      ‘takeWhile (not . isSpace)’
    In the expression:
      bindings ^? ix "save" & takeWhile (not . isSpace)

Aw darn. Okay, & can be replaced with <&> (which is defined in lens as the flipped version of <$>):

> bindings ^? ix "save" <&> takeWhile (not . isSpace)
Just "C-x"

Even so, trying to add an additional lens after takeWhile would finally break our solution. There must be a simpler way... A-ha! Remember about lens?

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

Well, since all lenses can be composed, and we don't intend to use the lens for setting anyway – only for convenient getting – it should be possible to use lens to create a “getting lens” out of an s -> a function.

to :: (s -> a) -> Lens' s a
to getter = lens getter setter
  where setter = error "to: tried to set a getting-only lens"

Will it work now? Will it crash for some unexpected reason? Let's check:

> bindings ^? ix "save" . to (takeWhile (not . isSpace))
Just "C-x"

> bindings ^? ix "save" . to (takeWhile (not . isSpace)) . _last
Just 'x'

Hooray, it doesn't crash. Our quest is complete... or is it?

A note regarding readability

Let's compare the lens version and the version which doesn't use lenses:

-- Lens version.
bindings ^? ix "save" . to (takeWhile (not . isSpace)) . _last

-- Usual version.
last . takeWhile (not . isSpace) <$> M.lookup "save" bindings

The “usual version” seems to be shorter and cleaner. So, is it true that the lens library should only be used for setting, and getting is easier done with already existing functions?

Not quite. Getting tends to become clumsy mainly because everybody has a different idea about how to write getting functions! If you want to get something from a structure, you have to consider this:

It's not even that findWithDefault is a long name (it is, tho). The real problem is that too many different functions do the same thing – you can't really write code without having to constantly consult Hoogle or documentation. Not only that, but people can't read your code that easily, because they might not have been trained to recognise findWithDefault, lastDef, fromMaybe [] (spoonify tail), etc. as conceptually similar things.

Lens solves this problem by providing a (mostly) uniform vocabulary. Getting can fail? Use ^?. Can't fail? ^.. Possibly can, but you don't care? ^?!. Want all elements and not only the first one? ^... After that, it's just a combination of predefined traversals, lenses, prisms, simple functions used with to, and -functions ending in Of- (no idea how to call them). The “what happens in case of a failure” and “how do I get the value” parts are separated.

Back to to

If you remember, just one section ago we wrote to – it “lifts” a function into a getter, which can be composed with other lenses.

to :: (s -> a) -> Lens' s a
to getter = lens getter setter
  where setter = error "to: tried to set a getting-only lens"

It's not very nice – Lens' usually indicates, well, lenses, and not getters. Liar liar. There should be some type which would indicate that the resulting “lens” can't be used for setti— a-ha, but we have such a type:

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

And functions which work with Getting have types like this:

view     :: Getting  a        s a -> s -> a
toListOf :: Getting [a]       s a -> s -> [a]
preview  :: Getting (First a) s a -> s -> Maybe a
has      :: Getting Any       s a -> s -> Bool

If we want all these functions to work with our to-created getter, it has to have a type which would match all of these:

Can some type match all of these? Sure!

type Getter s a = forall r . Getting r s a

So, our better to would have this type signature:

to :: (s -> a) -> Getter s a

Implementing to is simple:

Great, now instead of a runtime exception there'll be a more-or-less nice compile-time error:

> False & to id .~ True

    Couldn't match typeConst r0 Bool’ with ‘Identity BoolExpected type: Setting Bool c Bool Bool
      Actual type: Getting r0 Bool Bool
    In the first argument of ‘(.~)’, namely ‘to id’
    In the second argument of ‘(&)’, namely ‘to id .~ True


Reader, Writer, State, related stuff

There isn't much left to say about getters (without touching indexed getters, of course). However, if you browse the Control.Lens.Getter module [like I did when I had thought I was done writing this section], you can notice that there are 5 functions left unexplained: views, use, uses, listening and listenings. What do they do?

To find out, look closely at the function which isn't among these 5 – view. Its type signature is:

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

However, back in part 1 we defined it with a different signature:

view :: Getting a s a -> s -> a

(by the way, please take a second right now and reimplement view without looking anywhere or querying any types or anything of the sort). What does MonadReader do here? In fact, how come lens's view works at all, after all, we aren't using any monads..?

I promised that nothing beyond Functor, Applicative, Monoid and Monad is “required knowledge” for these posts, and “reader” isn't any of those 4 things. So, I'll take a moment to explain it.

The previous paragraph was written more than a month ago, and I guess that constantly thinking “and now I have to explain reader/writer/state, da-arn” somehow took all my motivation away. I still don't think there's a good explanation of reader/writer/state anywhere, but apparently I can't write my own. Sorry.

Have some links instead!

view (and preview)

Remember that view makes a function out of a lens/traversal/getter – which means that with its help we can use lenses and getters when reading just as we use ordinary functions. Here's a contrived example showing how swap can be written Reader-style:

ex3 :: (a, b) -> (b, a)
ex3 = do
  a <- fst
  b <- snd
  return (b, a)

Or with view (because view _1 is the same as fst):

ex3' :: (a, b) -> (b, a)
ex3' = do
  a <- view _1
  b <- view _2
  return (b, a)

By the way, you can use preview in the same way (it'll just return a Maybe). I won't give an example because, honestly, I dislike giving completely contrived examples and I can't think of a good one (“let's define a huge thing which depends on some configuration which is a record with lots of fields which have other fields” doesn't count as a particularly good example).

Bonus task: figure out what pre does, why preview = view . pre, and write it.

So, why MonadReader?

Because x -> a isn't the only reader monad. There's also Reader, which is simply a newtype over x -> a:

newtype Reader r a = Reader
  { runReader :: r -> a }

And then there's ReaderT, which is a monad transformer – it can be used to turn other monads into reader monads.

So, we make view work in any reader monad so that you wouldn't have to use asks (view ...). At this point I could rant about how maybe every function should be made work in any reader monad – like, why not length too? if you want to abstract over such a thing as Getting A Function Argument, go on, abstract everywhere! – but I'm willing to accept that it's just for convenience. Ahem.

Old implementation:

view :: Getting a s a -> s -> a
view l = getConst . l Const

New implementation:

import Control.Monad.Reader

view :: MonadReader s m => Getting a s a -> m a
view l = asks (getConst . l Const)


There's also views, which is to view what asks is to ask. That is, it takes a getter and a function to apply to the value returned by the getter. So, if there's a config like Config, which contains field name (well, let's say the field is called _name and name is the name of the corresponding lens), and you want to get length of said field – in some reader monad – you have at least 6 slightly different ways:

  1. Straightforward – length . view name <$> ask.
  2. With asksasks (length . view name).
  3. Moving length into getter – asks (^. name . to length).
  4. Using view as askslength <$> view name.
  5. Again, moving length into getter – view $ name . to length
  6. With viewsviews name length.

I think #4 looks the best in this particular case, but choose for yourself.

By the way, what is the type of views? You might think it's

views :: MonadReader s m => Getter s a -> (a -> r) -> m r

but it's not – remember that Getter s a is the same as Getting x s a which has to work for any x, and we surely don't need any x, right? We only need some specific x.

What about Getting a s a? It would've made sense – with Getting a s a we can get a out of an s, and then simply apply the a -> r function to it. Like this:

views :: MonadReader s m => Getting a s a -> (a -> r) -> m r
views l f = f <$> view l

However, it's not the right answer either.

The right answer is this:

views :: MonadReader s m => Getting r s a -> (a -> r) -> m r
views l f = view (l . to f)
-- Or “asks (getConst . l (Const . f))”.

Can you answer – without thinking much – why l is required to have type Getting r s a in the final definition of views? I couldn't – so here we go, yet another bullet list:

Open question: it's impossible to write a function of the type (x -> y) -> Getting x s a -> Getting y s a, but it's possible to write a function of the type (r -> r) -> Getting r s a -> Getting r s a, which would do pretty much what you expect. Can it be of any use?

Open question: are there non-contrived situations where it would matter whether views takes Getting r s a or Getting a s a?


A reader-style version of quicksort deforested tree sort left from my attempted explanation of reader:

sort' = do
  empty <- null
  if empty then id else do
    pivot <- head
    local tail $ do
      left  <- local (filter (<  pivot)) sort'
      right <- local (filter (>= pivot)) sort'
      return (left ++ pivot : right)

listening and listenings

I haven't ever seen anybody use them, so I guess I'll just skip them. It's not that hard to figure out what they do by looking at the signatures and implementation, anyway:

-- | This is a generalized form of 'listen' that only extracts the portion
-- of the log that is focused on by a 'Getter'.
listening :: MonadWriter w m => Getting u w u -> m a -> m (a, u)
listening l m = do
  (a, w) <- listen m
  return (a, view l w)

listenings :: MonadWriter w m => Getting v w u -> (u -> v) -> m a -> m (a, v)
listenings l uv m = do
  (a, w) <- listen m
  return (a, views l uv w)

Actions 101

And now for something completely different! I'm tired of sounding like I know what I'm talking about, which means that right now I'll tell you what I don't know, and then we'll try to unravel everything toge— well, I'll be unravelling things and imagining that you're sitting with me in this room and asking questions or participating or something.

[Ye-eah, it was another I'm-bored-at-night-so-why-not-be-a-bit-awkward paragraph.]

Take a look at the Control.Lens.Action module (that's what I'm doing right now) (note that you need the lens-action package for it). What questions could you ask?

type Action m s a = forall f r. Effective m r f => (a -> f a) -> s -> f s

What's Effective?

An Action is a Getter enriched with access to a Monad for side-effects.

Should putStrLn (which prints given value but returns ()) be made into an Action? What are canonical uses of actions? What interesting actions can there be using other monads? Are all monads allowed, or maybe not?

Every Getter can be used as an Action.

Remember that

type Getter s a = forall r . (a -> Const r a) -> s -> Const r a

How does this unify with Effective m r f => (a -> f a) -> s -> f s? Is there an instance of Effective for Const or something?

act :: Monad m => (s -> m a) -> Action m s a
act sma afb a = effective (sma a >>= ineffective . afb)

What do effective and ineffective do?

acts: a self-running Action, analogous to join.

acts ≡ act id

What can this be useful for?

liftAct: apply a Monad transformer to an Action.

Again, need examples of usage.

[Skipping indexed stuff and monadic folds.]

type Acting m r s a = LensLike (Effect m r) s s a a

Used to evaluate an Action.

Acting to Action is the same as Getting to Getter, right?

class (Monad m, Functor f, Contravariant f) => Effective m r f | f -> m r

An Effective Functor ignores its argument and is isomorphic to a Monad wrapped around a value.

Ignores its argument... Um, like Const? Is Const (m r) an Effective?


  • Effective Identity r (Const r)
  • Monad m => Effective m r (Effect m r)

What's Effect? What if Effect m r is the same as Const (m r)? And what's the Identity instance for?

Action and Effect

Let's say we're only given the definition:

An Action is a Getter enriched with access to a Monad for side-effects.

Can we deduce everything else from it? I think we can.

A getter is conceptually the same as s -> a:

type Getter s a = s -> a

In this spirit, an action is, logically:

type Action m s a = s -> m a
-- Where “m” is a Monad.

However, the actual Getter is turned around, like this:

type Getter s a = forall r . (a -> r) -> s -> r

We can turn around Action in the same way:

type Action m s a = forall r . (a -> m r) -> s -> m r

And since Getter uses Const r instead of r to be compatible with lenses...

type Getter s a = forall r . (a -> Const r a) -> s -> Const r s

...Action should too.

type Action m s a = forall r . (a -> Const (m r) a) -> s -> Const (m r) s

The final step is giving a name for Const (m r) (guess what it's going to be):

type Effect m r = Const (m r)

type Action m s a = forall r . (a -> Effect m r a) -> s -> Effect m r s

No, wait, this is not the final step. The final step would be adding Acting, to achieve the symmetry with Getter/Getting.

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

type Getter s a = forall r . Getting r s a

(Just reminding you that Getter is to show that a function is a getter, and Getting is for functions which need to accept a getter as an argument.)

type Acting m r s a = (a -> Effect m r a) -> s -> Effect m r s
-- Or “LensLike (Effect m r) s s a a”.

type Action m s a = forall r . Acting m r s a

Making actions

Since Effect m r is just Const (m r), we can actually represent Acting as Getting:

type Getting  r  s a = (a -> Const   r   a) -> s -> Const   r   s
type Acting  m r s a = (a -> Const (m r) a) -> s -> Const (m r) s

-- From which follows...

type Acting m r s a = Getting (m r) s a

However, we can't represent Action as Getter, and thus can't make actions with to. We need a separate combinator for this (which I'll call act because in lens it's called act):

act :: Monad m => (s -> m a) -> Action m s a

...“implementing act is left as an exercise for the reader”, yeah.

Using actions

What is to ^. as Acting is to Getting? Let's call it ^! and implement it.

(^.) :: s -> Getting a s a -> a

(^!) :: s -> Acting m a s a -> m a
--   :: s -> Getting (m a) s a -> m a

infixr 8 ^!

Again, left as an exercise or something.

Finally, let's define perform just because ^. has view and ^! wants to have something too (and also because it's sometimes better to use a function than an operator):

perform = flip (^!)

With ^! and act, we can write extremely cool stuff, like hello world:

> "Hello, world!" ^! act putStrLn
Hello, world!

Actions and traversals

We probably can do something like each . act putStrLn. Well, we should be able to, at least, because otherwise actions are just useless. So, let's try:

> ["a", "b", "c"] ^! each . act putStrLn

    No instance for (Monoid (IO ())) arising from a use of ‘each’
    In the first argument of ‘(.)’, namely ‘each’
    In the second argument of ‘(^!)’, namely ‘each . act putStrLn’
    In the expression: ["a", "b", "c"] ^! each . act putStrLn

Ouch. What's going o— a-ha, right, the Monoid trouble from the past strikes again – each is a traversal, each uses Applicative, and the Applicative instance for Const (which we're using) requires a Monoid. There's no general instance like

instance (Monad m, Monoid a) => Monoid (m a)

for various reasons; if we want Effects to combine, we'll have to resort to newtypes. Behold, the actual definition of Effect:

-- | Wrap a monadic effect with a phantom type argument.
newtype Effect m r a = Effect { getEffect :: m r }

instance Functor (Effect m r) where
  fmap _ (Effect m) = Effect m

instance (Monad m, Monoid r) => Applicative (Effect m r) where
  pure _ = Effect (return mempty)
  Effect ma <*> Effect mb = Effect (liftM2 mappend ma mb)

(Now you'll have to make corresponding changes to ^! and act.)

Just in case, I'll spell out what's going on here:

With this new definition, will the original each thing work? Sure:

> ["a", "b", "c"] ^! each . act putStrLn

The Monoid used here is (), but any other could be used as well:

> -- A helper for generating a random string.
> let rand n = (++ ".") <$> sample (replicateM n (uniform 'a' 'z'))
> forM [1..5] rand

> -- Let's create 10 files containing random strings:
> forM_ [1..10] $ \i -> writeFile ("/tmp/rand" ++ show i) =<< rand i

> -- Just in case, let's also check they exist:
> :! du -b /tmp/rand*
2   /tmp/rand1
11  /tmp/rand10
3   /tmp/rand2
4   /tmp/rand3
5   /tmp/rand4
6   /tmp/rand5
7   /tmp/rand6
8   /tmp/rand7
9   /tmp/rand8
10  /tmp/rand9

> -- Okay, now we'll read them all and concat their contents.
> let files = ["/tmp/rand" ++ show i | i <- [1..10]]
> files ^! each . act readFile

(The instance in this case is Monoid [a], since readFile returns a String.)

Actions and getters

In the last example, you might've noticed that the definition of files could've been combined with the action, like this:

[1..10] ^! each . to (\i -> "/tmp/rand" ++ show i) . act readFile

Well, for one, it's clumsy and you shouldn't do it. However, it also doesn't typecheck (spoiler: yet!). Observe:

> [1..10] ^! each . to (\i -> "/tmp/rand" ++ show i) . act readFile

    Couldn't match typeConst r0 t0’ with ‘Effect IO String t0’
    Expected type: ([Char] -> Const r0 [Char])
                   -> t0 -> Effect IO String t0
      Actual type: Getting r0 t0 [Char]
    In the first argument of ‘(.)’, namely
      ‘to (\ i -> "/tmp/rand" ++ show i)’
    In the second argument of ‘(.)’, namely
      ‘to (\ i -> "/tmp/rand" ++ show i) . act readFile’

    Couldn't match typeEffect IO String [Char]’
                  with ‘Const r0 [Char]’
    Expected type: (String -> Effect IO String String)
                   -> [Char] -> Const r0 [Char]
      Actual type: Acting IO String [Char] String
    In the second argument of ‘(.)’, namely ‘act readFile’
    In the second argument of ‘(.)’, namely
      ‘to (\ i -> "/tmp/rand" ++ show i) . act readFile’

Um, no, don't observe. Better try something simpler:

> :t to show . act putStrLn

    Couldn't match typeEffect IO r1 String’ with ‘Const r StringExpected type: (() -> Effect IO r1 ()) -> String -> Const r String
      Actual type: Acting IO r1 String ()
    In the second argument of ‘(.)’, namely ‘act putStrLn’
    In the expression: to show . act putStrLn

A-ha, now that's tolerable. So, what's the problem?

The problem is actually very simple: if Effect and Const are no longer the same thing (and we made sure they aren't), we can't unify them:

> -- This is type of “to show”:
> :t to show
to show :: Show s => Getting r s String

> -- And now expanded:
> :kind! Show s => Getting r s String
Show s => Getting r s String :: *
= forall r s.
  Show s =>
  ([Char] -> Const r [Char]) -> s -> Const r s

> -- Type of “act putStrLn”:
> :t act putStrLn
act putStrLn :: Acting IO r String ()

> -- Expanded:
> :kind! forall r . Acting IO r String ()
forall r . Acting IO r String () :: *
= forall r. (() -> Effect IO r ()) -> [Char] -> Effect IO r [Char]

So, in order for to show . act putStrLn to work, the result of act putStrLn must be the same as the argument of to show:

“act putStrLn” (result)   : [Char] -> Effect IO r [Char]
“to show”      (argument) : [Char] -> Const r [Char]

What to do now?

A possible solution is to create a class for Const-like functors, and make getters and actions accept any of those functors instead of Const and Effect specifically. Turns out, however, that we don't need a special class for Const-like functors – it exists already as a combination of 2 other classes, Functor and Contravariant (originally defined in contravariant).

Contravariant is a class with a single method:

class Contravariant f where
  contramap :: (a -> b) -> f b -> f a

Compare with Functor:

class Functor f where
  fmap :: (a -> b) -> f a -> f b

There aren't many interesting instances of Contravariant. Functions could've been one – you can “reverse-apply” a function to the argument of another function – but due to the lack of type-level lambdas they can't, so you have to resort to newtypes:

newtype Op b a = Op (a -> b)

instance Contravariant (Op b) where
  contramap g (Op f) = Op (f . g)

-- contramap :: (x -> y) -> Op b y -> Op b x

-- Or without newtypes:
-- contramap :: (x -> y) -> (y -> b) -> (x -> b)

Or you can focus on some specific functions – e.g. on comparisons:

newtype Comparison a = Comparison (a -> a -> Ordering)

instance Contravariant Comparison where
  contramap g (Comparison comp) = Comparison (comp `on` g)

Const is a Contravariant too:

instance Contravariant (Const a) where
  contramap f (Const x) = Const x

Now, the interesting thing is that you can prove that any Functor which is also Contravariant doesn't actually contain any values – that is, it's like Const. How to prove it? By writing the coerce function:

coerce :: (Functor f, Contravariant f) => f a -> f b

You can't get N values of type b out of thin air unless N is 0, so the existence of such function would prove it.

Don't look ahead before trying to implement coerce on your own. Mindless brute-force search -of a composition of a small number of functions which has the type you want- is fun!

We-ell, maybe not that mindless... anyway,

coerce :: (Functor f, Contravariant f) => f a -> f b
coerce = contramap (const ()) . fmap (const ())

-- fmap      (const ()) :: Functor       f => f a  -> f ()
-- contramap (const ()) :: Contravariant f => f () -> f a

The definition in lens uses absurd instead, and I feel obliged to explain it as well:

Do you still remember what we wanted to do? We wanted to find a way to show that a Functor is Const-like – i.e. it doesn't hold any values – and we found that putting an additional Contravariant constraint on it is enough. Now the Getter definition can be updated:

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

-- Old:
--   type Getter s a = forall r . (a -> Const r a) -> s -> Const r s

As well as the definition of to:

to :: (s -> a) -> Getter s a
to getter f s = coerce (f (getter s))

-- Old:
--   to getter f s = Const (getConst (f (getter s)))

The next step is adding an instance of Contravariant for Effect, which is the same as the Functor instance:

instance Contravariant (Effect m r) where
  contramap _ (Effect m) = Effect m

This is enough for to show . act putStrLn to work:

> True ^! to show . act putStrLn

What about the rest of the questions?

Umh. What if I just asked Whoever Happens To Read This Post to tell me the answers? I admit that I haven't ever actually used actions, which makes it pretty hard for me to invent usecases for them. So, what I'm looking for is any snippets of code which use actions, liftAct, acts, etc. (and it'd be kinda nice if you could send me them if you have any).

As for Effective... I suspect that it's needed when you start implementing “indexed stuff”, but I can't be sure because I'm actively trying not to learn anything about it before time comes to write about it.

Getter and Setter laws

Getters have no laws, because they're just functions in disguise.

set l y (set l x a) ≡ set l y a

You can't view a Setter in general, so the other two laws are irrelevant.

However, two Functor laws apply to a Setter:

over l id ≡ id
over l f . over l g ≡ over l (f . g)
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