In the first article in the series about
we’ve looked at the motivation behind the lens library, and we also
derived the basic type of
Lens s a.
In this article we’ll go deeper and explain the reasoning beheind the more
Lens s t a b type. We’ll also take a look at how we can get a multi
focus lens using a
Just to reiterate, here’s how looks the type we derived in the previous article.
type Lens s a = forall f. Functor f => (a -> f a) -> s -> f s
What we’ll do here is further generalize it so that we can change the type of the focus.
type Lens s t a b = forall f. Functor f => (a -> f b) -> s -> f t
Now you might be thinking that four type parameters is a bit much, but bear
with me here. If we compare the our
Lens s t a b to something like
can see a bit resemblance there.
λ> :t fmap :: Functor f => (a -> b) -> f a -> f b
Much like a function
a -> b can be applied on
f a to change it’s
structure to become an
f b. In the same way a
Lens s t a b allows us to
b, which changes the shape of
t. We can also read it
as: A lens allows us to look at
a inside an
s, and if we can also replace
a with a
b, which will make the
t. Here’s a simple example
λ> :t ("hello", "world") :: (String, String) λ> :t over _1 length ("hello", "world") :: (Int, String) λ> over _1 length ("hello", "world") (5,"world")
Initially we started out with
s :: (String, String) and ended up with
t :: (Int, String) by applying a
String -> Int function on the first element of
the tuple. The specific type of the
_1 lens in this case would be
Lens (String, String) (Int, String) String Int.
It’s important to understand that all of the derivations we made for
Lens s a
still hold for
Lens s t a b, since it’s just a bit more generic. In fact you
can write the following (as it is done in the lens library.)
type Lens' s a = Lens s s a a
I’ll leave it as an exercise to the reader to go through all of the steps we
did previously and use
Lens s t a b instead.
Traversal - the multi foci lens
Disclaimer: When I say list I really mean
using a list makes things easier to understand. I also wrote an article on
Traversable if you’re unfamiliar with
While the lenses we’ve established so far are useful, they do have their shortcomings. One example are nested lists, let’s see an example.
data User = User String [Post] deriving Show data Post = Post String deriving Show
Now if I give you a list of users and ask you to give me all of the names of their posts, you’ll probably not be very happy about that. Not that it’s difficult, but some work involved.
traverse we can focus on all elements of a list and do
this in a single step. But first, let’s define us some lenses to work with the
types. In a real world application we’d use Template Haskell to generate the
lenses automatically, but for the sake of exercise let’s do it manually here.
posts :: Lens' User [Post] posts f (User n p) = fmap (\p' -> User n p') (f p) title :: Lens' Post String title f (Post t) = fmap Post (f t)
We got two lenses, one that focuses on
User’s posts, and another one for the
post’s title. Let’s also define us some test data to play around with.
users :: [User] users = [User "john" [Post "hello", Post "world"], User "bob" [Post "foobar"]]
Now lets open up GHCi, load these definitions file, import
see what we can do.
λ> view (traverse.posts) users [Post "hello",Post "world",Post "foobar"]
This seems to do what we want, we gave it a list of users and pulled out a list
of posts. Note that we used
traverse every time the current focus was a
list, which is just in the first step on
The next step is to go deeper to fetch the post title. If you look at the type
of our current lens
traverse.posts, you’ll see that it focuses on
λ> :t traverse.posts :: (Traversable t, Applicative f) => [Post] -> f [Post]) -> t User -> f (t User)
In order to reach out to each post, we need to use
traverse again. You can
traverse as something that allows us to focus on multiple targets at
once, in a similar way that
map allows us to apply a function to all elements
of a list.
- We started with
- We can’t directly apply the
postslens, since that requires a
traversechanges to focus on
traverse.postsnow works, since our target is just a
User, so we can compose to get a lens of
It is also important to note here that the lens composition works backwards
than what is usual in Haskell. You can think of it as a sort of object accessor
notation in an object-oriented language, where you’d do
Just to make this point crystal clear, here’s how function composition works
for regular functions. The
*2 gets applied before the
λ> ((+1).(*2)) 1 3
With lenses it goes the other way and the
traverse goes before the
Traversing deeper and deeper
Our previous example worked out just as we wanted, so let’s try to go deeper
and actually fetch the title of each
Post from our
λ> view (traverse.posts.traverse.title) users "helloworldfoobar"
Huh? This isn’t what we wanted at all! Lens must be completely broken?!!?1!
Much like we got
traverse.posts, it would make sense to get
traverse.posts.traverse.title, but instead we got one big
String with all of the titles combined. In order to understand why this is
happening we need to look more closely at how
Here’s a simpler example that we can use to reproduce what we had previously.
λ> view traverse ["hello", "world"] "helloworld"
The reason for this behavior is that if we use
view together with
it will use the
Monoid instance of our focus and smash them together.
Let’s see how this works by inlining the definition of
view :: Lens s a -> s -> a view ln s = getConst $ ln Const s
Inlining the arguments we get the following.
λ> view traverse ["hello", "world"] "helloworld" λ> getConst $ traverse Const ["hello", "world"] "helloworld"
We can already see that it is not the
lens library that does the magic, it’s
traverse combined with
view just picks the
applicative to be used with the
Now moving on to inlining definition of
traverse, which for a list look like
traverse _  = pure  traverse f (x:xs) = (:) <$> f x <*> traverse f xs
Since this is a recursive function and our list has two elements, we need to inline it in multiple steps.
-- Inlined the arguments into the definition. (:) <$> Const "hello" <*> traverse f ["world"] -- First recursive call to traverse inlined. (:) <$> Const "hello" <*> ((:) <$> Const "world" <*> traverse f ) -- Second recursive call to traverse inlined. (:) <$> Const "hello" <*> ((:) <$> Const "world" <*> pure )
This whole expression will return a type of
Const String [a], from which we
need to extract the
getConst, as shown above.
λ> getConst $ (:) <$> Const "hello" <*> ((:) <$> Const "world" <*> pure ) "helloworld"
As you can see we’re still getting the same result as in the case of
view traverse ["hello", "world"], which means we’re on the right track. But this
still doesn’t explain why are the two strings being concatenated together.
Const as a Monoid
To understand the concatenation we need to take a look at how the
Const is implemented, but let’s think about this first.
Const a b acts as an
Functor that pretends to contain a value of type
b, but in reality hides a value of type
a. That’s why if we have
Const Int String and
fmap a function of type
String, we’ll get a
Const Int Int,
even though there was no actual value for
λ> let a = Const 3 :: Const Int String λ> :t a :: Const Int String λ> :t fmap length a :: Const Int Int λ> getConst $ fmap length a 3
If you’re having trouble understanding this, check out my first article on Lenses which explains this in a bit more detail.
Now we’re faced with the problem of implementing an
Applicative instance. The
problem being that
pure :: a -> f a, which takes a
value and lifts it into the
Applicative. But because we’re working with
Const, there is no actual value being lifted, as in the case of a
where we didn’t really apply the function.
Const Int String does not contain any
String, it only contains the
Int. That’s why if we do
pure 3 to get back a
Const String Int, we must
throw away the
3 and somehow create a
String to hide it into the
We need to have a way to create a value for the type we’re hiding. But how do
we do that when we have nothing?
We use a
instance Monoid m => Applicative (Const m) where pure _ = Const mempty
We just throw away the argument to
pure and create a new
Const hiding the
value returned by
mempty, which for a
String in our previous example would
λ> getConst $ (pure 3 :: Const String Int) ""
Next up is the definition of
<*>, which is rather simple now that we know
that our hidden value is a
Monoid. The way that
<*> works is that it takes
Applicatives and smashes them together. In a general case it would mean
applying the function in the first one to the value in the second one, but
Const is just pretending to have a function while it has none, we
do not need to apply it. We just need to find a way to combine our two hidden
monoidal values, which is exactly where
mappend will come to play.
We simply extract the hidden values and
mappend them together to create a new
instance Monoid m => Applicative (Const m) where pure _ = Const mempty Const f <*> Const x = Const (f `mappend` x)
Finally we can get back to our
traverse example and understand why it does
what it does. We ended up with the following expression.
(:) <$> Const "hello" <*> ((:) <$> Const "world" <*> pure )
With the recently gained knowledge we can see that it doesn’t matter what
function we apply to our
Const. In this case it is
(:) but it might as well
λ> getConst $ undefined <$> Const "hello" "hello"
This means that the whole
(:) <$> has absolutely no meaning. It’s just there
so that our
Const "hello" can take on a type of a function application, so
that we can use
<*>. In fact the only thing that does something is the
combinator, which calls
mappend on the hidden values, but let’s take this
step by step.
First we replace
pure  with the actual value it returns in this case.
(:) <$> Const "hello" <*> ((:) <$> Const "world" <*> Const "")
Next we can evaluate the expression in the parentheses, which if you look at
our definition of
Const will just reduce to the following.
Const $ "world" `mappend` ""
Which evaluates to just
Const "world". Now we’re left with the following.
(:) <$> Const "hello" <*> Const "world"
Which again just ends up being:
Const $ "hello" `mappend` "world"
Which evaluates to
Const "helloworld". Our initial expression applied
getConst to the result of this expression, which would just yield
λ> getConst $ Const $ "hello" `mappend` "world" "helloworld"
There we go, now we have a full understanding of why
view traverse requires
the traversed values to be a Monoid.
In the next article we’ll focus on some other use cases for
traverse and how
to use it with combinators like
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