each tries to be a universal Traversal – it behaves like traversed in most situations, but also adds support for e.g. tuples with same-typed values:

>>> (1,2) & each %~ succ
(2,3)

>>> ["x", "y", "z"] ^. each
"xyz"

However, note that each doesn't work on every instance of Traversable . If you have a Traversable which isn't supported by each , you can use traversed instead. Personally, I like using each instead of traversed whenever possible – it's shorter and more descriptive.

You can use each with these things:

each :: Traversal [a] [b] a b

each :: Traversal (Maybe a) (Maybe b) a b
each :: Traversal (Either a a) (Either b b) a b  -- since 0.4.11

each :: Traversal (a,a) (b,b) a b
each :: Traversal (a,a,a) (b,b,b) a b
each :: Traversal (a,a,a,a) (b,b,b,b) a b
each :: Traversal (a,a,a,a,a) (b,b,b,b,b) a b

each :: (RealFloat a, RealFloat b) => Traversal (Complex a) (Complex b) a b

You can also use each with types from array , bytestring , and containers by using microlens-ghc , or additionally with types from vector , text , and unordered-containers by using microlens-platform .

This traversal lets you access (and update) an arbitrary element in a list, array, Map , etc. (If you want to insert or delete elements as well, look at at .)

An example for lists:

>>> [0..5] & ix 3 .~ 10
[0,1,2,10,4,5]

You can use it for getting, too:

>>> [0..5] ^? ix 3
Just 3

Of course, the element may not be present (which means that you can use ix as a safe variant of ( !! )):

>>> [0..5] ^? ix 10
Nothing

Another useful instance is the one for functions – it lets you modify their outputs for specific inputs. For instance, here's maximum that returns 0 when the list is empty (instead of throwing an exception):

maximum0 = maximum & ix [] .~ 0

The following instances are provided in this package:

ix :: Int -> Traversal' [a] a

ix :: Int -> Traversal' (NonEmpty a) a

ix :: (Eq e) => e -> Traversal' (e -> a) a

You can also use ix with types from array , bytestring , and containers by using microlens-ghc , or additionally with types from vector , text , and unordered-containers by using microlens-platform .

This lens lets you read, write, or delete elements in Map -like structures. It returns Nothing when the value isn't found, just like lookup :

Data.Map.lookup k m = m ^. at k

However, it also lets you insert and delete values by setting the value to Just value or Nothing :

Data.Map.insert k a m = m & at k .~ Just a

Data.Map.delete k m = m & at k .~ Nothing

Or you could use ( ?~ ) instead of ( .~ ):

Data.Map.insert k a m = m & at k ?~ a

Note that at doesn't work for arrays or lists. You can't delete an arbitrary element from an array (what would be left in its place?), and you can't set an arbitrary element in a list because if the index is out of list's bounds, you'd have to somehow fill the stretch between the last element and the element you just inserted (i.e. [1,2,3] & at 10 .~ 5 is undefined). If you want to modify an already existing value in an array or list, you should use ix instead.

at is often used with non . See the documentation of non for examples.

Note that at isn't strict for Map , even if you're using Data.Map.Strict :

>>> Data.Map.Strict.size (Data.Map.Strict.empty & at 1 .~ Just undefined)
1

The reason for such behavior is that there's actually no “strict Map ” type; Data.Map.Strict just provides some strict functions for ordinary Map s.

This package doesn't actually provide any instances for at , but there are instances for Map and IntMap in microlens-ghc and an instance for HashMap in microlens-platform .

Gives access to the 1st field of a tuple (up to 5-tuples).

Getting the 1st component:

>>> (1,2,3,4,5) ^. _1
1

Setting the 1st component:

>>> (1,2,3) & _1 .~ 10
(10,2,3)

Note that this lens is lazy, and can set fields even of undefined :

>>> set _1 10 undefined :: (Int, Int)
(10,*** Exception: Prelude.undefined

This is done to avoid violating a lens law stating that you can get back what you put:

>>> view _1 . set _1 10 $ (undefined :: (Int, Int))
10

The implementation (for 2-tuples) is:

_1 f t = (,) <$> f    (fst t)
             <*> pure (snd t)

or, alternatively,

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

(where ~ means a lazy pattern ).

_2 , _3 , _4 , and _5 are also available (see below).

strict lets you convert between strict and lazy versions of a datatype:

>>> let someText = "hello" :: Lazy.Text
>>> someText ^. strict
"hello" :: Strict.Text

It can also be useful if you have a function that works on a strict type but your type is lazy:

stripDiacritics :: Strict.Text -> Strict.Text
stripDiacritics = ...

>>> let someText = "Paul Erdős" :: Lazy.Text
>>> someText & strict %~ stripDiacritics
"Paul Erdos" :: Lazy.Text

strict works on ByteString and StateT / WriterT / RWST if you use microlens-ghc , and additionally on Text if you use microlens-platform .

lazy is like strict but works in opposite direction:

>>> let someText = "hello" :: Strict.Text
>>> someText ^. lazy
"hello" :: Lazy.Text