Safe Haskell | None |
---|---|
Language | Haskell2010 |
This module is needed to give other packages from the microlens family (like microlens-ghc) access to functions and classes that don't need to be exported from Lens.Micro (because they just clutter the namespace). Also, okay, uh, e.g. traversed
is here because otherwise there'd be a dependency cycle.
Classes like Each
, Ixed
, etc are provided for convenience – you're not supposed to export functions that work on all members of Ixed
, for instance. Only microlens can do that. You mustn't declare instances of those classes for other types, either; these classes are incompatible with lens's classes, and by doing so you would divide the ecosystem.
If you absolutely need to define an instance (e.g. for internal use), only do it for your own types, because otherwise I might add an instance to one of the microlens packages later and if our instances are different it might lead to subtle bugs.
- traversed :: Traversable f => Traversal (f a) (f b) a b
- folded :: (Foldable f, Applicative (Const r)) => Getting r (f a) a
- foldring :: Applicative (Const r) => ((a -> Const r a -> Const r a) -> Const r a -> s -> Const r a) -> (a -> Const r b) -> s -> Const r t
- foldrOf :: Getting (Endo r) s a -> (a -> r -> r) -> r -> s -> r
- foldMapOf :: Getting r s a -> (a -> r) -> s -> r
- sets :: ((a -> b) -> s -> t) -> ASetter s t a b
- (#.) :: Coercible c b => (b -> c) -> (a -> b) -> a -> c
- (.#) :: Coercible b a => (b -> c) -> (a -> b) -> a -> c
- class Each s t a b | s -> a, t -> b, s b -> t, t a -> s where
- type family Index s :: *
- type family IxValue m :: *
- class Ixed m where
- ix :: Index m -> Traversal' m (IxValue m)
- class Ixed m => At m where
- class Field1 s t a b | s -> a, t -> b, s b -> t, t a -> s where
- class Field2 s t a b | s -> a, t -> b, s b -> t, t a -> s where
- class Field3 s t a b | s -> a, t -> b, s b -> t, t a -> s where
- class Field4 s t a b | s -> a, t -> b, s b -> t, t a -> s where
- class Field5 s t a b | s -> a, t -> b, s b -> t, t a -> s where
- class Cons s t a b | s -> a, t -> b, s b -> t, t a -> s where
- class Snoc s t a b | s -> a, t -> b, s b -> t, t a -> s where
Documentation
traversed :: Traversable f => Traversal (f a) (f b) a b Source
traversed
traverses any Traversable
container (list, vector, Map
, Maybe
, you name it):
>>>
Just 1 ^.. traversed
[1]
traversed
is the same as traverse
, but can be faster thanks to magic rewrite rules.
foldring :: Applicative (Const r) => ((a -> Const r a -> Const r a) -> Const r a -> s -> Const r a) -> (a -> Const r b) -> s -> Const r t Source
class Each s t a b | s -> a, t -> b, s b -> t, t a -> s where Source
Nothing
each :: Traversal s t a b Source
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 beach
::Traversal
(Maybe
a) (Maybe
b) a beach
::Traversal
(a,a) (b,b) a beach
::Traversal
(a,a,a) (b,b,b) a beach
::Traversal
(a,a,a,a) (b,b,b,b) a beach
::Traversal
(a,a,a,a,a) (b,b,b,b,b) a beach
:: (RealFloat
a,RealFloat
b) =>Traversal
(Complex
a) (Complex
b) a b
Additionally, you can use each
with types from array, bytestring, and containers by importing Lens.Micro.GHC
from the microlens-ghc package.
Each [a] [b] a b Source | |
Each (Complex a) (Complex b) a b Source | |
Each (Maybe a) (Maybe b) a b Source | |
((~) * a b, (~) * q r) => Each (a, b) (q, r) a q Source | |
((~) * a b, (~) * a c, (~) * q r, (~) * q s) => Each (a, b, c) (q, r, s) a q Source | |
((~) * a b, (~) * a c, (~) * a d, (~) * q r, (~) * q s, (~) * q t) => Each (a, b, c, d) (q, r, s, t) a q Source | |
((~) * a b, (~) * a c, (~) * a d, (~) * a e, (~) * q r, (~) * q s, (~) * q t, (~) * q u) => Each (a, b, c, d, e) (q, r, s, t, u) a q Source |
Nothing
ix :: Index m -> Traversal' m (IxValue m) Source
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] aix
:: (Eq
e) => e ->Traversal'
(e -> a) a
Additionally, you can use ix
with types from array, bytestring, and containers by importing Lens.Micro.GHC
from the microlens-ghc package.
class Ixed m => At m where Source
at :: Index m -> Lens' m (Maybe (IxValue m)) Source
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
or Just
valueNothing
:
Data.Map.insert k a m = m&
at k.~
Just a Data.Map.delete k m = m&
at k.~
Nothing
at
doesn't work for arrays, because you can't delete an arbitrary element from an array.
If you want to modify an already existing value, you should use ix
instead because then you won't have to deal with Maybe
(ix
is available for all types that have at
).
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 you can import Lens.Micro.GHC
from the microlens-ghc package and get instances for Map
and IntMap
.
class Field1 s t a b | s -> a, t -> b, s b -> t, t a -> s where Source
Nothing
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).