A lenses library for Haskell.
An extensive library which provides families of lenses, isomorphisms, folds, traversals, getters and setters.
For more info visit:
Questions tagged [haskell-lens]
454 questions
44
votes
3 answers
What are Prisms?
I'm trying to achieve a deeper understanding of lens library, so I play around with the types it offers. I have already had some experience with lenses, and know how powerful and convenient they are. So I moved on to Prisms, and I'm a bit lost. It…
Michail
- 1,843
- 1
- 17
- 21
31
votes
1 answer
What is the difference between `ix` and `at` in the Lens library of Haskell
All I know is that one works and the other doesn't.
Context:
I have one data structure F which contains a Data.Map.Map k S to another data structure S. My goal was to build a Lens that given an F and k would describe a field in S.
The difficulty is…
John F. Miller
- 26,961
- 10
- 71
- 121
30
votes
4 answers
What are the differences between lenses and zippers?
This is an example of using a zipper in Haskell:
data Tree a = Fork (Tree a) (Tree a) | Leaf a
data Cxt a = Top | L (Cxt a) (Tree a) | R (Tree a) (Cxt a)
type Loc a = (Tree a, Cxt a)
left :: Loc a -> Loc a
left (Fork l r, c) = (l, L c r)
right ::…
hawkeye
- 34,745
- 30
- 150
- 304
25
votes
1 answer
Why class constraint in type synonym needs RankNTypes
This compiles fine:
type List a = [a]
But when I introduce a class constraint, the compiler asks for RankNTypes to be included:
type List2 a = Num a => [a]
After including that extension, it compiles fine. Why is that extension required for…
Sibi
- 47,472
- 16
- 95
- 163
22
votes
2 answers
Isn't it redundant for Control.Lens.Setter to wrap types in functors?
I'm watching the Control.Lens introduction video.
It makes me wonder why is it needed for the Setter type to wrap things in functors.
It's (roughly) defined like this:
type Control.Lens.Setter s t a b = (Functor f) => (a -> f a) -> s -> f t
Let's…
qwe
- 766
- 1
- 5
- 15
21
votes
1 answer
How to combine lenses (not compose)
In haskell without lenses I can do things like :
data Item = Item { quantity :: Double, price ::Double }
cost :: Item -> Double
cost = (*) <$> quantity <*> price
If I use lenses instead how can I do the equivalent ? The best I can do is
cost = to…
mb14
- 22,276
- 7
- 60
- 102
21
votes
2 answers
Using a Lens to read multiple fields
Given the types
data Prisoner = P { _name :: String
, _rank :: Int
, _cereal :: Cereal }
data Cereal = C { _number :: Int
, _percentDailyValue :: Map String Float
…
rampion
- 87,131
- 49
- 199
- 315
20
votes
4 answers
Is it possible to get all contexts of a Traversable lazily?
lens offers holesOf, which is a somewhat more general and powerful version of this hypothetical function:
holesList :: Traversable t
=> t a -> [(a, a -> t a)]
Given a container, holesList produces a list of elements of the container along…
dfeuer
- 48,079
- 5
- 63
- 167
20
votes
1 answer
Lens / Prism with error handling
Let's say I have a pair of conversion functions
string2int :: String -> Maybe Int
int2string :: Int -> String
I could represent these fairly easily using Optics.
stringIntPrism :: Prism String Int
However if I want to represent failure reason, I'd…
James Davies
- 9,602
- 5
- 38
- 42
20
votes
2 answers
Getting multiple results from map with "lens"
Having these imports:
> import Control.Lens
Control.Lens> import qualified Data.Map as Map
and a map value defined as follows:
Control.Lens Map> let m = Map.fromList [('a', 1), ('c', 3), ('b', 2)]
I can get it's elements one by one like…
Nikita Volkov
- 42,792
- 11
- 94
- 169
19
votes
3 answers
Generic LensLike like mapped and traverse
Suppose I wanted to create an "optic" for the contents of MaybeT m a:
maybeTContents = _Wrapped .something. _Just
Is there such a something?
maybeTContents would for example be a Traversal when m is [],
but only a Setter when m is (->) Int.
Example…
yairchu
- 23,680
- 7
- 69
- 109
18
votes
2 answers
Could someone explain the diagram about the `lens` library?
If you browse through Lens entry on hackage, Lens Github's repo, or even google about Lens, you will find a lot of partial references such as introductory tutorials/videos, examples, overviews and so on. Since I already know most of the basics, I am…
kqr
- 14,791
- 3
- 41
- 72
18
votes
4 answers
When manipulating immutable datastructures, what's the difference between Clojure's assoc-in and Haskell's lenses?
I need to manipulate and modify deeply nested immutable collections (maps and lists), and I'd like to better understand the different approaches. These two libraries solve more or less the same problem, right? How are they different, what types of…
Dustin Getz
- 21,282
- 15
- 82
- 131
18
votes
1 answer
How do I make lenses from a record in GHCi
I want to play around with the Lens library a bit. I've loaded it into GHCi and created a record data type with the appropriate underscores:
> data Foo a = Foo {_arg1 :: Int, _arg2 :: [a]}
I would like to make the lenses for Foo using the…
John F. Miller
- 26,961
- 10
- 71
- 121
17
votes
2 answers
Combining lenses
Using a lens library I can apply a modification function to individual targets, like so:
Prelude Control.Lens> (1, 'a', 2) & _1 %~ (*3)
(3,'a',2)
Prelude Control.Lens> (1, 'a', 2) & _3 %~ (*3)
(1,'a',6)
How can I combine those individual lenses (_1…
Nikita Volkov
- 42,792
- 11
- 94
- 169