A lenses library for Haskell.
An extensive library which provides families of lenses, isomorphisms, folds, traversals, getters and setters.
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Questions tagged [haskell-lens]
454 questions
6
votes
1 answer
What is the purpose of united lens?
While reading through the Lens over Tea #1 article, I found the united lens. My implementation is
united :: Lens' a ()
united f v = const v <$> f ()
which is pretty much the same as the implementation in the Lens library.
What leaves me completely…
Lubomír Sedlář
- 1,580
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6
votes
1 answer
Modifying the target of a Lens conditionally
I have a function that produces an updated board from an input and a board, if the move is permitted by the rules of the game:
move :: Input -> Board -> Maybe Board
The board is wrapped in the GameState type with some additional data:
type…
vtan
- 75
- 4
6
votes
1 answer
zippers: mapping over last breadcrumb
I've bumped into an issue using zippers and lens. Consider following example:
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeOperators #-}
import Control.Lens
import Control.Zipper
data A = AA { _aa :: A }
| AB { _ab :: B }
…
remdezx
- 2,939
- 28
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6
votes
1 answer
stepping into zipper with `to` lens
I'm struggling with lens and zippers. Consider below code run in ghci
> import Control.Lens
> import Control.Zipper
>
> :t within (ix 1) $ zipper ([1,2,3] :: [Int])
> within (ix 1) $ zipper ([1,2,3] :: [Int])
:: Control.Monad.MonadPlus m => m…
remdezx
- 2,939
- 28
- 49
6
votes
0 answers
Idiomatic way to share lens field definitions among modules
If I have two modules that both use Control.Lens.TH' makeFields to generate fields from a record, and a record in each of the different modules has the same field name, what's the best way of ensuring that the two modules are using the same…
Fraser
- 1,521
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6
votes
1 answer
Aeson and Lenses with error handling
I'm very new to the magic of lenses, so I'm having some trouble with this.
With reference to: https://www.fpcomplete.com/user/tel/lens-aeson-traversals-prisms
a JSON object can be traversed in the following way:
val ^? nth 0 . key "someObject" .…
iamnat
- 4,056
- 1
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6
votes
1 answer
Implementing polymorphic 'deep' function for traversals and folds
I'm using lens together with xml-lens. I'd like to make the following function more polymorphic, so that it also works for Folds and not only Traversals:
-- | Traverse a plated structure recursively, trying to match a fold at each level. Don't…
bennofs
- 11,873
- 1
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6
votes
3 answers
Composing lenses with `at` and `ix`
Let's say I have some fairly simple data type Person with a couple of fields, and a type that holds a collection of Persons.
data Person = Person { _name :: String, _age :: Int }
data ProgramState = PS { _dict :: IntMap Person }
makeLenses…
Chris Taylor
- 46,912
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6
votes
1 answer
How to zoom in acid-state?
data Foo = Foo {
_bar :: Map String Integer
} deriving (Eq, Ord, Read, Show, Data, Typeable)
$(deriveSafeCopy 0 'base 'Foo)
$(makeLenses ''Foo)
Given the above code I am under the impression that it should be possible to do this:
addEntry ::…
fho
- 6,787
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- 71
6
votes
1 answer
How to work around inability to use lenses with existential types?
I am using Edward Kmett's lens library for the first time, and finding it rather nice, but I ran into a snag...
The question at [1] explains that existential quantifiers disrupt makeLenses. I really rather want to use an existential with lenses in…
Irene Knapp
- 145
- 8
6
votes
1 answer
How is anamorphism related to lens?
How is the Lens, the record accessor, e.g.
http://hackage.haskell.org/packages/archive/lens/3.9.0.2/doc/html/Control-Lens-Type.html#t:Lens
related to anamorphism?…
nushio
- 753
- 3
- 12
5
votes
1 answer
How to get sublist's head using lens?
I have a value:
my :: [(A, Either B [C])]
and I want to get [(A, C)] from it using lens. The result items are items from my that:
the second item in tuples is Right [C]
the list in this Right [C] has at least 1 item, so the result get the first…
RandomB
- 3,367
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5
votes
2 answers
How to use lens to access a record field behind a sum type
I am trying to access a nested record using lenses and prisms in Haskell:
import Data.Text (Text)
import Control.Lens.TH
data State = State
{ _stDone :: Bool
, _stStep :: StateStep
}
data StateStep
= StatePause
| StateRun
…
ruben.moor
- 1,876
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5
votes
1 answer
Is there a standard combinator packing the result of a lens in functor?
While trying to apply a function with multiple layers of functors to a member of a host data structure, as in the following example:
update ::
(SomeProductField -> Maybe (Int, SomeProductField)) ->
(SomeProduct -> Maybe (Int, SomeProduct))
I…
Nikita Volkov
- 42,792
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- 169
5
votes
1 answer
Returning ‘Nothing’ if an indexed list traversal doesn’t match
I’m new to lens and trying to use it to compose many small modifications to a nested structure, which may fail and possibly return additional results:
element -> Maybe element
element -> Maybe (result, element)
How do I modify an inner structure by…
Jon Purdy
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