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
4
votes
1 answer
Design code to avoid rewriting during maintenance
I have lens based code that expects a certain data structure:
makeLenses ''Pool
...
pool ^. objects ^? _head :: Maybe Object
This code assumes that:
objects ::
Functor f =>
([Object] -> f [Object]) -> Pool -> f Pool
But I want to introduce new…
sevo
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4
votes
3 answers
How would I use lens in Haskell to duplicate Python's enumerate?
Python's enumerate on lists can be written as zip [0..]. I looked at Control.Lens.Traversal and Control.Lens.Indexed, but I couldn't figure out how to use lenses to generalize this to any reasonable container (I hesitate to say "Traversable").
I'm…
Alex R
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4
votes
1 answer
Type error with rank-n types and lenses
I have a simple polymorphic datatype Foo
{-# LANGUAGE TemplateHaskell #-}
import Control.Lens
data Foo c =
Foo {
_bar :: c,
_baz :: c,
_quux :: c
}
makeLenses ''Foo
The generated lenses are, of course,…
tjhance
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4
votes
1 answer
How to define settable lens
I have a record type in my Haskell code to which I want to define a lens that can be used as a getter and as a setter. The code looks like this:
data Players = Players { _white :: Player
, _black :: Player
…
Lemming
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4
votes
2 answers
How to remove an item from a list by index using the lens library?
I can view the, say, 4th item in a list using a lens like this:
preview (ix 3) myList
Is there something that could replace "preview" in order to remove the fourth item from the list instead of viewing it? The return list should be the same as the…
Richard Parsons
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4
votes
1 answer
How do you fmap a Getter?
As discussed on reddit, you can't just lift a Lens' a b to Lens' (Maybe a) (Maybe b). But for the special case Getter a b, this is obviously possible, since it's isomorphic to a->b. But unlike with Iso, there appears to be no standard function to…
leftaroundabout
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4
votes
1 answer
How can I express `mapM` with `concat` using Lenses to concatenate results of an IO operation?
I'm trying to figure out a way how to combine traverseOf with >>= in such a way that would allow the following.
TLDR; A simple example in plain Haskell would be something like this, but using lenses deep inside a data structure.
λ> fmap concat $…
Jakub Arnold
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4
votes
1 answer
Aeson Prisms with "free" references
Just read the brilliant "Lens/Aeson Traversals/Prisms" article and have a real-world application. Given the following anonymized JSON structure, how would I prism out a collection rather than a specific value?
{"Locations" : [ {"id" : "2o8434",…
Brandon
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4
votes
2 answers
Lenses: Composing backwards and (.) in Lens context
I have been reading this article and in one of their section it is stated:
Lenses compose backwards. Can't we make (.) behave like functions?
You're right, we could. We don't for various reasons, but the
intuition is right. Lenses should combine…
Sibi
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4
votes
2 answers
How does get function typecheck in lens
I have been reading this post to understand lens. They initially define a
type synonym like this:
type RefF a b = forall f. Functor f => (b -> f b) -> (a -> f a)
Const is defined like this:
newtype Const a b = Const { getConst :: a }
How does the…
Sibi
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4
votes
0 answers
How is using the lens library different than programming in an imperative language?
What advantages do we get by using the lens library over a language like C or Python?
I get that state is still immutable with the lens library, but from a practical perspective, how is it any different?
Examples would be very useful.
EDIT:
What I…
Emil
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4
votes
2 answers
How to check if map has a key using lens syntax?
How to check if map has a key using lens syntax?
import qualified Map as Map
let x = Map.member "bla" m
How to write this using lenses?
Vagif Verdi
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4
votes
2 answers
How can I use Lenses to perform read-only monadic operation over a sequence held in some state?
My data structure looks more or less like this (simplified for the purpose of the question)
data GameObject = GameObject { _num :: Int }
data Game = Game { _objects :: [GameObject] }
I use makeLenses to generate the accessors for both of those.…
Bartek Banachewicz
- 38,596
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4
votes
1 answer
Is there some method to construct value for record with lenses without underscore identifiers?
For example I have the following record
data Rec = Rec
{ _a :: Int
, _b :: Int
, _c :: Int
} deriving (Show, Eq)
makeLenses ''Rec
and I see only 2 ways to constuct new values:
Rec{_a=1,_b=2,_c=3}
Rec 1 2 3
The second variant does…
Qrilka
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4
votes
1 answer
How does one use an IndexedTraversal in Control.Lens to perform an index-aware action for each element?
I have an IndexedTraversal (from the Control.Lens package) and I would like to apply an index-aware monadic action to each element in it. Unfortunately, all of the convenient ways that I see of doing something like this --- such as ^! combined with…
Gregory Crosswhite
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