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
10
votes
1 answer
Isomorphisms between 3 and more types using lens
Inspired by a question on polymorphic function between ADTs I'm trying to create isomorphisms between multiple (not just 2) types, so that every time I need an isomorphic but not the same type, I can sprinkle my code with some convert.
Suppose I…
Mirzhan Irkegulov
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10
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2 answers
How to make the product of two lenses?
If I have two lenses:
foo :: Lens' X Foo
bar :: Lens' X Bar
Is there a way to construct a product lens:
foobar :: Lens' X (Foo, Bar)
foobar = ... foo bar
or is it impossible?
phadej
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How to avoid default return value when accessing a non-existent field with lenses?
I love Lens library and I love how it works, but sometimes it introduces so many problems, that I regret I ever started using it. Lets look at this simple example:
{-# LANGUAGE TemplateHaskell #-}
import Control.Lens
data Data = A { _x :: String,…
Wojciech Danilo
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10
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2 answers
Lenses and Monomorphism Restriction
I have been working out on the examples given out in this article for creating Lenses.
I created Lens as stated in the article and the following is my code:
{-# LANGUAGE TemplateHaskell #-}
import Control.Lens
type Degrees = Double
type Latitude =…
Sibi
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How do I combine lenses and functors?
I'm trying to get used to the lens library for Haskell, and find myself struggling at some simple problems. For instance, let's say (for convenience) that at and _1 have the following types (this is how I understand them, at least):
at :: Ord k => k…
wen
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1 answer
Getting Maybe from lens with default value
A lot of lens getters return Maybe values. And i often need to replace them with some default.
Say map lookup but with default.
fromMaybe "" $ Map.fromList [(1,"Foo")] ^? at 1
Can this be written with lens syntax? Maybe something close to…
Vagif Verdi
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2 answers
Why would my datatype need to an instance of Monoid to use this lens?
I'm using the code below on a record that has a field '_scene' of type SceneGraph. I've created lenses for it using makeLenses.
inputGame :: Input -> Game -> Game
inputGame i g = flip execState g $ do
let es = g ^. userInput . events
sg…
schellsan
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9
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What's the reason behind the name Market in Control.Lens?
Edward Kmett's optics library; Control.Lens defines a large number of types.
Most of these have relatively self explanatory names, like Traversal and Fold.
It also defines some types with less obvious names, like Bazaar
From the Bazaar…
Joe
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How can I write a lens for a sum type
I have a type like this:
data Problem =
ProblemFoo Foo |
ProblemBar Bar |
ProblemBaz Baz
Foo, Bar and Baz all have a lens for their names:
fooName :: Lens' Foo String
barName :: Lens' Bar String
bazName :: Lens' Baz String
Now I'd like to…
Paul Johnson
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9
votes
2 answers
Traversal over last element of 'filtered'
I'm looking to filter a traversal, then select the last element to use with over.
e.g. something like this (but which will actually compile):
[1,2,3,4] & traverse . filtered even . _last +~ 10
> [1,2,3,14]
Any ideas?
P.S. I'm aware that filtered…
Chris Penner
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3 answers
Zipping Traversals
I have a Traversable with holes in it -- imagine this binary tree:
/ \
/ \ Nothing
Just 1 / \
Nothing Just 3
I also have a list of values to fill the holes with -- [2, 4] -- resulting in:
/ …
Joel Burget
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9
votes
1 answer
Haskell: Reusing FromJSON instances with lenses, lens-aeson, and nested JSON
I have been playing with Aeson and the lens package (lens-aeson, migrated from the core lens package), and have been sruggling to get them to work together.
As a toy example, I have a type:
data Colour = Yellow | Green | Blue
and the FromJSON…
jsdw
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9
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1 answer
How to set the range of the axis in the Haskell Chart library
In haskell-chart, how do you change the range of an axis? It seems like it has something to do with lenses and viewports, but as a beginner in Haskell, I'm having a hard time reading the…
Craig
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9
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Haskell - Lenses, use of 'to' function
I have the following code. I'd like to be able to modify the active player's life when given a game state. I came up with an activePlayer lens, but when I try and use it in combination with the -= operator I receive the following error:
> over…
Dwilson
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indexing list with Control.Lens requires Monoid constraint
The following code doesn't compile:
{-# LANGUAGE TemplateHaskell #-}
import Control.Lens
data MyType = MyType Int
data Outer = Outer { _inners :: [ Inner ] }
data Inner = Inner { _val :: MyType }
$(makeLenses ''Outer)
$(makeLenses ''Inner)
i1 =…
ajp
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