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
7
votes
2 answers
How to compose lenses that return Maybe (Haskell)
Suppose I have a lens like at _ that needs some Maybe a:
import Data.Map as M
m = M.fromList [(1,(2,3))]
--set 2nd element
m ^. at 1 .~ Just (4,5)
--gives fromList [(1,(4,5))]
m ^. at 1 .~ Nothing
--gives fromList ()
Now suppose I want to compose…
holdenlee
- 969
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7
votes
1 answer
Applying functions that depend on multiple fields using the lens
Let A, B, C be types and there are two functions f :: (A , B) -> A and g :: (A , B) -> B. Consider following record type
data Rec = Rec{_a :: A, _b :: B, _c :: C}.
What would be the most elegant way to define function that maps (Rec a b c) to (Rec…
Katty J.
- 686
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- 11
7
votes
2 answers
Export only getter or setter from a module
Is there a way for me to only export specific getters xor setters from a module with a lens?
For example, let's assume a data structure that has an invariant of being always >= 0, being modified only by incrementing it and being created only with…
Shoe
- 74,840
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7
votes
3 answers
Using a lens twice
I'm struggling with using the lens library for a particular problem. I'm trying to pass
an updated data structure
a lens focussed on part of that updated structure
to another function, g. I pass both the lens and the data structure because g needs…
dbeacham
- 986
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- 16
7
votes
2 answers
Using Lens in Haskell to modify values
I find myself using this pattern often:
do
let oldHeaders = mail ^. headers
put $ (headers .~ (insert header value oldHeaders)) mail
which seems like the kind of thing Control.Lens should be able to do, but I guess I just haven't found the…
Drew
- 12,578
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6
votes
1 answer
Is there a way to compose reified lenses?
I was trying to use the lens library to solve the following problem:
Given the list version of a tree, make a tree.
Example:
Given:
[1,2,3,4,5,6,7]
I should make a tree:
1
2 3
4 5 6 7
My solution was to create nodes according to…
user821596
- 77
- 6
6
votes
2 answers
Why does Setter have Traversable constraint?
When you look at setting in lens-family-core package, you'll find that its type is Identical f => ((a -> b) -> s -> t) -> LensLike f s t a b, and Identical is defined as class (Traversable f, Applicative f) => Identical f.
I understand it needs…
snak
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6
votes
2 answers
Haskell - iso on newtype
If I have a newtype
newtype Foo = Foo Int
is there an automatic way to get an Iso' Foo Int?
I saw I could use makeLenses ''Foo, but I don't know what is the name of the generated iso.
marcosh
- 8,780
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- 74
6
votes
1 answer
Apply two Folds or Getters and only succeed when both succeed
Imagine I have the following list:
lst :: [(Bool, Maybe Integer)]
lst = [(True, Just 3), (True, Nothing), (False, Just 12)]
Using the lens library, I want to extract the elements of the tuples, but I only want it to succeed when the second element…
Alexis King
- 43,109
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6
votes
2 answers
Is there a direct way to combine setters for multiple record fields to a single setter?
import Control.Lens
import Control.Lens.TH
data Foo = Foo {
_bar, _baz :: Int
}
makeLenses ''Foo
Now if I want to modify both int fields, I can do
barbaz :: Setter' Foo Int
barbaz = sets $ \foo f -> foo & bar %~ f
…
leftaroundabout
- 117,950
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6
votes
2 answers
Use Haskell's lenses library to fmap a lens
In the code below, my question concerns the top-most function someFunc (everything below is just to provide a complete example). I use a record-syntax getter and fmap there. What's the lens way to implement someFunc?
import …
ruben.moor
- 1,876
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- 27
6
votes
1 answer
What's the best way to represent a short bit string?
I want to represent a string of up to around 120 bits, and speed is critical. I need to be able to build a bitstring by repeated snoc operations, and then to consume it with repeated uncons operations. One idea is to steal the implementation of…
dfeuer
- 48,079
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6
votes
1 answer
resource that explains vocabulary used in Edward Kmett's lens package
I am trying to read the documentation in Edward Kmett's Lens package. I am not familiar with a lot of the terms used (profunctor, isomorphism, monomorphic, contravariant, bifunctor, etc...)
What would be a good resource to go to learn some of this…
John F. Miller
- 26,961
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6
votes
1 answer
Use a lens to replace a specific element of a (key,value) list
I want to use Kmett's lens library to access an element of a (key, value) list under a specific key. In other words, I would like to replace this code with something more idiomatic and perhaps shorter:
type Headers = [ (ByteString, ByteString)…
dsign
- 12,340
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6
votes
1 answer
Is this expected behavior of Template Haskell?
Can anyone tell why this code doesn't compile
data A = A {
_b :: B
}
makeLenses ''A
type B = String
with message
Not in scope: type constructor or class B
and this does:
type B = String
data A = A {
_b :: B
}
makeLenses ''A
Without…
Alexey Vagarenko
- 1,206
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