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
0
votes
1 answer
Lens: combine children values (xml-lens)
I have an xml that looks like this:
AAA
100
...
BBB
200
...
I need to get a list of A and B values (for example as a list of…
WHITECOLOR
- 24,996
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votes
0 answers
Lens through the maybe field
I have a data similar to, but a bit more nested and of course named differently:
data AppModel = AppModel
{ _foo :: Maybe String
} deriving (Eq, Show)
I would like to pass it to the Monomer textField
textField :: WidgetEvent e => ALens' s Text ->…
majkrzak
- 1,332
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0
votes
1 answer
How to fix compilation re haskell rigid type variable
I have a an aggregate type named Step and from it, some concrete types are created:
class (Eq q, User a, User b, User c) => (Step q a b c) where
{-# MINIMAL performerA, performerB, performerC, completionDate #-}
performerA :: Lens' q…
Fiftywebs
- 185
- 10
0
votes
1 answer
Haskell Lens : lexical error in string/character literal at end of input
I'm working the "Optics by Example" haskell book.
Below is my dependencies in package.yaml of stack project-
dependencies:
- base >= 4.7 && < 5
- aeson
- containers
- lens
- lens-aeson
- mtl
- text
Below is my Haskell Code -
{-# LANGUAGE…
user51
- 8,843
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0
votes
1 answer
functor on data type with two constructors - occurs check
I have a data type
data Pnt2d i v = Pnt2d {_p2id:: i, _v2:: V2 v}
and would like to construct a functor to apply to the second component (V2) as
instance Functor (Pnt2d i) where
fmap f (Pnt2d i v) = Pnt2d i (f v)
but I get an "occurs check". I…
user855443
- 2,596
- 3
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- 37
0
votes
2 answers
How do I convert indices to a lens for a Forest to a Tree?
I am trying to make a function that take a index into a forest, and a list of integers to choose a way down a tree, and turns that into a lens that focuses that same element.
Effectively, it is the opposite of what (t :: Data.Tree.Forest) ^@..…
Janus Troelsen
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1 answer
How to use iset with nested datatype and lenses?
I can't get the types in the last function to line up. Point is to set the all price Doubles in connections with a function that depends only on the index of the 3-tuple. The original Double value in the tuple can be discarded.
{-# LANGUAGE…
LHurttila
- 37
- 6
0
votes
1 answer
How to modify fields of a nested custom data type with lenses, when modifications depend on indices
Considering the following :
{-# LANGUAGE TemplateHaskell #-}
import Control.Lens
data Typex = Typex
{ _level :: Int
, _coordinate :: (Int, Int)
, _connections :: [(Int,(Int,Int))]
} deriving Show
makeLenses…
LHurttila
- 37
- 6
0
votes
0 answers
Is there a string-based lens for runtime field access?
Similar to this question, but even more runtime-ey.
Is there a way to automatically generate lenses, or any other mechanism, for accessing fields by name at runtime?
I'm looking for something similar to field from generic-lens, except taking the…
Isaac van Bakel
- 1,772
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- 22
0
votes
1 answer
how to deal with duplicate record field with lenses?
given this piece of code:
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE DuplicateRecordFields #-}
{-# LANGUAGE FunctionalDependencies #-}
module Foo where
import Control.Lens ((^.), makeFieldsNoPrefix)
import…
itsu
- 222
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0
votes
1 answer
How does Lens.Internal.Indexed connect to keyed containers?
A traversable may be labelled. To take this idea one step further, one may apply a function to any element of a traversable by its index.
import Control.Monad.State
updateAt :: forall a. Int -> (a -> a) -> [a] -> [a]
updateAt i f = flip evalState…
Ignat Insarov
- 4,660
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- 37
0
votes
0 answers
How to fix "Couldn't match type ‘Item a0’ with ‘[Item [Char]]’" when reading using lens
I'm writing markov chain generator with Haskell, using weighted list for randomly selecting elements. When I'm testing the configuration, I get an error "Couldn't match type ‘Item a0’ with ‘[Item [Char]]’". I suspect this is somehow related to the…
Tuukka Turto
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votes
3 answers
Haskell - A better way of keeping track of the (initial) state of a record
I'm working on some functions that take a record and return a slightly modified record.
For example
import Control.Lens ((%~), (^.), (&))
modifyRecord :: SomeRecord -> SomeRecord -> SomeRecord
modifyRecord baseR currentR = currentR & thisPart %~…
atis
- 881
- 5
- 22
0
votes
0 answers
How can one export a datatype record in haskell without exporting its fields?
I saw the somewhat older question at Avoiding namespace pollution in Haskell and am looking for a modern solution using lenses.
I think the approach would come in a few steps:
Rewrite fields to be lens friendly: field -> _field.
Use lenses.
Avoid…
bbarker
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0
votes
1 answer
How do I add an Applicative context to a type expected by Lens' MyDataType?
I have a function generalized over a function func to be applied on MyDataType1 and another MyDataType1 like this
setThis :: (Functor f0, Applicative f0) => Lens' MyDataType1 (f0 Float) -> Float -> MyDataType1 -> MyDataType1 -> MyDataType1 …
atis
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- 22