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Questions tagged [haskell-lens]

A lenses library for Haskell.

An extensive library which provides families of lenses, isomorphisms, folds, traversals, getters and setters.

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454 questions
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Do I need Applicative Pretext for using Traversal with Lens?

I have a function that I believe is stricter than lens because of Applicative f context. Prelude Text.XML.Lens Data.Maybe Data.String Text.XML> :t f f :: Control.Applicative.Applicative f => (Element -> f Element) -> Document -> f…
sevo
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Update nested maps in scala

In haskell it is possible to do the following. import Control.Lens import qualified Data.Map as M m :: Map Int (Map Int String) m = M.empty & at 1 . non M.empty . at 2 .~ Just "one-two" Is it possible to do something similar in scala? Maybe using…
Pavel
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cabal install lens: library not found for -lHStransformers-compat-0.3.3.4-ghc7.8.3

This is on Mac OS X with brew installed ghc and cabal-install. latest versions. ghc 7.8.3 When I run cabal install lens I get this error output: Resolving dependencies... [1 of 1] Compiling Main (…
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Using a Prism as a smart constructor

I was browsing the 'lens' documentation concerning prisms, and I noticed a line in Control.Lens.Review stating that the infix review operator # 'is commonly used when using a Prism as a smart constructor.'. This got my a bit intrigued, because the…
Kasper
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wreq not compiling with cabal sandbox

I'm using cabal sandbox and I got this error when compiling the wreq library: Network/Wreq/Lens/Machinery.hs:20:58: Couldn't match type `[Name]' with `Name' Expected type: Name -> [Name] -> Name -> [DefName] Actual type: [Name] -> Name…
z1naOK9nu8iY5A
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Trouble when composing lenses

First of all, a snippet of code: λ> let applicationState = ('a','b',(M.fromList $ zip [1..3] [11,22,33],M.fromList $ zip [4,5,6] [44,55,66],M.fromList $ zip [7,8,9] [S.fromList ["77","777","7777"],S.fromList ["88","888","8888"],S.fromList…
FallingSkies
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Some potential and difficulties in the use of lenses in MonadState

What follows is a series of examples/exercises upon Lenses (by Edward Kmett) in MonadState, based on the solution of Petr Pudlak to my previous question. In addition to demonstrate some uses and the power of the lenses, these examples show how…
Alberto Capitani
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Saving a position with Lenses

I'm trying to use xml-conduit and xml-lens for parsing and traversing an XML document. Instead of having to traverse the same parts of the document multiple times, I would like to store the traversal up until the previous point and then drill down…
Adam
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Is it possible to nest `at` with setters?

I'd like to set a value in 2 level map - i.e. to a key in a map which is a value for some key in an "outer" map. For ordinary Map I could use something like m & at 42 ?= "value" But I could not find any way to nest 2 at's What is the easiest way to…
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Lensified Entity System

Inspired by this two comments on reddit I set out to create a 'lensified entity system'. The basic idea is to have Lens' Entity Value lenses, but although there is Action to create Getters with side effects there is no Setter or a combined LensM'…
fho
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How does `lens` work?

I mean, not the simple stuff like this (from here): strike :: StateT Game IO () strike = do lift $ putStrLn "*shink*" boss.health -= 10 But things like using lens to map over types from Linear. How would I express this in terms of…
fho
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Different setter and getter types in Haskell's lenses

I've got a data type G, which have got field _repr :: Data.Graph.Inductive.Gr String String. The normal way, when adding new node into Gr graph, we have to provide an LNode a object, which basically is defined as a tuple of (Int, a), where Int is…
Wojciech Danilo
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How to conditionally set values in a nested record + list using lenses?

What's a succinct way to implement the ensureRecAFlag function using lenses? If RecB contains a RecA with recaFlag=True then do nothing Else, if RecB contains a RecA with recaName="internal_code" then set recaFlag=True for that RecA Else, if RecB…
Saurabh Nanda
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Implement a ReaderT monad Type for mocking network api calls

data TestEnv = TestEnv { rateLimiter' :: !TokenBucket , apiManager :: !Manager , apiKey :: !BS.ByteString } type BunnyReaderT m = ReaderT TestEnv m class MonadIO m => HasBunny m where runRequest :: Request -> m (Response BSL.ByteString) …
Vikram malik
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Elegant way to apply curried function to indexed traversal

I want to compose a curried function f :: i -> a -> b With an indexed traversal l :: IndexedTraversal' i s a apply it to an s and get back a [b]. I came up with s ^.. l . withIndex . to (uncurry f) But I would like a combinator itoListByOf that…
pat
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