lens

I recently discovered the lens package on Hackage and have been trying to make use of it now in a small test project that might turn into a MUD/MUSH server one very distant day if I keep working on it. Here is a minimized version of my code illustrating the problem I am facing right now with the at lenses used to access Key/Value containers (Data.Map.Strict in my case) {-# LANGUAGE OverloadedStrings, GeneralizedNewtypeDeriving, TemplateHaskell #-} module World where import Control.Applicative ((<$>),(<*>), pure) import Control.Lens import Data.Map.Strict (Map) import qualified Data.Map.Strict

If I have a record type, I can do pretty much anything I want to it with lenses. If I have a sum type, I can do pretty much whatever I want to it with prisms. But if I have a sum that includes a record, makeFields doesn't give me lenses into the fields (of course), but only traversals for them. declarePrisms seems a bit more promising. According to the documentation, declarePrisms [d| data Exp = Lit Int | Var String | Lambda{ bound::String, body::Exp } |] will create data Exp = Lit Int | Var String | Lambda { bound::String, body::Exp } _Lit :: Prism' Exp Int _Var :: Prism' Exp String _Lambda :

Let's say, I have an object with two fields: data Example = Example { _position :: Int , _storage :: [Int]} how do I construct a lens that focuses on position element inside storage ? Also, will it be possible to restrict position values being modified via lenses to a range based on storage size? It seems like alongside could be used somehow, since Example is isomorphic to a tuple, but I can't comprehend the way to do that. I'm not sure how to phrase the question, so I was unable to find much relevant info. Edit: I misunderstood the problem, original answer follows at the end. I don't know any

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 = Inner (MyType 1) i2 = Inner (MyType 2) o = Outer [i1, i2] x = o ^. inners . ix 0 . val giving this error Toy.hs:17:23: No instance for (Data.Monoid.Monoid MyType) arising from a use of `ix' Possible fix: add an instance declaration for (Data.Monoid.Monoid MyType) In the first argument of `(.)', namely `ix 0' In the second argument of `(.)', namely `ix 0

I need to manipulate and modify deeply nested immutable collections (maps and lists), and I'd like to better understand the different approaches. These two libraries solve more or less the same problem, right? How are they different, what types of problem is one approach more suitable for over the other? Clojure's assoc-in Haskell's lens Clojure's assoc-in lets you specify a path through a nested data struture using integers and keywords and introduce a new value at that path. It has partners dissoc-in , get-in , and update-in which remove elements, get them without removal, or modify them

Suppose I have a struct with many fields: (struct my-struct (f1 f2 f3 f4)) If I am to return a new struct with f2 updated, I have to rephrase every other fields: (define s (my-struct 1 2 3 4)) (my-struct (my-struct-f1 s) (do-something-on (my-struct-f2 s)) (my-struct-f3 s) (my-struct-f4 s)) Which is redundant and would be a source of bugs if I update the number of the fields or changed their orders. I really wonder if there's a such way I can update a specific field for a struct like: (my-struct-f2-update (my-struct 1 2 3 4) (lambda (f2) (* f2 2))) ;; => (my-struct 1 4 3 4) Or I can just set

Let's say I have a pair of conversion functions string2int :: String -> Maybe Int int2string :: Int -> String I could represent these fairly easily using Optics. stringIntPrism :: Prism String Int However if I want to represent failure reason, I'd need to keep these as two separate functions. string2int :: String -> Validation [ParseError] Int int2string :: Int -> String` For this simple example Maybe is perfectly fine, since we can always assume that a failure is a parse failure, thus we don't actually have to encode this using an Either or Validation type. However imagine, in addition to my

可以将文章内容翻译成中文,广告屏蔽插件可能会导致该功能失效(如失效，请关闭广告屏蔽插件后再试): 问题: I know the default OpenCV Calibration systems model a Pinhole camera, but I'm working with a system using extremely wide FOV lens (187-degrees). If there any existing way to do this in OpenCV, or to work with just wide lenses? Or will I have to rewrite all the calibration/undistort for my system? 回答1: Seems there's no good OpenCV way to do this. I wound up using OCamLib to do the actual calibration, then writing my own "undistortPoints" function (using Scaramuzza's algorithms) to undistort 2D image points into 3D unit vectors (rather than 2D