How to modify fields of a nested custom data type with lenses, when modifications depend on indices

问题

Considering the following :

{-# LANGUAGE TemplateHaskell   #-}

import Control.Lens

data Typex = Typex 
    { _level       :: Int
    , _coordinate  :: (Int, Int)
    , _connections :: [(Int,(Int,Int))]
    } deriving Show
makeLenses ''Typex

initTypexLevel :: Int -> Int -> Int -> [Typex] 
initTypexLevel a b c = [ Typex a (x, y) [(0,(0,0))]
                       | x <- [0..b], y <- [0..c]
                       ]

buildNestedTypexs :: [(Int, Int)] -> [[Typex]]
buildNestedTypexs pts
     = setConnections [ initTypexLevel i y y
                      | (i,(_,y)) <- zip [0..] pts
                      ]

setConnections :: [[Typex]] -> [[Typex]]
setConnections = ?

How can I uses lenses to modify the connections in allTypexs with a function of type [[Typex]] -> [[Typex]] in such a way that in each Typex

connections = [(level of Typex being modified +1, (x, y))] where
x,y = 0..(length of next [Typex] in [[Typex]])/2

X and y both need to go through that length of the next [Typex]. The final [Typex] should be left unchanged if possible. So all the connections of each Typex in the same [Typex] are the same.

Output for setConnections $ buildNestedTypexs [(0,1),(1,1)] should be:

[ [ Typex { _level = 0
          , _coordinate = (0,0)
          , _connections = [(1,(0,0)), (1,(0,1)), (1,(1,0)), (1,(1,1))] }
  , Typex { _level = 0
          , _coordinate = (0,1)
          , _connections = [(1,(0,0)), (1,(0,1)), (1,(1,0)), (1,(1,1))] }
  , Typex { _level = 0
          , _coordinate = (1,0)
          , _connections = [(1,(0,0)), (1,(0,1)), (1,(1,0)), (1,(1,1))] }
  , Typex { _level = 0
          , _coordinate = (1,1)
          , _connections = [(1,(0,0)), (1,(0,1)), (1,(1,0)), (1,(1,1))] }
  ]
 ,[ Typex { _level = 1
          , _coordinate = (0,0)
          , _connections = [(0,(0,0))] }
  , Typex { _level = 1
          , _coordinate = (0,1)
          , _connections = [(0,(0,0))] }
  , Typex { _level = 1
          , _coordinate = (1,0)
          , _connections = [(0,(0,0))] }
  , Typex { _level = 1
          , _coordinate = (1,1)
          , _connections = [(0,(0,0))] }
  ]]

I suppose I'll need to import Control.Lens.Indexed but that's about it so all help is appreciated.

回答1:

Is this what you want?

{-# LANGUAGE TupleSections #-}

setConnections :: [[Typex]] -> [[Typex]]
setConnections (x:rest@(y:_)) = map (connect y) x : setConnections rest
  where connect :: [Typex] -> Typex -> Typex
        connect txs tx
          = tx & connections .~ (map ((tx ^. level) + 1,) $ txs ^.. traverse.coordinate)
setConnections lst = lst

This isn't a pure lens solution, but I find that as a general rule when working with lenses, it's not always a good idea to get the lenses to do everything. It just makes things difficult to write and hard to understand.

Here, I've used "plain Haskell" in a lot of places: to pattern matching with a manual recursion to process pairs x,y of consecutive [Typex]s and I've used map to connect each Typex in the first x :: [Typex] with the second y :: [Typex]. I've also used map to add the new level to the coordinate list to generate the new connections value.

The only lens expressions used here are:

tx & connections .~ (...) which replaces the connections field of tx :: Typex with a new value
tx ^. level which fetches the level of the current tx :: Typex
txs ^.. traverse.coordinate which fetches the coordinate fields of all Typex values in the list txs :: [Typex] and returns them as a list [(Int,Int)]

In my opinion, this sort of balance between lenses and "plain Haskell" is the best way of dealing with complex transformations.

来源：https://stackoverflow.com/questions/60545053/how-to-modify-fields-of-a-nested-custom-data-type-with-lenses-when-modification

标签

haskell

nested

indices

lens

custom-data-type