Haskell Substitution Problem

Example.hs

{-# LANGUAGE TemplateHaskell #-}

module Main where

import Control.Lens
import Data.Map (Map)

type SessionID = String

data Player = Player {
                _outbox :: [String]
              } deriving (Show,Eq)

data Board = Board {
                _players :: Map SessionID Player
             } deriving (Show,Eq)

makeLenses ''Player
makeLenses ''Board

--

-- | Pull the messages out of the given player's outbox.
dequeue :: SessionID -> Board -> (Board,[String])
dequeue sessionID board = (board',messages)
  where path = (players . ix sessionID . outbox)
        board' = set (players . ix sessionID . outbox) [] board
        messages = view path board

main :: IO ()
main = undefined

Example_Snippet.hs

-- | Pull the messages out of the given player's outbox.
dequeue :: SessionID -> Board -> (Board,[String])
dequeue sessionID board = (board',messages)
  where path = (players . ix sessionID . outbox)
        board' = set path [] board
        messages = view path board

And for future generations, here's the winning solution:

dequeueForPlayer :: SessionID -> Board -> (Board,[String])
dequeueForPlayer sessionID board = (board',messages)
  where path :: Traversal' Board [String]
        path = players . ix sessionID . outbox
        board' = set path [] board
        messages = view path board