Example.elm

import Html exposing (Html, button, div, text)
import Html.Events exposing (onClick)
import Game exposing (..)
import Game.Lenses exposing(..)

main =
  Html.beginnerProgram { model = game, view = view, update = update }

type Msg = UpdateGame

update msg model =
    case msg of
        UpdateGame ->
            updateGame model

view model =
    div []
        [ div [] [ text (toString (model |> inventory_.get)) ]
        ]

player = Player 10 [Item "Sword" "it cuts"]
slime = Monster "Slime" 1 (Item "goo" "it goos")
game = Game player [slime]



module Game exposing (..)

type alias Game =
    { player: Player
    , monsters: List Monster
    }

type alias Player =
    { health: Int -- [lens : playerHealth]
    , inventory: List Item
    }

type alias Item =
    { name: String -- [lens : itemName]
    , description: String
    }

type alias Monster =
    { name: String -- [lens :monsterName]
    , health: Int -- [lens :monsterHealth]
    , drops: Item
    }

updateGame : Game -> Game
updateGame game = game

model |> (player_ >. inventory_).get

  inv_ = player_ >. inventory_ 

Then you can just do model |> inv_.set [...] or whatever

To append to the inventory:

 model |> (player_ >. inventory_ >. (nth_ hugenumber defaultItem)).set myitem

where hugenumber is either a fixed huge number or is the length of the current inventory as determined by an erlier step, say stored in a let expression, and defaultItem is some fixed default item that is needed for formal purposes in this example.