缩放是否有"双重"？

Question

zoom 允许我们在实际定义了更多变量的上下文中使用仅使用某些状态变量的状态操作.

{-# LANGUAGE TemplateHaskell #-}

import Control.Lens

import Control.Monad.Trans.State
import Control.Monad.IO.Class

data Galaxy = Galaxy {
    _solarSys :: SolarSystem
  , _otherStars :: String
  } deriving (Show)
data SolarSystem = SolarSystem {
    _sun :: Float
  , _planets :: Int
  } deriving (Show)

makeLenses ''SolarSystem
makeLenses ''Galaxy

main = (`runStateT`Galaxy (SolarSystem 2e+30 8) "") $ do
   zoom solarSys $ do
      sun -= 1e+23
      planets += 1
   liftIO . print =<< get

Galaxy {_solarSys = SolarSystem {_sun = 1.9999999e30, _planets = 9}, _otherStars = ""}

但是,如果我想在一个只定义了一些状态变量的环境中做一些事情,然后运行一个具有一些额外的本地状态变量的计算呢？喜欢

data Expedition = Expedition {
    _environment :: SolarSystem
  , _spacecraft :: Char
  } deriving (Show)
makeLenses ''Exploration

main = (`runStateT`Galaxy (SolarSystem 2e+30 8) "Milky") $ do
   zoom solarSys $ do
      spectralFilter environment (spacecraft ???~= '') $ do
         spacecraft .= '?'
         environment . planets -= 1
   liftIO . print =<< get

我怀疑初始化spacecraft实际上需要一些其他光学器件,但我看不出哪个.

Answer 1

这个功能怎么样？

cram :: Monad m => Iso' s' (s,x) -> x -> StateT s' m r -> StateT s m r
cram someiso extra action =
    StateT (\small0 -> do let big0 = view (from someiso) (small0,extra)
                          (r,big) <- runStateT action big0
                          let (small,_) = view someiso big
                          pure (r,small))

“如果你让我相信扩展状态是小状态加上额外的东西，并且你给我一些最初的额外东西，我可以将扩展状态计算塞进小状态计算中。”

你必须写一个Iso' Expedition (SolarSystem,Char).