Reader Monad (2)

updated 2009-11-07 23:13:
加個「筆記」標籤，因為 comments 很重要 @@

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真糟，看半天試半天還是不懂 ask/asks 怎麼取到 Env 的？？

data Tem = Txt String | Var Tem deriving Show
type Env = [(String, String)]

lookupVar :: String -> Env -> Maybe String
lookupVar = lookup

resolve :: Tem -> Reader Env String
resolve (Txt str) = return str

重點來了：

resolve (Var tem) = do
                       name <- resolve tem
                       val  <- asks (lookupVar name)
                       return (maybe "" id val)

這三者是等價的，但是 ask/asks 怎麼把 Env 變出來的啊？？

resolve (Var tem) = resolve tem >>= \name -> asks (lookupVar name)
                                >>= \val  -> return (maybe "" id val)

resolve (Var tem) = resolve tem >>= \name -> (ask >>= \env -> return (lookupVar name env))
                                >>= \val  -> return (maybe "" id val)

Reader 定義在這：

instance Monad (Reader e) where 
    return a         = Reader $ \e -> a 
    (Reader r) >>= f = Reader $ \e -> f (r e) e

明明 e 就沒有餵給 f, 怎麼變出來的 @@

是說其實可以把 ask 忘掉，然後記得 asks 可以取得目前 Env,
這樣好像也是可以寫程式..... 但背後完全不懂行嗎？ @@