Haskell monad鏈接中的類型檢查錯誤

Question

我是Haskell的新手，我只是遵循RWH上的示例。我有在第14章有以下程序問題：

import qualified Data.Map as M 

type PersonName = String 
type PhoneNumber = String 
type BillingAddress = String 
data MobileCarrier = Honest_Bobs_Phone_Network 
        | Morrisas_Marvelous_Mobiles 
        | Petes_Plutocratic_Phones 
        deriving (Eq, Ord) 

findCarrierBillingAddress :: PersonName 
          -> M.Map PersonName PhoneNumber 
          -> M.Map PhoneNumber MobileCarrier 
          -> M.Map MobileCarrier BillingAddress 
          -> Maybe BillingAddress 
-- This will work 
findCarrierBillingAddress person phoneMap carrierMap addressMap = do 
    phone <- M.lookup person phoneMap 
    carrier <- M.lookup phone carrierMap 
    address <- M.lookup carrier addressMap 
    return address 

-- This will NOT work: 
findCarrierBillingAddress person phoneMap carrierMap addressMap = 
    return person >>= 
    lookup phoneMap >>= 
    lookup carrierMap >>= 
    lookup addressMap 
    where lookup = flip M.lookup 

看來，在單子寫findCarrierBillingAddres當鏈接的格式使用>> =，它只是不類型檢查：

/home/bruce/Programming/haskell/real/ch14/hello.hs:21:9: 
    Couldn't match type `[Char]' with `MobileCarrier' 
    Expected type: MobileCarrier -> Maybe BillingAddress 
     Actual type: PersonName -> Maybe BillingAddress 
    In the return type of a call of `lookup' 
    In the second argument of `(>>=)', namely `lookup addressMap' 
    In the expression: 
     return person >>= lookup phoneMap >>= lookup carrierMap 
     >>= lookup addressMap 

/home/bruce/Programming/haskell/real/ch14/hello.hs:21:16: 
    Couldn't match type `MobileCarrier' with `[Char]' 
    Expected type: M.Map PersonName BillingAddress 
     Actual type: M.Map MobileCarrier BillingAddress 
    In the first argument of `lookup', namely `addressMap' 
    In the second argument of `(>>=)', namely `lookup addressMap' 
    In the expression: 
     return person >>= lookup phoneMap >>= lookup carrierMap 
     >>= lookup addressMap 
Failed, modules loaded: none. 

問題是......爲什麼使用>> =的第二個格式不會輸入check？

Answer 1

這只是單態的限制又出毛病了。由於您有一個沒有類型簽名的模式綁定，因此推斷的類型是單形的，因此由第一次使用確定。

它只是改變

lookup m k = flip M.lookup m k 

甚至

lookup m = flip M.lookup m 

你只需要說服GHC來概括它時，它只是一個簡單的模式綁定不會做的功能。添加一個參數會將它變成一個函數綁定，這意味着它將被完全泛化。

如果我失去你有點起來了，我已經blogged about this

Answer 2

你被打與Monomorphism Restriction 當類型inferencer試圖推導出當地 lookup函數的類型。

而不是演繹最普通的類型，它的lookup第一次使用 ，它應該有類型lookup :: Map [Char] [Char] -> [Char] -> Maybe [Char]，不能統一時，你再嘗試使用它 上的Map [Char] MobileCarrier值決定。

您的第一個選擇是使用編注 {-# LANGUAGE NoMonomorphismRestriction #-}禁用單態限制。第二個選項是增加 類型簽名lookup因爲如此

findCarrierBillingAddress person phoneMap carrierMap addressMap = 
    return person >>= 
    lookup phoneMap >>= 
    lookup carrierMap >>= 
    lookup addressMap 
    where lookup :: Ord k => M.Map k a -> k -> Maybe a 
     lookup = flip M.lookup