haskell - Haskell monad 链接中的类型检查错误

Question

我是 Haskell 新手，我只是按照 RWH 上的示例进行操作。我在使用第 14 章的以下程序时遇到问题:

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

似乎当使用 >>= 以 monad 链接格式编写 findCarrierBillingAdres 时，它只是不进行类型检查:

/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.

问题是..为什么使用 >>= 的第二种格式不会进行类型检查？

Answer 1

这只是单态限制再次发挥作用。由于您有一个没有类型签名的模式绑定(bind)，因此推断的类型是单态的，因此由第一次使用决定。

直接改成

lookup m k = flip M.lookup m k

甚至

lookup m = flip M.lookup m

你只需要说服 GHC 概括该功能，而当它只是一个简单的模式绑定(bind)时它不会这样做。添加参数会将其变成函数绑定(bind)，这意味着它将被完全泛化。

如果我在那里有点失去你，我已经 blogged about this