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Question

Possible Duplicate:
Incomplete type signature

考慮以下幾點：

import Network.HTTP.Conduit 

(parseUrl "http://stackoverflow.com") :: Maybe a 

parseUrl回報Failure HttpException m => m (Request m')

它的文件說：

Since this function uses Failure , the return monad can be anything that is an instance of Failure , such as IO or Maybe .

然而，當我試圖迫使parseUrl使用Maybe，我得到以下錯誤：

main.hs:9:11: 
    Couldn't match type `a' with `Request m'0' 
     `a' is a rigid type variable bound by 
      an expression type signature: Maybe a at main.hs:9:10 
    Expected type: Maybe a 
     Actual type: Maybe (Request m'0) 

反正是有強制類型Maybe沒有指定完整確切類型？包括GHC擴展在內的答案很好。

需要注意的是這個工程：

f :: Maybe a -> Maybe a 
f x = x 

f (parseUrl "http://stackoverflow.com") 

但似乎醜陋的我。

Answer 1

您可以使用asTypeOf，

main = do 
    print (parseUrl "http://stackoverflow.com" `asTypeOf` Nothing) 

迫使單子是Maybe。並不是說這個收益大大超過

main = do 
    print (parseUrl "http://stackoverflow.com" :: Maybe (Request m))