haskell facebook示例

Question

我被困在haskell類型中。

{-# LANGUAGE OverloadedStrings #-} 

module Main (
    main 
) where 

import qualified Facebook as FB 
import Network.HTTP.Conduit (withManager) 
import Control.Monad.IO.Class (liftIO) 
import System.IO 

app :: FB.Credentials 
app = FB.Credentials "localhost" "249348058430770" "..." 

url :: FB.RedirectUrl 
url = "http://localhost/fb" 

perms :: [FB.Permission] 
perms = ["user_about_me", "email"] 

main :: IO() 
main = do 
    fbAuthUrl <- FB.getUserAccessTokenStep1 url perms 
    liftIO $ print fbAuthUrl 
    argument <- readLn 
    token <- FB.getUserAccessTokenStep2 url [argument] 
    withManager $ \manager -> do 
     FB.runFacebookT app manager $ do 
      u <- FB.getUser "me" [] token 
      liftIO $ print (FB.userEmail u) 

錯誤

src/Main.hs:23:18: 
    Couldn't match expected type `IO t0' 
       with actual type `FB.FacebookT 
            FB.Auth m0 text-0.11.2.0:Data.Text.Internal.Text' 
    In the return type of a call of `FB.getUserAccessTokenStep1' 
    In a stmt of a 'do' block: 
     fbAuthUrl <- FB.getUserAccessTokenStep1 url perms 
    In the expression: 
     do { fbAuthUrl <- FB.getUserAccessTokenStep1 url perms; 
      liftIO $ print fbAuthUrl; 
      argument <- readLn; 
      token <- FB.getUserAccessTokenStep2 url [argument]; 
      .... } 

包http://hackage.haskell.org/package/fb

Answer 1

首先，讓我前言本答案由我從來沒有實際使用Facebook的API或Conduits庫中的免責聲明，所以我不知道這段代碼實際上是否做了任何明智的事情，但通過只輸入類型信息，我認爲這是你正在嘗試做的事

main :: IO() 
main = withManager $ \manager -> FB.runFacebookT app manager $ do 
    fbAuthUrl <- FB.getUserAccessTokenStep1 url perms 
    liftIO $ print fbAuthUrl 
    argument <- liftIO $ readLn 
    token <- FB.getUserAccessTokenStep2 url [argument] 
    u <- FB.getUser "me" [] (Just token) 
    liftIO $ print (FB.userEmail u) 

主要缺陷是，在Haskell main必須總是有型IO a，但你要使用FacebookT Auth m()類型的值作爲你main。你的實現是正確的，但runFacebookT和withManager需要是該函數中的第一件事。

從類型上看，實際的do-block的類型爲FacebookT Auth (ResourceT IO)()。 runFacebookT功能用於解開FacebookT變壓器，產生一個ResourceT IO()值，該值依次由withManager進行處理以產生一個普通的舊的IO()。

另外一個問題是，你最初在你的do-block中有一個readLn沒有liftIO，這是令人困惑的類型推理。我還將缺少的Just添加到FB.getUser調用中。