Haskell抓取http-conduit问题

Question

我一直在尝试编写我的第一个"真正的"Haskell程序，该程序旨在最终从 http://www.boxofficemojo.com/weekly/chart/?yr=2012&wk=52&p=.htm 形式的页面中抓取有关电影的信息。我这样做的第一步是创建一个函数，该函数能够查询两个日期之间的每周信息。我想出的代码不起作用，错误消息有点超出我目前的 haskell 能力。

代码：

import Network.HTTP.Conduit
import Data.Time.Clock
import Data.Time.Calendar.WeekDate
import Data.Time.Calendar (Day, addDays, fromGregorian)
import Control.Monad.Trans.Resource (runResourceT)
import qualified Data.ByteString.Char8 as C
import qualified Data.ByteString.Lazy as L
import qualified Data.ByteString as S

curDate :: IO Day
curDate = fmap utctDay getCurrentTime
dayToWkYr :: Day -> (S.ByteString, S.ByteString)
dayToWkYr day = (C.pack (show year), C.pack (show week))
                where (year, week, _) = toWeekDate day
mkDateList :: Day -> Day -> [Day] -> [Day]
mkDateList start end lst
    | start == end = lst
    | otherwise    = mkDateList (addWk start) end (start:lst)
    where addWk = addDays 7
getMovies' :: Manager -> [Day] -> [Response L.ByteString] -> [Response L.ByteString]
getMovies' manager (d:ds) results = runResourceT $ do
    let (year, week) = dayToWkYr d
    initreq <- parseUrl "http://boxofficemojo.com/weekly/chart/"
    let request = initreq { queryString = "?yr=" `S.append` year `S.append`
                                            "&wk=" `S.append` week}
    response <- httpLbs request manager
    getMovies' manager ds (response:results)
getMovies' _ [] results = results

错误：

scraper.hs:27:37:
    Couldn't match type `[]' with `IO'
    When using functional dependencies to combine
      Control.Monad.Trans.Control.MonadBaseControl [] [],
        arising from the dependency `m -> b'
        in the instance declaration in `Control.Monad.Trans.Control'
      Control.Monad.Trans.Control.MonadBaseControl IO [],
        arising from a use of `runResourceT' at scraper.hs:27:37-48
    In the expression: runResourceT
    In the expression:
      runResourceT
      $ do { let (year, week) = dayToWkYr d;
             initreq <- parseUrl "http://boxofficemojo.com/weekly/chart/";
             let request = ...;
             response <- httpLbs request manager;
             .... }
scraper.hs:33:5:
    Couldn't match type `[]'
                  with `Control.Monad.Trans.Resource.ResourceT []'
    Expected type: Control.Monad.Trans.Resource.ResourceT
                     [] (Response L.ByteString)
      Actual type: [Response L.ByteString]
    In the return type of a call of getMovies'
    In a stmt of a 'do' block:
      getMovies' manager ds (response : results)
    In the second argument of `($)', namely
      `do { let (year, week) = dayToWkYr d;
            initreq <- parseUrl "http://boxofficemojo.com/weekly/chart/";
            let request = ...;
            response <- httpLbs request manager;
            .... }'

如果有人能说明我做错了什么，我将不胜感激！

Answer 1

我绝不是 Haskell 专家，但这是我为了编译而更改的内容。

问题在于函数getMovies'。首先，返回类型应为 IO [响应 L.ByteString]。第二个问题在于你对管道资源Monad的处理，函数runResourceT返回你在管道流中所做的一切，在你的例子中应该是来自httpLbs请求管理器的返回值。因此，您需要将递归调用从资源单体中移出以获取电影。

getMovies' :: Manager -> [Day] -> [Response L.ByteString] -> IO [Response L.ByteString]
getMovies' manager (d:ds) results = do
  response <- runResourceT $ do  -- we get the response here instead
    let (year, week) = dayToWkYr d
    initreq <- parseUrl "http://boxofficemojo.com/weekly/chart/"
    let request = initreq { queryString = "?yr=" `S.append` year `S.append`
                                            "&wk=" `S.append` week}
    httpLbs request manager
  getMovies' manager ds (response:results)
getMovies' _ [] results = return results -- wrap results in the IO monad.