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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=形式的页面中抓取有关电影的信息.htm。我为此所做的第一步是创建一个能够查询两个日期之间的每周信息的函数。我想出的代码不起作用，并且错误消息有点超出我目前的 haskell 能力。

编码：

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;
            .... }'

如果有人能阐明我做错了什么，我将不胜感激！

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1 回答 1

2

我绝不是 haskell 专家，但这就是我为使其编译而进行的更改。

问题在于函数 getMovies'。首先返回类型应该是 IO [Response L.ByteString]。第二个问题在于您对管道 Resource Monad 的处理，函数runResourceT返回您在管道流中所做的任何事情，在您的情况下应该是httpLbs 请求管理器的返回值。因此，您需要将getMovies的递归调用从 Resource monad 中移出。 

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.
于 2013-09-23T19:42:05.673 回答