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!