http conduit - Haskell不评估块

Question

我正在编写简单的站点地图.xml爬虫。代码如下。我的问题是为什么main末尾的代码不打印任何内容。我怀疑是因为哈斯克尔的懒惰，但不知道如何处理这里：

import Network.HTTP.Conduit
import qualified Data.ByteString.Lazy as L
import Text.XML.Light
import Control.Monad.Trans (liftIO)
import Control.Monad
import Data.String.Utils
import Control.Exception
download :: Manager -> Request -> IO (Either HttpException L.ByteString)
download manager req = do
  try $
    fmap responseBody (httpLbs req manager)
downloadUrl :: Manager -> String -> IO (Either HttpException L.ByteString)
downloadUrl manager url = do
  request <- parseUrl url
  download manager request
getPages :: Manager -> [String] -> IO [Either HttpException L.ByteString]
getPages manager urls =
  sequence $ map (downloadUrl manager) urls
main = withManager $  manager -> do
  -- I know simpleHttp is bad here
  mapSource <- liftIO $ simpleHttp "http://example.com/sitemap.xml"
  let elements = (parseXMLDoc mapSource) >>= Just . findElements (mapElement "loc")
      Just urls = liftM (map $ (replace "/#!" "?_escaped_fragment_=") . strContent) elements
      mapElement name = QName name (Just "http://www.sitemaps.org/schemas/sitemap/0.9") Nothing
  return $
    getPages manager urls >>=  pages -> do
      print "evaluate me!"
      sequence $ map print pages

Answer 1

你遇到了我在这里描述的同样的问题，至少就错误代码而言，当它实际上应该给出类型错误时进行类型检查：为什么类型是"Main.main"，"IO（）"而不是"IO a"？这就是为什么您应该始终明确地main类型签名main :: IO ()的原因。

要解决此问题，您需要将return替换为lift（请参阅 http://hackage.haskell.org/package/transformers/docs/Control-Monad-Trans-Class.html#v:lift），并将sequence $ map ...替换为mapM_。 mapM_ f相当于sequence_ . map f。

Answer 2

将最后的return替换为 runResourceT （http://hackage.haskell.org/package/resourcet-1.1.1/docs/Control-Monad-Trans-Resource.html#v:runResourceT）。正如其类型所暗示的那样，它会将资源T转换为IO操作。