http - 如何使用 wreq 只下载部分响应？

Question

如何使用 wreq 仅下载响应的前 1 KiB？图书馆？

理论上我可以使用 Range HTTP header 。但是，这需要服务器的支持，我不能依赖它。

一旦客户端上有足够的数据，我该如何中止请求？

Answer 1

一种选择是使用 foldGet函数，在数据到达时累积数据 block ，并在下载足够的数据后抛出异常。累积的数据将在异常中传递出去。

import qualified Control.Exception           as E
import qualified Data.ByteString             as BS
import qualified Data.ByteString.Lazy        as BL
import           Data.ByteString.Builder

data EnoughException = Enough Builder

instance Show EnoughException where
    show _ = "EnoughException"

instance E.Exception EnoughException

getFile :: Int -> String -> IO BL.ByteString
getFile maxLen url = ((extract . fst) <$> foldGet process (mempty, 0) url) `E.catch` enough
  where
    extract :: Builder -> String
    extract = BL.take (fromIntegral maxLen) . toLazyByteString

    process :: (Builder, Int)   -- ^(Accumulated data, current length)
            -> BS.ByteString    -- ^Downloded chunk
            -> IO (Builder, Int)
    process (acc, cur) now = do
        let acc' = acc <> byteString now
        let cur' = cur + BS.length now
        if cur' > maxLen
            then E.throw (Enough acc')
            else return (acc', cur')

    enough (Enough d) = return (extract d)

(注意:此函数不处理可能由 wreq 抛出的异常。对于实际使用，这是必要的。)