懒惰的 ByteString 奇怪的行为或错误？

Question

当我在 GHCI 中测试我的函数 intervalFinder 时，它似乎正在工作，但是当我尝试编译它时，我没有输出：

函数作用于输入：

*Main> intervalFinder $[B.pack"first",B.pack"second",B.empty,B.pack"third",B.emp
ty]
Loading package bytestring-0.9.2.1 ... linking ... done.
["Start Time: first","End   Time: second","Start Time: third","End   Time: third
"]

并运行 main：

*Main> main
Loading package bytestring-0.9.2.1 ... linking ... done.
*Main> :q
Leaving GHCi.

在 results.txt 中打印：

Start Time: firstEnd   Time: secondStart Time: thirdEnd   Time: third 

但如果我运行 ghc test3.hs，输出文件为 0kb（显然其中没有数据！）

我做错了吗？

代码：

{-# LANGUAGE OverloadedStrings #-}
import qualified Data.ByteString.Char8 as B
import qualified Data.ByteString.Lazy.Char8 as Bl
import System.IO 
import System.Environment


intervalFinder :: [B.ByteString]->[B.ByteString]
intervalFinder x = helper x ""
    where
    helper (x:xs) "" 
        | x /= ""   = ((B.append (B.pack("Start Time: ")) x)):(helper xs x)
        | otherwise = helper xs ""
    helper (x:xs) y
        | x == ""   = ( (B.append (B.pack("End   Time: ")) y)):(helper xs "")
        | otherwise = helper xs x
    helper _ _      = []

main = do
    filehandle <- openFile "result.txt" WriteMode
    Bl.hPutStr (filehandle) .  Bl.fromChunks . intervalFinder $[B.pack"first",B.pack"second",B.empty,B.pack"third",B.empty]

谢谢！

Answer 1

main = do
    filehandle <- openFile "result.txt" WriteMode
    Bl.hPutStr (filehandle) .  Bl.fromChunks . intervalFinder
          $[B.pack"first",B.pack"second",B.empty,B.pack"third",B.empty]

输出是缓冲的，因此程序退出时不会使用runghc 或已编译的二进制文件刷新缓冲区。在 ghci 中，退出 ghci 时会刷新所有缓冲区¹。

当你openFile一个文件句柄时，你应该在使用后hClose它。如果输出缓冲区以写入或追加模式打开，这也会刷新输出缓冲区。

main = do
    filehandle <- openFile "result.txt" WriteMode
    Bl.hPutStr (filehandle) .  Bl.fromChunks . intervalFinder
          $[B.pack"first",B.pack"second",B.empty,B.pack"third",B.empty]
    hClose filehandle

或者，您可以使用writeFile

main = Bl.writeFile "result.txt" $ Bl.fromChunks ...

¹我不能 100% 确定这一点，但经验支持它。