在 Haskell 中，对 Lazy ByteString 调用 length 会强制将整个字符串放入内存吗？
我正在使用惰性字节串读取大数据流，并且想知道在解析它时是否至少有 X 个字节可用。也就是说，我想知道字节串是否至少有 X 个字节长。
对其调用 length 是否会导致整个流被加载，从而违背使用惰性字节串的目的？
如果是，那么后续操作将是：如何在不加载整个流的情况下判断它是否至少有 X 个字节？
编辑：最初我在读取文件的上下文中询问，但了解有更好的方法来确定文件大小。然而，我需要的最终解决方案不应依赖于惰性字节串源。
I am reading a large data stream using lazy bytestrings, and want to know if at least X more bytes is available while parsing it. That is, I want to know if the bytestring is at least X bytes long.
Will calling length on it result in the entire stream getting loaded, hence defeating the purpose of using the lazy bytestring?
If yes, then the followup would be: How to tell if it has at least X bytes without loading the entire stream?
EDIT: Originally I asked in the context of reading files but understand that there are better ways to determine filesize. Te ultimate solution I need however should not depend on the lazy bytestring source.
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它将必须迭代整个字符串，但如果您不在其他任何地方保留对整个惰性字节字符串的引用，我相信它应该能够在字符串向尾部前进时释放字符串的头部。
It will have to iterate the entire string, but if you don't keep a reference to the entire lazy byte-string anywhere else, I believe it should be able to free the head of the string as it progresses towards its tail.
是的。
<代码>长度。取x。
Yes.
length . take x.您不使用 hFileSize :: Handle -> 有什么原因吗？ IO Integer 用于获取文件的长度？
Is there a reason you're not using
hFileSize :: Handle -> IO Integerfor getting the length of the file?编辑：抱歉。我想我认为字节串是列表。字节串没有 genericLength。
length是严格的，因为它返回的Int类型是严格的。您可以使用 Data.List 中的genericLength并导入一个定义惰性 Peano 数字的库，并为其提供一个 Num 实例，例如 numbers 库：这可以让您以您想要的方式表达您的功能，但 ehemient 的答案在功能上是相同的，并且不需要导入新的库。
我刚刚在这里发表了一篇关于该主题的博客文章，如果这听起来像您可能感兴趣的方法：
http://coder.bsimmons.name/blog/2010/03/lazy-arithmetic-in-haskell/
EDIT: sorry. I guess I was thinking bytestrings were lists. There is no genericLength for bytestrings.
lengthis strict because the type it returnsIntis strict. You can usegenericLengthfrom Data.List and import a library that defines lazy Peano numbers and gives you a Num instance for them, e.g the numbers library:That would let you express your function in the way you would like, but ephemient's answer is functionally the same, and doesn't require importing a new library.
I just did a blog post on the subject here, if that sounds like an approach you might be interested in:
http://coder.bsimmons.name/blog/2010/03/lazy-arithmetic-in-haskell/