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<How laziness changes thinking in Haskell ⁄ >




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One of the great benefits of lazy evaluation is that your code doesn't need to account for the scale of an operation. Let's take a simple example: checksumming a large file whose contents are being read, on-demand, over HTTP.

In C++, if I wanted to checksum a large file read over HTTP, I couldn't buffer it memory because I don't know how large it might get. Nor do I want my checksumming code to know anything about HTTP, or where the data comes from. The answer here is to use I/O streams. By passing a generic istream interface around, I can hide any knowledge of where the data came from. The checksumming algorithm just reads data from the stream as it needs to, and the HTTP layer downloads more bytes as required (typically caching to avoid constant network access).

However, there's a downside to this: the checksumming code now knows something about I/O. In actuality, a checksumming algorithm only cares about the bytes being checksummed and little else. It shouldn't have to know about I/O, or strings, or any of the details of where data comes from or how it's structured. It should ideally receive a pointer to an arbitrary large sequences of 8-bit bytes, and return a fixed size checksum representing a fingerprint of those bytes.

Yet this naive approach can't really be done in C++. If it were a file I was checksumming, I could memory map the file and pass around a byte pointer, and the OS would take care of lazily reading in the bytes for me as needed. But for a file being accessed over HTTP this would require first downloading the file and then checksumming it, when I specifically wanted to "checkum as I go". Who knows, maybe I'll discover a reason to stop summing beyond a certain point and I'd like to stop downloading at that point as well.



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