Haskell has a data type known as a lazy bytestring. By utilizing laziness, you can create large values without exhausting memory. Using lazy I/O, you can do such tricks as having a value which represents the entire contents of a file, yet only occupies a small memory footprint. In theory, a lazy bytestring is the only representation necessary for a response body.

In practice, while lazy byte strings are wonderful for generating “pure” values, the lazy I/O necessary to read a file introduces some non-determinism into our programs. When serving thousands of small files a second, the limiting factor is not memory, but file handles. Using lazy I/O, file handles may not be freed immediately, leading to resource exhaustion. To deal with this, WAI uses conduits.