In a conventional GCed language, you need to minimise heap allocations to avoid putting too much pressure on the garbage collector. The OxCaml extensions allows values to be passed 'locally' (that is, on the callstack) as an alternative to heap allocation. When the function returns, the values are automatically deallocated (and the type system guarantees safety).

This means that I can pass in a buffer, parse it, do my business logic, and then return, without ever allocating anything into the global heap. However, if I do need to allocate into it (for example, a complex structure), then it's still available.

It's kind of Rust in reverse: OxCaml has a GC by default, but you can write very high-performance code that effectively never uses a GC. There's also emerging support for data-race-free parallelisation as well.

The webserver I'm putting together also uses io_uring, which provides zero-copy buffers from kernel to userspace. The support for one-shot effect handlers in OCaml allows me to directly resume a blocked fiber straight from the io_uring loop, and then this httpz parser operates directly on that buffer. Shared memory all the way with almost no syscalls!