How to you manage the API when caller doesn't know the right size to allocate? I mean you could pass an allocator object (ala zig/C++ stdlib etc), though I'm not sure I like the extra arg on functions... I guess a quasi-global-variable approach (e.g. a "context" object, passed to all functions, which can hold an allocator, or equivalently implicit arguments in a functional language) would be one way to achieve that without the "noise" of explicit allocators passed everywhere.