The problem and solution are broadly fine, but having done a bit of performance critical work, I usually dislike when a function allocates without a) me asking, b) me having the option to tell it not to.
I like passing in pre-allocated containers to house the results of my computation. That way I have not only predictable memory consumption, but I can also avoid costly allocation (and de-allocation) in tight loops. Having worked with Go a bit, there were times when I used this (like passing in byte slices to readers), but oftentimes you couldn't just tell a function not to allocate (which it did for safety), because you and only you knew it'd be safe to modify the structure in place.
The bottom line is that people should not be "always copy/always use pure functions" or "always use mutable structures, because performance!!!", but be aware of what the upsides and downsides of each are.
I like passing in pre-allocated containers to house the results of my computation. That way I have not only predictable memory consumption, but I can also avoid costly allocation (and de-allocation) in tight loops. Having worked with Go a bit, there were times when I used this (like passing in byte slices to readers), but oftentimes you couldn't just tell a function not to allocate (which it did for safety), because you and only you knew it'd be safe to modify the structure in place.
The bottom line is that people should not be "always copy/always use pure functions" or "always use mutable structures, because performance!!!", but be aware of what the upsides and downsides of each are.