I don't understand why Zig's `Io` is a "monad". In fact I discussed that with the author of this article and the author of Zig here, but no conclusion was reached (https://news.ycombinator.com/item?id=46129568).
But, flipping the script, if you want to see something like Zig's `Io` interface in Haskell then have a look at my capability system Bluefin, particularly Bluefin.IO. The equivalent of Zig's `Io` is called `IOE` and you can't do IO without it!
Regarding custom allocators and such, well, that could fit into the same pattern, in principle, since capabilities/regions/lifetimes are pretty much the same pattern. I don't know how one would plug that into Haskell's RTS.
But, flipping the script, if you want to see something like Zig's `Io` interface in Haskell then have a look at my capability system Bluefin, particularly Bluefin.IO. The equivalent of Zig's `Io` is called `IOE` and you can't do IO without it!
https://hackage-content.haskell.org/package/bluefin-0.5.1.0/...
Regarding custom allocators and such, well, that could fit into the same pattern, in principle, since capabilities/regions/lifetimes are pretty much the same pattern. I don't know how one would plug that into Haskell's RTS.