A couple of notes on Marshal, which I don’t think have been covered
- Although the guarantee is only between identical versions of OCaml, the implementation actually goes to considerable lengths to maintain backwards compatibility (so a value written by older OCaml remains readable in newer OCaml). Our own testsuite, for example, indirectly includes a test which unmarshals a 3.12.1 value. I don’t know exactly how far back the support goes.
- As it happens, the change which affected Unison in 4.08 was the first breaking change to Marshal since either 4.00 or 4.01. The fact that it doesn’t break often (and that the two code paths - at least at present - are small) meant I have suggested a few months back that we could in future add an additional flag in the style of
Compat_32to allow values to be written in a way which should be readable on older versions of OCaml. Indeed, it’s small enough that flags could be added for the changes in 4.08 (PR#1683) and in 4.11 (PR#8791). - Neither point undermines using alternative formats either for network serialisation or persistent storage, for the many reasons discussed above!