> As a fall-back solution, there is MD5 hashing. The problem is that
> you're then mixing information for all the contents of a module.
> Any change will produce a new incompatible hash value.
> For instance, every time you add a function to a library, it becomes
> incompatible. And there are new functions in every release of ocaml.
>
> Note that for C, compatibility policies generally allow adding extra
> functions to a library without changing the version number, since the
> problem, should it arise, can be detected at link time.

IMHO this a perfect research problem:

Find a mapping H:S->B where S is the set of module signatures and
B is the set of binary (arbitrary length) strings. Such that if and only if
s_1 is a subset of s_2 then there is some relation between H(s_1) and
H(s_2), thus  s_1<s_2 iff H(s_1) R H(s_2).

Perhaps you could drop "and only if" and let H(s_1) R H(s_2) imply
s_1 < s_2 with 99.9...% certainty.

Finding an efficient pair H and R would really make life easier for
software maintainers. I guess reasonable demands on H and R are that
H(s) should have a binary length that is a fraction of the size of the
corresponding .cmi and that b_1 R b_2 is big ordo the size of b_1 plus b_2.
In any case R must be significantly faster than big ordo size of b_1 times
b_2.



Does this make since? Is it possible? Is it already done?

Yours,

- Johan Granström

Ps. Using hashes of modules for compability checks is a pain in the
ass. Microsoft .NET framework does it - and it doesn't work in real
life. I know from personal experience...


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