I think we'd all like it if someone could use GHC 9.10 to compile a library L that depends on base-4.9 and either L doesn't work at all with base-4.10, or L's dependency bounds have not yet been adjusted to allow base-4.10.

We'd like to have a version of `base`, say `base-4.9.1` that has the exact same API as `base-4.9` but works with GHC 9.10.

Today, GHC 9.10 comes with a specific version of base, and you can't change it. The original reason for that was, I recall, that GHC knows the precise place where (say) the type Int is declared, and it'll get very confused if that data type definition moves around.

But now we have `ghc-internal`, all these "things that GHC magically knows" are in `ghc-internal`, not `base`.

Hence my question: what (now) stops us making `base` behave like any other library?  That would be a big step forward, because it would mean that a newer GHC could compile old libraries against their old dependencies.

(Some changes would still be difficult.  If, for example, we removed Monad and replaced it with classes Mo1 and Mo2, it might be hard to simulate the old `base` with a shim.  But getting 99% of the way there would still be fantastic.)

Simon