A very large proportion of libraries, and virtually all end-user applications, transitively depend on Template Haskell. Whether they use Template Haskell directly or not. So if we're saying “base is reinstallable, except when you have Template Haskell somewhere”, we're effectively saying “base is not reinstallable”. Now, it could be a good stepping-stone, from an engineering standpoint, but I don't think we could deliver this and be satisfied that we've accomplished anything.

No one has yet answered my naive question (from 3 days ago) asking why Template Haskell stops base being reinstallable. I'll quote it here for completeness.

Let's say that

An old library mylib (which uses TH) depends on base-4.7.

A new GHC, say GHC 9.10, depends on a newer version of base-4.9, which in turn depends on ghc-internal-9.10.

At the same time, though, we release base-4.7.1, which depends on ghc-internal-9.10, and exposes the base-4.7 API.

At this point we use ghc-9.10 to compile L, against base-4.7.1. (Note that the ghc-9.10 binary includes a compiled form of `base-4.9`.)

That produces compiled object files, such as, mylib:M.o.

To run TH we need to link them with the running binary

So we need to link the compiled `base-4.7.1` as well. No problem: it contains very little code; it is mostly a shim for ghc-internal-9.10

So the only thing we need is the ability to have a single linked binary that includes (the compiled form for) two different versions/instantiations of `base`. I think that's already supported: each has a distinct "installed package id".

(End of quote)

What am I missing?

Simon