On 10/20/23 04:00, Simon Peyton Jones wrote:
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
Simon I think you are right on the level of GHC itself: GHC can indeed
cope with multiple versions of libraries just fine. However if we do
this we run the risk of the user getting "base.x.y FooType is not the
same as base.x.(y+1) FooType" errors, and I don't think that is good.
cabal-install and stack current enforce build plans such that those
sorts of errors are not possible, and I think that is a good thing we
should not revisit at this time.
I don't think this should stop anything about reinstallable base,
however. We just need to make a nice error when someone tries to splice
in code using the wrong version of template haskell (or otherwise
interact with GHC) using ABI hashes. Then we have the following situation:
1. Users that use stack/cabal-install plans only get one version of
base, and if they have template-haskell using the the "wrong"
version of base (identified via API hashes) they will get a nice error.
2. Users that "go rogue" and manually set up their project to mix base
versions and avoid other errors will have their TH work as you
describe./
So the thing Simon talks about is not /encouraged/, but it is a "free
feature" we shouldn't go out of our way to prevent either.
John
_______________________________________________
ghc-devs mailing list
ghc-devs@haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
