Re: Reinstallable - base

Something I haven’t gotten around to but only preliminary experiments with is dynamically built iserv binaries. Using -fexternal-interpreter can decouple the symbols the interpreter sees and those the compiler sees (They can even be of different architectures). iserv could be linked against the base the project wants to use, whereas GHC itself could use a different base. I’m not sure this covers everything, but it covers at least the case where we don’t need to load two different packages into the same process. Wrt to TH, I’m a bit behind on reading all the prior work to solve this, but conceptually I still believe template-haskell itself should not expose the internal ast, but only a combinator API to it. Regarding DSO’s: let’s please not make the existence of DSO a hard dependency. There are platforms for which we don’t have DSO capabilities, and where we are forced to use the in-memory loader and linker. On Wed, 18 Oct 2023 at 4:17 AM, Simon Peyton Jones < simon.peytonjones@gmail.com> wrote:

(Meta-question: on reflection, would this discussion perhaps be better on a ticket? But where? GHC's repo? Or HF's?)

The difficulty is that, as a normal Haskell library, ghc itself will be

...

compiled against a particular verson of base. Then when Template Haskell is used (with the internal interpreter), code will be dynamically loaded into a process that already has symbols for ghc's version of base, which means it is not safe for the code to depend on a different version of base.

I'm not understanding the difficulty yet.

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 the 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".

What am I missing?

Simon

On Tue, 17 Oct 2023 at 16:54, Adam Gundry wrote:

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Hi Simon,

Thanks for starting this discussion, it would be good to see progress in this direction. As it happens I was discussing this question with Ben and Matt over dinner last night, and unfortunately they explained to me that it is more difficult than I naively hoped, even once wired-in and known-key things are moved to ghc-internal.

The difficulty is that, as a normal Haskell library, ghc itself will be compiled against a particular version of base. Then when Template Haskell is used (with the internal interpreter), code will be dynamically loaded into a process that already has symbols for ghc's version of base, which means it is not safe for the code to depend on a different version of base. This is rather like the situation with TH and cross-compilers.

Adam

On 17/10/2023 11:08, Simon Peyton Jones wrote:

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Dear GHC devs

Given the now-agreed split between ghc-internal and base https://github.com/haskellfoundation/tech-proposals/pull/51, what stands in the way of a "reinstallable base"?

Specifically, suppose that

* GHC 9.8 comes out with base-4.9 * The CLC decides to make some change to `base`, so we get base-4.10 * Then GHC 9.10 comes out with base-4.10

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

-- Adam Gundry, Haskell Consultant Well-Typed LLP, https://www.well-typed.com/

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