Reinstallable - base

Wed Oct 18 02:30:54 UTC 2023

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 at 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 <adam at well-typed.com> wrote:
>
>> 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:
>> > 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/
>>
>> Registered in England & Wales, OC335890
>> 27 Old Gloucester Street, London WC1N 3AX, England
>>
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