Reinstallable - base

[Arnaud Spiwack]

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.

On Thu, 19 Oct 2023 at 13:47, Oleg Grenrus <oleg.grenrus at iki.fi> wrote:

> For what it worth, `template-haskell` itself depends on a `base`. So if
> `base` if different base is used, different `template-haskell` is to be
> used.
>
> In my opinion is not *too unfair* to require that if you actually splice
> in (i.e. the code not only provides template-haskell combinators to
> create/modify splices) then you must have base and template-haskell
> versions aligned with host GHC used versions.
>
> The same restriction is GHC plugins, isn't it, except `template-haskell`
> is replaced with `ghc`?
>
> - Oleg
>
> On 17.10.2023 18.54, Adam Gundry 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
> >
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-- 
Arnaud Spiwack
Director, Research at https://moduscreate.com and https://tweag.io.
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