Why can't we re-install the template-haskell package?

[Ryan Scott]

If you're reinstalling the same template-haskell version, then I can't
immediately think of anything that would go wrong. Then again, I'm not
intimately familiar with all of the little details that might be relevant
here.

Out of curiosity, are you asking this question because you've tried
implementing this and failed somewhere? Or are you trying to figure out is
there's some obvious problem with the idea that's been overlooked? In the
absence of anyone chiming in with such an obvious problem, I'd be inclined
to just try implementing it and seeing what happens. If it does break, that
could provide some valuable insight.

Ryan S.

On Thu, Mar 7, 2019 at 10:12 PM Moritz Angermann <moritz.angermann at gmail.com>
wrote:

> Hi Ryan, hi Richard,
>
> > My (limited) understanding is that template-haskell is not reinstallable
> for the same reasons that base and ghc-prim aren't reinstallable: the GHC
> codebase directly wires in several definitions several functions, types,
> and data constructors from template-haskell. See THNames [1]. If one were
> to, say, use GHC 8.6 but install a different version of template-haskell
> than the one that came bundled with 8.6, then it's almost certain that lots
> of code in THNames would no longer work, since they could be referencing
> identifiers that no longer exist (if the new version of template-haskell
> doesn't have them).
>
> Right, I'm mostly concerned about re-installing the *same* version again.
> For the motivation:
> template-haskell depends on pretty, deepseq and array.  Let's assume there
> is some off-by-one issue
> in array that only affects my application right now.  Now I'm going to
> patch my array package, but
> if my application depends on template-haskell, I end up with two different
> array packages in my dependency
> tree that are not identical.  What I'd ideally like to do here is to
> re-install deepseq, pretty and
> template-haskell based on my fixed array package.  Now I don't have two
> different array packeds in my
> dependencies anymore; it would however require me to be able to rebuild
> (the same version) of template-haskell.
>
>
> > I have to admit I don't have a strong grasp on what "reinstallable"
> implies. Does a package get the same hash after reinstalling? What could
> make a package not reinstallable? Why aren't packages reinstallable today?
> Why isn't ghc-prim reinstallable?
>
> We can't re-install packages that depend on build-time values. The RTS and
> ghc-prim right now
> include files that are generated during the build process and they have no
> capability on their
> own to create those files, as such re-installing them is not possible
> right now.
>
> > My concern stems from the fact that ghc is interlinked with TH in at
> least two ways:
> > - GHC imports definitions from template-haskell. But this is the same as
> the way GHC is involved with, say, `base`.
> > - GHC also wires in some template-haskell definitions. This is the
> aspect I am worried about. Is `base` reinstallable? If so, then perhaps
> template-haskell could be, too.
>
>
> Now especially with TH I might see an issue when *running/using* TH, as at
> that point the compiler
> and the produces code have to interact.  That is we are compiling splices
> against a different
> template-haskell package than the compiler is built against.  This is
> where I see *upgrading* (that
> is building a newer version of Template Haskell) could be an issue, but I
> feel I don't fully grasp
> why rebuilding the same version should pose an issue.
>
> If we go one step further and use iserv (-fexternal-interpreter) outright,
> then I think we could just
> rebuild iserv against the re-installed template-haskell, and would not
> even run into the above
> mentioned issue. Might this potentially even be allow us to upgrade
> template-haskell?
>
> Cheers,
>  Moritz
>
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