Viktor Dukhovni
On Tue, Oct 17, 2023 at 04:54:41PM +0100, Adam Gundry wrote:
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.
To avoid that problem, GHC's own dependency on "base" could be indirect via a shared object with versioned symbol names and a version-specific SONAME (possibly even a private to GHC SONAME and private symbol version names). Say "libbase.so.4.19.1".
The problem here is deeper than simply the symbol names. For instance, the `List` type provided by `base-X.Y.Z` and `base-X.Y.Z'` may differ. Since lists are used in the `template-haskell` AST, we would be unable to share lists between `template-haskell` and `ghc`. As noted in my recent reply elsewhere in the thread, this can be avoided by communicating via serialisation instead of heap objects. Cheers, - Ben