Quoting Troy Sankey (2017-02-18 12:40:15)
> There is one more problem, and it is not a minor problem...
> 
> ------------------------------- cut here -------------------------------
> $ du -hs tmp/all-cabal-files
> 862M    tmp/all-cabal-files
> $ du -hs tmp/all-cabal-files/.git
> 140M    tmp/all-cabal-files/.git
> $ echo '862 - 140' | bc
> 722
> ------------------------------- cut here -------------------------------
> 
> The filesize of the package output will be 722 MiB.  We could prune it
> of all the unrelated versions according to the stackage LTS, but this
> starts to get complicated and error-prone...

On second thought, this is not too much of an issue since
ghc-all-cabal-files would only be an implicit *native* input.
Bystanders who install ghc-old-time (or any other package with a revised
cabal file) from a guix substitute will not need to download
ghc-all-cabal-files.

Troy