Make the Erlang and Java grades optional for Mercury

[zopsicle]

The mercury package has a dependency on erlang and jdk so that the Mercury compiler can generate code for the Erlang and Java targets. However these are huge dependencies adding up to 1.5 GB to be downloaded from the Nix cache, even if these features are not going to be used.

I’d like to make a change to the Nix expression that makes these dependencies optional, but I’m not sure how to incorporate that into Nixpkgs, any suggestions? Perhaps have four packages: mercury, mercury-erlang, mercury-jdk, mercury-erlang-jdk, all defined in terms of a single parameterized package that allows you to pass null for the erlang and jdk packages.

[dtzWill]

+1, I made a change like this myself as well as a handful of tweaks to how we build mercury (such as adding a more recent ROTD variant).

In case it's useful, and feel free to take what you like and ignore the rest or ask questions:

• https://github.com/dtzWill/nixpkgs/commits/nixos-dtz/pkgs/development/compilers/mercury (starting with dtzWill@73993d6#diff-21df3ce5db84cd1eff425f94ad653199 which is relevant 😇)
• Current expression producing variants for use elsewhere: https://github.com/dtzWill/nixpkgs/blob/nixos-dtz/pkgs/development/compilers/mercury/default.nix

---

I'm not sure what I have is best, it was written while experimenting with mercury and grades and such so was originally exploratory and what's left is good enough to build the only mercury package I use and hack on, notmuch-bower :).

That said, minor feedback to your proposed four variants: I don't think it's worth enumerating the combinations as top-level attributes, but making their use optional sounds good to me. I'm unclear what the viability of the erlang or jdk backends is for either 14 or the current direction of mercury development, if you know or have any experience I'm interested. Mentioning mostly because if it seems they don't work and no one wants them, might be best to just nuke instead of pretending they're supported. (but dunno).

Hope this helps! Thanks for filling the issue, I think we can likely do better one way or another along the lines you outline!
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