Omit deriving Generic on LicenseId

[ndmitchell]

Compiling the LicenseId file on my Windows GHC 6.8.3 takes ~30s (measured while running Hadrian). Experimenting with a cut down version, it turned out that most of the time was due to the Generic derivation on LicenseId. That generic derivation is used to provide a Binary instance, but can be replaced trivially by:

instance Binary LicenseId where
    put = put . fromEnum
    get = toEnum <$> get

In my measurements this reduces the compilation time significantly, from 9s to 3s at -O0 and 22s to 8s on my cutdown file. That seems a worthwhile speedup.

Given this is an exported type, it's technically a user visible API change, hence raising a bug rather than a PR.

[23Skidoo]

I believe it should be fine to apply this change to master.
6.8.3 is quite old though, how do more recent versions fare?

[ndmitchell]

Sorry, I meant 8.6.3 - the latest released one 🤦‍♂️

[23Skidoo]

OK, let's do it.

[ndmitchell]

Hmm, actually, it's going to change the Binary instance for LicenseId, so things saved with the old won't still work. Is that an issue?

[23Skidoo]

It should be fine, since we don't support reading local build infos generated by old Cabal versions (you get a message telling you to re-run configure). May be worth bumping the master version to 2.5.1 so that people using master snapshots don't get weird errors.

[hvr]

I'm strongly against this, as this would mean that we'd end up with orphan instances somewhere, cause Generic instances are useful especially for non-trivial types such as LicenseId. Compile times are no reason to cripple an API, and 30s more are not much in the overall build time of Cabal tbh. If anything, we should try to optimise GHC than to cause regressions in APIs.

[23Skidoo]

I don't understand the argument about orphan instances.

[harpocrates]

I think Herbert is suggesting that consumers of Cabal are likely to be interested in Generic instances of non-trivial types such as LicenseId and may end up declaring their own orphan instances of Generic LicenseId.

[23Skidoo]

OK, makes sense.

[23Skidoo]

For reference: Neil's post about the problem; GHC ticket.

[23Skidoo]

Since there are strong objections, I'm closing this as wontfix.

[hsyl20]

Binary instance not relying on Generics has been implemented in #6001 / d8740cd

[andreasabel]

Recent reinkarnations in:

• Reducing the memory footprint of compiling lib:Cabal #8074
• Remove the Generic instance for LicenseId #8117