I wrote:

...

How do I get a working GHC 7.4.2 on Trusty?

Thanks to all for the suggestions. Daniel Trstenjak wrote, off-list:

I can only recommend: https://launchpad.net/~hvr/+archive/ubuntu/ghc

Thanks Daniel! I hadn't looked at Herbert's ppa for a while. Despite the comments, which say it is only updated for precise but should also work for trusty, the ppa actually *is* updated for trusty now. This is trivially simple to use and worked great, so that's what I ended up using. But still - something really really needs to be done about GHC portability. This thread makes it clear what a mess we have inherited from the days when GHC was primarily a research compiler. Let's face it - GHC is now also a serious production compiler, and this urgently needs to be cleaned up. Thanks, Yitz