Warning: Some installed formulae are not readable: ghc@8.0: unknown version :lion

brew doctor

で
Warning: Some installed formulae are not readable: ghc@8.0: unknown version :lion
と出たので

brew uninstall ghc@8.0

した。

すると、

brew doctor

で
Please note that these warnings are just used to help the Homebrew maintainers
with debugging if you file an issue. If everything you use Homebrew for is
working fine: please don't worry or file an issue; just ignore this. Thanks!

Warning: Your Xcode (10.1) is too outdated.
Please update to Xcode 10.2.1 (or delete it).
Xcode can be updated from the App Store.

Warning: Your Xcode (10.1) is outdated.
Please update to Xcode 10.2.1 (or delete it).
Xcode can be updated from the App Store.

となった。

次にやること

Xcodeのダウンロード。